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Telecommunications Fundamentals, Chapter 4A: Fundamentals of Switching
#1ESS
In 1968 AT&T introduced a new family of switches called Electronic Switching Systems (ESS). These switches differed from their predecessors in several ways: Stored Program Control (SPC) represents software that allows increased flexibility in making changes to the switch. Use of flexible concentration ratios that allow a designer to change the concentration ratio of the switch based on need. Introduction of redundancy in the major components of the switch (dramatically increased reliability) Introduction of the Switching Control Center (SCC) which allows for automatic monitoring and maintenance capabilities