Private Line covers what has occurred, is occurring, and will ocurr in telecommunications. Since communication technology constantly changes, you can expect new content posted regularly.
Consider this site an authoritative resource. Its moderators have successful careers in the telecommunications industry. Utilize the content and send comments. As a site about communicating, conversation is encouraged.
Thomas Farely
Tom has produced privateline.com since 1995. He is now a freelance technology writer who contributes regularly to the site.
His knowledge of telecommunications has served, most notably, the American Heritage Invention and Technology Magazine and The History Channel.
His interview on Alexander Graham Bell will air on the History Channel the end of 2006.
Ken Schmidt
Ken is a licensed attorney who has worked in the tower industry for seven years. He has managed the development of broadcast towers nationwide and developed and built cell towers.
He has been quoted in newspapers and magazines on issues regarding cell towers and has spoke at industry and non-industry conferences on cell tower related issues.
He is recognized as an expert on cell tower leases and due diligence processes for tower acquisitions.
by John Scourias
jscouria@www.shoshin.uwaterloo.ca
(Reprinting rights and assistance
kindly provided by John Scourias)
Tom Farley comments in maroon.
(Extended quotes by others in blue)
During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe, particularly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. This was an undesirable situation, because not only was the mobile equipment limited to operation within national boundaries, which in a unified Europe were increasingly unimportant, but there was also a very limited market for each type of equipment, so economies of scale and the subsequent savings could not be realized.
The Europeans realized this early on, and in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study and develop a pan-European public land mobile system. The proposed system had to meet certain criteria:
* Good subjective speech quality
* Low terminal and service cost
* Support for international roaming
* Ability to support handheld terminals
* Support for range of new services and facilities
* Spectral efficiency
* ISDN compatibility
Pan-European means European-wide. ISDN throughput at 64Kbs was never envisioned, indeed, the highest rate a normal GSM network can achieve is 9.6kbs.
Europe saw cellular service introduced in 1981, when the Nordic Mobile Telephone System or NMT450 began operating in Denmark, Sweden, Finland, and Norway in the 450 MHz range. It was the first multinational cellular system. In 1985 Great Britain started using the Total Access Communications System or TACS at 900 MHz. Later, the West German C-Netz, the French Radiocom 2000, and the Italian RTMI/RTMS helped make up Europe's nine analog incompatible radio telephone systems. Plans were afoot during the early 1980s, however, to create a single European wide digital mobile service with advanced features and easy roaming. While North American groups concentrated on building out their robust but increasingly fraud plagued and featureless analog network, Europe planned for a digital future. Link to my mobile telephone history series
In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries [6]. Although standardized in Europe, GSM is not only a European standard. Over 200 GSM networks (including DCS1800 and PCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3 million subscribers worldwide [18], which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications.
According to the GSM Association as of 2002, here are the current GSM statistics:
* No. of Countries/Areas with GSM System (October 2001) - 172
* GSM Total Subscribers - 590.3 million (to end of September 2001)
* World Subscriber Growth - 800.4 million (to end of July 2001)
* SMS messages sent per month - 23 Billion (to end of September 2001)
* SMS forecast to end December 2001 - 30 Billion per month
* GSM accounts for 70.7% of the World's digital market and 64.6% of the World's wireless market
http://www.gsmworld.com/membership/mem_stats.html (external link, now dead.)
The developers of GSM chose an unproven (at the time) digital system, as opposed to the then-standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom. They had faith that advancements in compression algorithms and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost. The over 8000 pages of GSM recommendations try to allow flexibility and competitive innovation among suppliers, but provide enough standardization to guarantee proper interworking between the components of the system. This is done by providing functional and interface descriptions for each of the functional entities defined in the system.
The United States suffered no variety of incompatible systems as in the different countries of Europe. Roaming from one city or state to another wasn't difficult . Your mobile usually worked as long as there was coverage. Little desire existed to design an all digital system when the present one was working well and proving popular. To illustrate that point, the American cellular phone industry grew from less than 204,000 subscribers in 1985 to 1,600,000 in 1988. And with each analog based phone sold, chances dimmed for an all digital future. To keep those phones working (and producing money for the carriers) any technological system advance would have to accommodate them.
GSM was an all digital system that started new from the beginning. It did not have to accommodate older analog mobile telephones or their limitations. American digital cellular, first called IS-54 and then IS-136, still accepts the earliest analog phones. American cellular networks evolved slowly, dragging a legacy of underperforming equipment with it. Advanced fraud prevention, for example, was designed in later for AMPS, whereas GSM had such measures built in from the start. GSM was a revolutionary system because it was fully digital from the beginning.
From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signalling used. However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.
Isn't this a shame? What many wireless customers need most is a high speed data connection and this is what GSM provides least. Only 9.6kbs if everything works right. It is possible the GSM designers in the early 1980s never envisioned the need for such bandwidth. It may be true, too, that in most countries the radio spectrum needed to give every caller a 64kbs channel was never available. The add on technology EDGE (external link) promises higher data speed rates in the near to mid-term for GSM. Highest data rates will come in the long term when GSM changes into a radio service based on wide band code division multiple access, and not TDMA.
Using the ITU-T definitions (external link), telecommunication services can be divided into bearer services, teleservices, and supplementary services. The most basic teleservice supported by GSM is telephony. As with all other communications, speech is digitally encoded and transmitted through the GSM network as a digital stream. There is also an emergency service, where the nearest emergency-service provider is notified by dialing three digits (similar to 911).
* Bearer services: Typically data transmission instead of voice. Fax and SMS are examples.
* Teleservices: Voice oriented traffic.
* Supplementary services: Call forwarding, caller ID, call waiting and the like.
A variety of data services is offered. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Since GSM is a digital network, a modem is not required between the user and GSM network, although an audio modem is required inside the GSM network to interwork with POTS.
GSM is an all digital network but many machines are still analog, as is most of the local loop. Thus, we need a modem, even though we are dealing with digital.
A FAX machine's digital signal processor converts an analog image into an instantaneous digital representation; a series of bits, all 0s and 1s. A modulator then turns these bits into audio tones representing the digital values. An analog FAX machine at the other end converts the tones received back into digital bits and then into an image.
This tedious process was required initially because local loops were and are primarily analog. In addition, digital services such as T1, fractional T1, or ISDN, where available, was and is extremely expensive. All digital equipment, such as Group 4 Fax machines, are far higher priced than their analog counterparts. The local loop will remain primarily analog for some time.
Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval [2].
Supplementary services are provided on top of teleservices or bearer services. In the current (Phase I) specifications, they include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), and call barring of outgoing or incoming calls, for example when roaming in another country. Many additional supplementary services will be provided in the Phase 2 specifications, such as caller identification, call waiting, multi-party conversations.
Excellent IEC tutorial on SMS is here: http://www.iec.org/online/tutorials/wire_sms/ (external link)
The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.
The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.
GSM phones use SIM cards, or Subscriber information or identity modules. Memory modules. They're the biggest difference a user sees between a GSM phone or handset and a conventional cellular telephone. With the SIM card and its memory the GSM handset is a smart phone, doing many things a conventional cellular telephone cannot. Like keeping a built in phone book or allowing different ringtones to be downloaded and then stored. Conventional cellular telephones either lack the features GSM phones have built in, or they must rely on resources from the cellular system itself to provide them. Let me make another, important point.
With a SIM card your account can be shared from mobile to mobile, at least in theory. Want to try out your neighbor's brand new mobile? You should be able to put your SIM card into that GSM handset and have it work. The GSM network cares only that a valid account exists, not that you are using a different device. You get billed, not the neighbor who loaned you the phone.
This flexibility is completely different than AMPS technology, which enables one device per account. No swtiching around. Conventional cellular telephones have their electronic serial number burned into a chipset which is permanently attached to the phone. No way to change out that chipset or trade with another phone. SIM card technology, by comparison, is meant to make sharing phones and other GSM devices quick and easy.
On the left above: Front of a Pacific Bell GSM phone. In the middle above: Same phone, showing the back. The SIM card is the white plastic square. It fits into the grey colored holder next to it. On the right above. A new and different idea, a holder for two SIM cards, allowing one phone to access either of two wireless carriers. Provided you have an account with both. :-) The Sim card is to the left of the body.
The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.
An explanation of the Abis interface is here
The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
The BTS or Base Transceiver Station is also called an RBS or Remote Base station. Whatever the name, this is the radio gear that passes all calls coming in and going out of a cell site.
The base station is under direction of a base station controller so traffic gets sent there first. The base station controller, described below, gathers the calls from many base stations and passes them on to a mobile telephone switch. From that switch come and go the calls from the regular telephone network.
Some base stations are quite small, the one pictured here is a large outdoor unit. The large number of base stations and their attendant controllers, are a big difference between GSM and IS-136.
The Base Station Controller
The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).
Another difference between conventional cellular and GSM is the base station controller. It's an intermediate step between the base station transceiver and the mobile switch. GSM designers thought this a better approach for high density cellular networks. As one anonymous writer penned, "If every base station talked directly to the MSC, traffic would become too congested. To ensure quality communications via traffic management, the wireless infrastructure network uses Base Station Controllers as a way to segment the network and control congestion. The result is that MSCs route their circuits to BSCs which in turn are responsible for connectivity and routing of calls for 50 to 100 wireless base stations."
Many GSM descriptions picture equipment called a TRAU, which stands for Transcoding Rate and Adaptation Unit. Of course. Also known as a TransCoding Unit or TCU, the TRAU is a compressor and converter. It first compresses traffic coming from the mobiles through the base station controllers. That's quite an achievement because voice and data have already been compressed by the voice coders in the handset. Anyway, it crunches that data down even further. It then puts the traffic into a format the Mobile Switch can understand. This is the transcoding part of its name, where code in one format is converted to another. The TRAU is not required but apparently it saves quite a bit of money to install one. Here's how Nortel Networks sells their unit:
"Reduce transmission resources and realize up to 75% transmission cost savings with the TCU."
"The TransCoding Unit (TCU), inserted between the BSC and MSC, enables speech compression and data rate adaptation within the radio cellular network. The TCU is designed to reduce transmission costs by minimizing transmission resources between the BSC and MSC. This is achieved by reducing the number of PCM links going to the BSC, since four traffic channels (data or speech) can be handled by one PCM time slot. Additionally, the modular architecture of the TCU supports all three GSM vocoders (Full Rate, Enhanced Full Rate, and Half Rate) in the same cabinet, providing you with a complete range of deployment options."
(PCM? To read more about that click here.)
Voice coders or vocoders are built into the handsets a cellular carrier distributes. They're the circuitry that turns speech into digital. The carrier specifies which rate they want traffic compressed, either a great deal or just a little. The cellular system is designed this way, with handset vocoders working in league with the equipment of the base station subsystem.
A GSM network is composed of several functional entities, whose functions and interfaces are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile services Switching Center (MSC), performs the switching of calls between the mobile users, and between mobile and fixed network users. The MSC also handles the mobility management operations. Not shown is the Operations and Maintenance Center, which oversees the proper operation and setup of the network. The Mobile Station and the Base Station Subsystem communicate across the Um interface, also known as the air interface or radio link. The Base Station Subsystem communicates with the Mobile services Switching Center across the A interface.
As John states, he presents a generic GSM architecture. Lucent, Ericsson, Nokia, and others feature their own vision in their own diagrams. But they all share the same main elements and parts from different vendors should all work together. The links below show how these vendors picture the GSM architecture. You can remember the different terms much better by looking at all these diagrams.
Lucent GSM architecture/ Ericsson GSM architecture / Nokia GSM architecture / Siemen's GSM architecture
Figure 1. General architecture of a GSM network
SIM: Subscriber identify module.
ME: Mobile equipment.
BTS: Base transceiver station.
BSC: Base station controller.
HLR: Home location register.
VLR: Visitor location register.
MSC: Mobile services switching center.
EIR: Equipment identity register.
AuC: Authentication Center.
UM: Represents the radio link.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data network.
The Mobile Switch
Picture of a 5ESSThe central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjunction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signalling between functional entities in the Network Subsystem uses Signalling System Number 7 (SS7), used for trunk signalling in ISDN and widely used in current public networks.
.pdf file on SS7 and mobile networking -- Good reading!
Mobile switches go by many names: mobile switch (MS), mobile switching center (MSC), or mobile telecommunications switching office (MTSO). They all do the same thing, however, and that is to process mobile telephone calls. This switch can be a normal landline switch like a 5ESS, a Nokia, an Alcatel, or an Ericsson AXE (Automatic Exchange Electric) or a dedicated switch, built just to handle mobile calls. Each mobile switch manages dozens to scores of cell sites. In GSM the mobile switch handles cell sites by first directing the base station controllers. Large systems may have two or more MSCs. It's easy understand what a switch does. What is harder to understand is the role the switch has to do with other network resources.
Home Location Register and the Visitor/ed Location Register
The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signalling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signalling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.
The Home Location Register and the Visitor or Visited Location Register work together -- they permit both local operation and roaming outside the local service area. You couldn't use your mobile in San Francisco and then Los Angeles without these two electronic directories sharing information. Most often these these two directories are located in the same place, often on the same computer.
The HLR and VLR are big databases maintained on computers called servers, often UNIX workstations. Companies like Tandem, now part of Compaq, make the servers, which they call HLRs when used for cellular. These servers maintain more than the home location register, but that's what they call the machine. Many mobile switches use the same HLR. So, you'll have many Home Location Registers. To operate its nationwide cellular system, iDEN, Motorola uses over 60 HLRs nationwide.
The HLR stores complete local customer information. It's the main database. Signed up for cellular service in Topeka? Your carrier puts your information on its nearest HRL, or the one assigned to your area. That info includes your international mobile equipment identity number or IMEI, your directory number, and the class of service you have. It also includes your current city and your last known "location area," the place you last used your mobile.
The VLR or visitor location registry contains roamer information. Passing through another carrier's system? Once the visited system detects your mobile, its VLR queries your assigned home location register. The VLR makes sure you are a valid subscriber, then retrieves just enough information from the now distant HLR to manage your call. It temporarily stores your last known location area, the power your mobile uses, special services you subscribe to and so on. Though traveling, the cellular network now knows where you are and can direct calls to you.
The equipment Identity Register and the Authentication Center
The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.
"The Equipment Identity Register (EIR) is a standard GSM network element that allows a mobile network to check the type and serial number of a mobile device and determine whether or not to offer any service." The EIR or equipment identity register is yet another database. It's first purpose is to deny stolen or defective mobiles service. Good mobiles are allowed on the network, of course, as is faulty but still serviceable equipment. In the latter case such mobiles are flagged for the cellular carrier to monitor.
The AC or AuC is the Authentication Center, a secured database handling authentication and encryption keys. Authentication verifies a mobile customer with a complex challenge and reply routine. The network sends a randomly generated number to the mobile. The mobile then performs a calculation against it with a number it has stored in its SIM and sends the result back. Only if the switch gets the number it expects does the call proceed. The AC stores all data needed to authenticate a call and to then encrypt both voice traffic and signaling messages.
The Interfaces
Cellular radio's most cryptic terms belong to these names: A, Um, Abis, and Ater. A telecom interface means many things. It can be a mechanical or electrical link connecting equipment together. Or a boundary between systems, such as between the base station system and the network subsystem. GSM calls that one Interface "A", remember? To be more specific, Smith says "A" is the signaling link between the two subsystems. Which brings us to the point I want to make.
Interfaces are standardized methods for passing information back and forth. The transmission media isn't important. Whether copper or fiber optic cable or microwave radio, an interface insists that signals go back and forth in the same way, in the same format. With this approach different equipment from any manufacturer will work together. See my page on standards.
Let's consider the the A-bis interface as an example. Tektronix says the A-bis "is a French term meaning 'the second A Interface.' " Good grief! In most cases the actual span or physical connection is made on a T1 line or in Europe its equivalent, the E1.But regardless of the material used, the transmission media, it is the signaling protocol that is most important.
Although the interface is unlabeled, the mobile switch communicates with the telephone network using Signaling System Seven, an internationally agreed upon standard. More specifically, it uses ISUP over SS7. As the Performance Technologies people tersely put in in their tutorial on SS7, "ISUP defines the protocol and procedures used to set-up, manage, and release trunk circuits that carry voice and data calls over the public switched telephone network (PSTN). ISUP is used for both ISDN and non-ISDN calls."
Using SS7 throughout is a big difference between conventional cellular and GSM. IS-136 and IS-95 also uses SS7 but to communicate between the HLR and VLR it uses a standard called IS-41.
What about the mysterious UM? That's the radio link between a mobile and a base station. Um are the actual radio frequencies that calls are put on. Possibly the letters stand for User Mobile. R.C. Levine clears up this matter nicely,
"Interface names (A, Abis, B, C, etc.) were arbitrarily assigned in alphabetical order. The Um label is taken from the customer-network U interface label used in ISDN. Although mnemonics have been proposed for these letters, they are after-the-fact."
.pdf file on SS7 and mobile networking -- Good reading!
Figure 1. General architecture of a GSM network
SIM: Subscriber identify module.
BSC: Base station controller.
MSC: Mobile services switching center.
UM: Represents the radio link.
ME: Mobile equipment.
HLR: Home location register.
EIR: Equipment identity register.
BTS: Base transceiver station.
VLR: Visitor location register.
AuC: Authentication Center.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data network.
The International Telecommunication Union (ITU), which manages the international allocation of radio spectrum (among many other functions), allocated the bands 890-915 MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base station to mobile station) for mobile networks in Europe. Since this range was already being used in the early 1980s by the analog systems of the day, the CEPT had the foresight to reserve the top 10 MHz of each band for the GSM network that was still being developed. Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.
Cellular Radio frequencies around the world
| American Cellular | ||
| AMPS, N-AMPS, D-AMPS (IS-136) CDMA |
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| Narrowband | 901-941 MHz | |
| Broadband |
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| E-TACS | ||
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| GSM | ||
| GSM has three main frequency bands around the world: 900 MHz, 1800 MHz, and 1900 MHz. It all depends on the country. Other bands may be used in the future or may be in trial right now. |
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| 1800MHz | ||
| 1900 MHz. | ||
| JDC | ||
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GSM frequency spacing is 200Khz, AMPS is 30 Khz
American PCS/GSM/ Cellular frequencies
(A more specific view)
Since radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible. The method chosen by GSM is a combination of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The FDMA part involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.
This is the correct, complete view of GSM. It's not enough to say, as I have too many times, that GSM and conventional cellular (IS-136) are TDMA based. While that it is true, it is more true to say such systems are TDMA and FDM based. First, we have a number of radio frequencies, each separated by 200khz. This is the frequency division multiplexing part. (Or the FDMA part, a minor semantic difference.) Secondly, we have the transmission technology, TDMA, by which we put several calls on a single frequency. These calls are broken into many pieces, each piece of each call sent one after another. Each call separated by slight differences in time. GSM is a TDMA/FDMA system.
Weick calls a burst "a sequence of signals counted as a unit in accordance with some specific criterion or measure." Bits are single pulses of electrical energy. Much like the single dash of a Morse Code key. With Morse code we use long and short pulses of energy to stand for letters. Although of uniform length, the pulses we use in digital radio do the same thing. Bits grouped in patterns represent voice and data. We also use bits, as shown in the diagram below, for signaling. In the channel depicted a burst of bits is a marker, an indicator, a signal within a signal. It's what the mobile first looks for in the digital stream flowing from the base station. More on this on the next page.
Channels are defined by the number and position of their corresponding burst periods. All these definitions are cyclic, and the entire pattern repeats approximately every 3 hours. Channels can be divided into dedicated channels, which are allocated to a mobile station, and common channels, which are used by mobile stations in idle mode.
Terminology alert! Cellular radio uses the word channel in many ways. It is a pair of radio frequencies. And channels are part of the digital stream that flows back and forth from the mobile to the base station. Channels, therefore, can be carried on a channel. Confusing, isn't it? The discussion below focuses on data channels, not radio channels.
A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused (see Figure 2). TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics.
We've seen these characters before. Reading the Channels page might help you understand what follows. We'll discuss them individually as they come up later in the article.
In addition to these full-rate TCHs, there are also half-rate TCHs defined, although they are not yet implemented. Half-rate TCHs will effectively double the capacity of a system once half-rate speech coders are specified (i.e., speech coding at around 7 kbps, instead of 13 kbps). Eighth-rate TCHs are also specified, and are used for signalling. In the recommendations, they are called Stand-alone Dedicated Control Channels (SDCCH).
Common channels can be accessed both by idle mode and dedicated mode mobiles. The common channels are used by idle mode mobiles to exchange the signalling information required to change to dedicated mode. Mobiles already in dedicated mode monitor the surrounding base stations for handover and other information. The common channels are defined within a 51-frame multiframe, so that dedicated mobiles using the 26-frame multiframe TCH structure can still monitor control channels. The common channels include:
Dedicated mode means a mobile is in use. Dedicated to service. Control and common channels seem to be synonymous terms. Speaking of terms, don't try to memorize these channel names and functions. You will remember them soon, especially when we go over call processing in GSM. Bookmark or make this page a favorite so you can come back later. The GSM standard covers more than 5,000 pages so expect this kind of complexity. But keep reading the discussion. I think after you've glanced at this table you will stay interested in the article. BTW, these are just some of the channels . . .
| Control Channels | Channel Types | Usage |
| Broadcast Control Channel (BCCH) |
Broadcast downlink (Base station to mobile) |
Continually broadcasts, on the downlink, information including base station identity, frequency allocations, and frequency-hopping sequences. |
| Frequency Correction Channel (FCCH) | Broadcast downlink | Used to synchronise the mobile to the time slot structure of a cell by defining the boundaries of burst periods, and the time slot numbering. Every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are by definition on time slot number 0 (within a TDMA frame). |
| Synchronisation Channel (SCH) | Broadcast downlink | |
| Random Access Channel (RACH) |
Common uplink (Mobile to base station) |
Slotted Aloha channel used by the mobile to request access to the network. (p.s. I love that term "Aloha"; appropriate and to the point) |
| Paging Channel (PCH) |
Common downlink (Base station to mobile) |
Used to alert the mobile station of an incoming call. |
| Access Grant Channel (AGCH) | Broadcast downlink | Used to allocate an SDCCH to a mobile for signalling (in order to obtain a dedicated channel), following a request on the RACH. |
| Slow Associated Control Channel (SACCH) | Uplink and downlink | In every traffic channel. Used for low rate, non critical signaling. |
| Fast Associated Control Channel (FACCH) | Uplink and downlink | "A high rate signaling channel, used during call establisment, subscriber authentication, and for handover comands." Macario |
There are four different types of bursts used for transmission in GSM [16]. The normal burst is used to carry data and most signalling. It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.
The F burst, used on the FCCH, and the S burst, used on the SCH, have the same length as a normal burst, but a different internal structure, which differentiates them from normal bursts (thus allowing synchronization). The access burst is shorter than the normal burst, and is used only on the RACH.
Whoa, whoa, whoa! Too much information too quickly. Let's go slow. Four bursts exist:
1) The normal burst
2) The "F" or frequency control burst
3) The "S" or synchronous control burst
4) The access control burst.
There are many references below to quarter bits, which is really an impossibility. They are instead an effective quarter bit. All bits have fixed sizes save the guard bits. As you'll see we need a total rate of 148 bits for a burst. But we can't come up with an even 148 bits without some "slop" or adjusting. That's where the guard bits come in. The time rate for those is equivalent to 8.25 bits. Don't let this put you off, you will see what I mean as you look over the diagrams.
Remember, too, that you don't need to commit this all to memory; bookmark this page or make it a favorite so you can come back for reference.
Now, let's take a look at the most common burst first, the normal burst.
1) The Normal Burst
Pictured above is a burst of bits. A poetic name, eh? One can also call it a data packet. This normal burst is just one of four possible within a single GSM TDMA time slot. We've already seen how this burst fits within the data stream in GSM. Now we look at the burst itself. Let's see, what did John say about this burst?:
The normal burst is used to carry data and most signaling. It has a total length of 156.25 bits, made up of two 57 bit information bits, a 26 bit training sequence used for equalization, 1 stealing bit for each information block (used for FACCH), 3 tail bits at each end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bits are transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.
This burst carries our conversation in digital form. That's what the two 57 information, message, or data bits are for. The normal burst also carries signaling information needed to manage call processing, that is, data for setting up, maintaining, and then ending a call. What then are training, tail, stealing, and guard bits? Once again we go step by step.
a.) Training sequence bits. Used for equalization. Bits which get the base station and mobile in "tune" with each other. You need some background. As John will write later on,
At the 900 MHz [and 1900 Mhz] range, radio waves bounce off everything -- buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections.
So while traffic is being transmitted, equalization bits in every time slot work to keep that traffic in phase with the base station and the mobile. It is a continuous, automatic, ongoing operation, as the equalizers try to compensate for the problems found in any radio path.
Click here to "see" the effects of equalization
b.) Stealing bits. Whereby a bit is stolen from message bits, just temporarily, to make way for the Fast Associated Channel. It runs in a blank and burst mode. It transmits during handovers or when the slow associated channel can't send information quickly enough.. Like when entering a tunnel or possibly when a large truck gets in front of you. At that point the data link might be broken so the FACCH acts quickly. As an engineer puts it, "The FACCH overrides the voice payload, degrading speech quality to convey control information." This keeps Mr. Mobile linked to the base station.
c.) Tail bits: It's my understanding that tail bits clear the code that has gone before, setting everything back to 0 or a null state.
d.) Guard bits: Empty time spaces separating data packets to make sure one burst does not run into another. Scourias is more specific. He says the guard period allows "the sender some freedom to shift transmission timing to allow the receiver to receive aligned bursts." Guard bits, in other words, permit some leeway or slack.
2) The "F" or Frequency Control Burst
Significant for its lack of significance. 142 "O" bits, essentially an empty frame. But it is so distinctive that it acts as an important marker in call processing.
3) The "S" or Synchronous Control Burst
Welcome to the synchronization burst. What the base station transmits to a mobile to get in order with the rest of the digital traffic. It exists, not surprisingly, on a channel called the Synchronization Channel or SCH. More on this in call processing.
More on frames, slots, and channels here
4) The Access Control Burst.
Another distinctive digital signature in the data stream from the handset to the base station. The access control burst is only broadcast on the random access channel or RACH. Macario says a mobile uses it to request for a "subsequent operation, e.g., to establish a call or perform a location update." This channel occurs only on the uplink, that is, from the mobile to base station.
Unsure about bits and bytes? This won't take long to read: http://www.privateline.com/bitsandbytes/bitsandbytes.htm
Speech coding means turning voice into digital. I've written much on this subject so be sure to click on the links below if there are points you don't understand . . .
GSM is a digital system, so speech which is inherently analog, has to be digitized. The method employed by ISDN, and by current telephone systems for multiplexing voice lines over high speed trunks and optical fiber lines, is Pulse Coded Modulation (PCM). The output stream from PCM is 64 kbps, too high a rate to be feasible over a radio link. The 64 kbps signal, although simple to implement, contains much redundancy. The GSM group studied several speech coding algorithms on the basis of subjective speech quality and complexity (which is related to cost, processing delay, and power consumption once implemented) before arriving at the choice of a Regular Pulse Excited -- Linear Predictive Coder (RPE--LPC) with a Long Term Predictor loop.
Conventional cellular uses an equally intimidating algorithm named Vector Sum Excited Linear Predictive speech compression. Ugh. Click here to learn about it.
Basically, information from previous samples, which does not change very quickly, is used to predict the current sample. The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.
This is the subject of digital signal processing. Read about it here.
This is the so-called Full-Rate speech coding. Recently, an Enhanced Full-Rate (EFR) speech coding algorithm has been implemented by some North American GSM1900 operators. This is said to provide improved speech quality using the existing 13 kbps bit rate.
Nokia said in January, 1997 that they would start shipping Enhanced Full Rate voice codecs by March 1997: http://press.nokia.com/PR/199701/775480_5.html; I must assume their use is now wide spread.
Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding and block interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.
Radio waves are a rough medium to transmit fragile data over; we need a way to protect that information. We do so with error checking, mathematical routines that check and then double-check the integrity of our data. These methods contribute greatly to the overhead in a digital stream, adding a tremendous amount of bits, and thus dramatically cutting down on data speed. It's one reason data transfer rates are only 9.6kbs. This is a complex subject, one I haven't written much on.
Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this block were more important for perceived speech quality than others. The bits are thus divided into three classes:
* Class Ia 50 bits - most sensitive to bit errors
* Class Ib 132 bits - moderately sensitive to bit errors
* Class II 78 bits - least sensitive to bit errors
Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightly attenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder of constraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remaining Class II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.
To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from two different speech samples.
Recall that each time-slot burst is transmitted at a gross bit rate of 270.833 kbps. This digital signal is modulated onto the analog carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK). GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related to power consumption, which should be minimized for the mobile station. The spurious radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit adjacent channel interference, and allow for the co-existence of GSM and the older analog systems (at least for the time being).
For much, much more on GMSK, read Professor Levine's comments by clicking here. This discussion is quite advanced.
You can read my writing on modulation by clicking here.
At the 900 MHz range, radio waves bounce off everything - buildings, hills, cars, airplanes, etc. Thus many reflected signals, each with a different phase, can reach an antenna. Equalization is used to extract the desired signal from the unwanted reflections. It works by finding out how a known transmitted signal is modified by multipath fading, and constructing an inverse filter to extract the rest of the desired signal. This known signal is the 26-bit training sequence transmitted in the middle of every time-slot burst. The actual implementation of the equalizer is not specified in the GSM specifications.
Here are two old Western Union images. The top graphic shows transmission without a delay equalizer. The image below it shows the same transmission corrected by a delay equalizer.
Above. No equalizer.
Above. Delay equalizer introduced. Pretty dramatic difference, eh?
The mobile station already has to be frequency agile, meaning it can move between a transmit, receive, and monitor time slot within one TDMA frame, which normally are on different frequencies. GSM makes use of this inherent frequency agility to implement slow frequency hopping, where the mobile and BTS transmit each TDMA frame on a different carrier frequency. The frequency hopping algorithm is broadcast on the Broadcast Control Channel. Since multipath fading is dependent on carrier frequency, slow frequency hopping helps alleviate the problem. In addition, co-channel interference is in effect randomized.
Here's a huge difference between conventional cellular (IS-136) and GSM: frequency hopping. When enabled, slots within frames can leapfrog from one frequency to another. In IS-136, by comparison, once assigned a channel your call stays on that pair of radio frequencies until the call is over or you have moved to another cell.
Minimizing co-channel interference is a goal in any cellular system, since it allows better service for a given cell size, or the use of smaller cells, thus increasing the overall capacity of the system. Discontinuous transmission (DTX) is a method that takes advantage of the fact that a person speaks less that 40 percent of the time in normal conversation [22], by turning the transmitter off during silence periods. An added benefit of DTX is that power is conserved at the mobile unit.
The most important component of DTX is, of course, Voice Activity Detection. It must distinguish between voice and noise inputs, a task that is not as trivial as it appears, considering background noise. If a voice signal is misinterpreted as noise, the transmitter is turned off and a very annoying effect called clipping is heard at the receiving end. If, on the other hand, noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased. Another factor to consider is that when the transmitter is turned off, there is total silence heard at the receiving end, due to the digital nature of GSM. To assure the receiver that the connection is not dead, comfort noise is created at the receiving end by trying to match the characteristics of the transmitting end's background noise.
Levine (link to his cellular .pdf file) says that Voice Activity Detection or VAD is the 'gimmick" that enables greater call capacity in CDMA based (IS-95) systems. Not anything special with CDMA. I will let the experts argue that point. The clipping that John mentions is just the thing that makes digital audio generally inferior to analog. Analog audio quality, where a signal mereley fades instead of cutting out, almost always sounds better than digital.
The chief benefit of TDMA to cellular operators is increasing call capacity by multiplexing. With GSM and conventional cellular you put eight calls on a frequency pair compared to one call per pair with analog. But increased capacity does not necessarily benefit the callers, since most digital routines play havoc with voice quality. An uncompressed, non-multiplexed, bandwidth hogging analog signal simply sounds better than its present day compressed, digital counterpart. As Consumers Digest put it:
"Digital cellular service does have a couple of drawbacks, the most important of which is audio quality. Analog cellular phones sound worlds better. Many folks have commented on what we call the 'Flipper Effect." It refers to the sound of your voice taking on an 'underwater-like' quality with many digital phones. In poor signal areas or when cell sites are struggling with high call volume, digital phones will often lose full-duplex capability (the ability of both parties to talk simultaneously), and your voice may break up and sound garbled." Consumers Digest, August, 2000.
One more thing to think about when considering digital, is that a digital signal increases bandwidth compared to analog. It is only compression that makes digital comparable in bandwidth to analog. As Fike says:
The most noticeable disadvantage that is directly associated with digital systems is the additional bandwidth necessary to carry the digital signal as opposed to its analog counterpart. A standard T1 transmission link carrying a DS-1 signal transmits 24 voice channels of about 4kHz each. The digital transmission rate on the link is 1.544 Mbps, and the bandwidth re-quired is about 772 kHz. Since only 96 kHz would be required to carry 24 analog channels (4khz x 24 channels), about eight times as much bandwidth is required to carry the digitally (722kHz / 96 = 8.04)." Fike, John L. and George Friend, Understanding Telephone Electronics SAMS, Carmel 1983. p. 164
I write more about this here.
Another method used to conserve power at the mobile station is discontinuous reception. The paging channel, used by the base station to signal an incoming call, is structured into sub-channels. Each mobile station needs to listen only to its own sub-channel. In the time between successive paging sub-channels, the mobile can go into sleep mode, when almost no power is used.
All of this increases battery life considerably when compared to analog phones.
There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).
We need only enough power to make a connection. Any more is superfluous. If you can't make a connection using one watt then two watts won't help at these near microwave frequencies. Using less power means less interference or congestion among all the mobiles in a cell.
The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in co-channel cells alternatingly increase their power in response to increased co-channel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.
Two points. The first is that the base station can reach out to the mobile and turn down the transmitting power the handset is using. Very cool. The second point is that a digital signal will drop a call much more quickly than an analog signal. With an analog radio you can hear through static and fading. But with a digital radio the connection will be dropped, just like your landline modem, when too many 0s and 1s go missing. You need more base stations, consequently, to provide the same coverage as analog
Ensuring the transmission of voice or data of a given quality over the radio link is only part of the function of a cellular mobile network. A GSM mobile can seamlessly roam nationally and internationally, which requires that registration, authentication, call routing and location updating functions exist and are standardized in GSM networks. In addition, the fact that the geographical area covered by the network is divided into cells necessitates the implementation of a handover mechanism. These functions are performed by the Network Subsystem, mainly using the Mobile Application Part (MAP) built on top of the Signalling System No. 7 protocol.
Mobiles can in fact only roam seamlessly if they are multi-band units. Most international phones have two bands, one for the Americas at 1900Mhz, and one for Europe at 900Mhz. Others such as the Ericsson R380 show below, cover the 1800Mhz band as well. This lets the phone roam on Asian and African networks.
The mobile switch communicates with the telephone network using Signaling System Seven, an internationally agreed upon standard. IS-136 and IS-95 also uses SS7. But it uses a standard called IS-41 when communicating between the Home Location Register and the Visitor Location register. (Source for this IS-41 information is http://www.mobilein.com/mobile_basics.htm)
.pdf file on SS7 and mobile networking -- Good reading!
The signalling protocol in GSM is structured into three general layers [1], [19], depending on the interface, as shown in Figure 3. Layer 1 is the physical layer, which uses the channel structures discussed above over the air interface. Layer 2 is the data link layer. Across the Um interface, the data link layer is a modified version of the LAPD protocol used in ISDN (external link), called LAPDm. Across the A interface, the Message Transfer Part layer 2 of Signalling System Number 7 is used. Layer 3 of the GSM signalling protocol is itself divided into 3 sublayers.
* Radio Resources Management
* Controls the setup, maintenance, and termination of radio and fixed channels, including handovers.
* Mobility Management
* Manages the location updating and registration procedures, as well as security and authentication.
* Connection Management
* Handles general call control, similar to CCITT Recommendation Q.931, and manages Supplementary Services and the Short Message Service.
Signalling between the different entities in the fixed part of the network, such as between the HLR and VLR, is accomplished throught the Mobile Application Part (MAP). MAP is built on top of the Transaction Capabilities Application Part (external link) (TCAP, the top layer of Signalling System Number 7. The specification of the MAP is quite complex, and at over 500 pages, it is one of the longest documents in the GSM recommendations [16].
Figure 3. Signalling protocol structure in GSM
I've not written on layers and feel they are beyond the scope of this site.
The radio resources management (RR) layer oversees the establishment of a link, both radio and fixed, between the mobile station and the MSC. The main functional components involved are the mobile station, and the Base Station Subsystem, as well as the MSC. The RR layer is concerned with the management of an RR-session [16], which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels including the allocation of dedicated channels.
An RR-session is always initiated by a mobile station through the access procedure, either for an outgoing call, or in response to a paging message. The details of the access and paging procedures, such as when a dedicated channel is actually assigned to the mobile, and the paging sub-channel structure, are handled in the RR layer. In addition, it handles the management of radio features such as power control, discontinuous transmission and reception, and timing advance.
Paging means an incoming call for a mobile.
In a cellular network, the radio and fixed links required are not permanently allocated for the duration of a call. Handover, or handoff as it is called in North America, is the switching of an on-going call to a different channel or cell. The execution and measurements required for handover form one of basic functions of the RR layer.
There are four different types of handover in the GSM system, which involve transferring a call between:
* Channels (time slots) in the same cell
* Cells (Base Transceiver Stations) under the control of the same Base Station Controller (BSC),
* Cells under the control of different BSCs, but belonging to the same Mobile services Switching Center (MSC), and
* Cells under the control of different MSCs.
The first two types of handover, called internal handovers, involve only one Base Station Controller (BSC). To save signalling bandwidth, they are managed by the BSC without involving the Mobile services Switching Center (MSC), except to notify it at the completion of the handover. The last two types of handover, called external handovers, are handled by the MSCs involved. An important aspect of GSM is that the original MSC, the anchor MSC, remains responsible for most call-related functions, with the exception of subsequent inter-BSC handovers under the control of the new MSC, called the relay MSC.
Handovers can be initiated by either the mobile or the MSC (as a means of traffic load balancing). During its idle time slots, the mobile scans the Broadcast Control Channel of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover, based on the received signal strength. This information is passed to the BSC and MSC, at least once per second, and is used by the handover algorithm.
The algorithm for when a handover decision should be taken is not specified in the GSM recommendations. There are two basic algorithms used, both closely tied in with power control. This is because the BSC usually does not know whether the poor signal quality is due to multipath fading or to the mobile having moved to another cell. This is especially true in small urban cells.
The 'minimum acceptable performance' algorithm [3] gives precedence to power control over handover, so that when the signal degrades beyond a certain point, the power level of the mobile is increased. If further power increases do not improve the signal, then a handover is considered. This is the simpler and more common method, but it creates 'smeared' cell boundaries when a mobile transmitting at peak power goes some distance beyond its original cell boundaries into another cell.
The 'power budget' method [3] uses handover to try to maintain or improve a certain level of signal quality at the same or lower power level. It thus gives precedence to handover over power control. It avoids the 'smeared' cell boundary problem and reduces co-channel interference, but it is quite complicated.
Power control is a fascinating if complex issue. Tim Holliday writes about it in a most lucid fashion:
"The problem of power control for wireless communications has been well studied. Consider the typical setup of a group of mobile devices transmitting data to a base station. These mobile devices are faced with time-varying wireless channels, where the path loss in the channel and interference from other users changes randomly over time. As the path loss or interference increases the probability of a mobile device successfully transmitting data goes down."
"Or put another way, think of trying to hold a conversation with a friend in a crowded room your voice is the mobile transmitter and your friend's ear is the base station. Interference is like the voices of other people in the room; if they are speaking at a high volume your friend will not be able to distinguish your voice. Path loss, on the other hand, results from the appearance of objects (e.g. a vase, table, or door) between you and your friend. Of course, in the context of wireless communications, path loss is caused by much larger objects like hills, buildings, and so forth."
"If the channel conditions (path loss and interference) in the crowded room are poor, you can attempt to communicate with your friend by shouting, or by using very simple words or hand signals. Another option is to wait for everyone else to quiet down or move to another part of the room. This is analogous to what we try to do for wireless devices if conditions are poor, we can raise the transmitter power (start shouting), reduce coding complexity (use simpler words), or withhold transmission until the channel improves."
Tim Holliday, Management Science and Engineering Department, Stanford University. The quotation was from this URL, now dead: http://sll.stanford.edu/projects/i-rite/body_holliday.html
Power control also has a bearing on equalizing, which was written about earlier in this article.
The Mobility Management layer (MM) is built on top of the RR layer (radio resources), and handles the functions that arise from the mobility of the subscriber, as well as the authentication and security aspects. Location management is concerned with the procedures that enable the system to know the current location of a powered-on mobile station so that incoming call routing can be completed.
A powered-on mobile is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. One extreme would be to page every cell in the network for each call, which is obviously a waste of radio bandwidth. The other extreme would be for the mobile to notify the system, via location updating messages, of its current location at the individual cell level. This would require paging messages to be sent to exactly one cell, but would be very wasteful due to the large number of location updating messages. A compromise solution used in GSM is to group cells into location areas. Updating messages are required when moving between location areas, and mobile stations are paged in the cells of their current location area.
In conventional cellular location messages are sent to the exact cell a mobile is in.
To review, the VLR Data Base, or Visited or Visitor Location Register, contains all the data needed to communicate with the mobile switch. Levine says this data includes:
* Equipment identity and authentication-related data
* Last known Location Area (LA)
* Power Class and other physical attributes of the mobile or handset
* List of special services available to this subscriber
* More data entered while engaged in a Call
* Current cell
* Encryption keys
The location updating procedures, and subsequent call routing, use the MSC and two location registers: the Home Location Register (HLR) and the Visitor Location Register (VLR). When a mobile station is switched on in a new location area, or it moves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR, which records the location area information, and then sends the location information to the subscriber's HLR. The information sent to the HLR is normally the SS7 address of the new VLR, although it may be a routing number. The reason a routing number is not normally assigned, even though it would reduce signalling, is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for incoming calls. If the subscriber is entitled to service, the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration.
All of these abbreviations are covered on this page.
For reliability reasons, GSM also has a periodic location updating procedure. If an HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the database up to date would cause overloading. Therefore, the database is updated as location updating events occur. The enabling of periodic updating, and the time period between periodic updates, is controlled by the operator, and is a trade-off between signalling traffic and speed of recovery. If a mobile does not register after the updating time period, it is deregistered.
SIM: Subscriber identify module.
BSC: Base station controller.
MSC: Mobile services switching center.
UM: Represents the radio link.
ME: Mobile equipment.
HLR: Home location register.
EIR: Equipment identity register.
BTS: Base transceiver station.
VLR: Visitor location register.
AuC: Authentication Center.
Abis: Represents the interface between the base stations and base station controllers.
"A": The interface between the base station subsystem and the network subsystem.
PSTN and PSPDN: Public switched telephone network and packet switched public data network.
Figure 1. General architecture of a GSM network
A procedure related to location updating is the IMSI (International Mobile Subscriber Identity) attach and detach. A detach lets the network know that the mobile station is unreachable, and avoids having to needlessly allocate channels and send paging messages. An attach is similar to a location update, and informs the system that the mobile is reachable again. The activation of IMSI attach/detach is up to the operator on an individual cell basis.
Since the radio medium can be accessed by anyone, authentication of users to prove that they are who they claim to be, is a very important element of a mobile network. Authentication involves two functional entities, the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key, one copy of which is stored in the SIM card and the other in the AuC. During authentication, the AuC generates a random number that it sends to the mobile. Both the mobile and the AuC then use the random number, in conjuction with the subscriber's secret key and a ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated [16].
The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key, together with the TDMA frame number, use the A5 algorithm to create a 114 bit sequence that is XORed with the 114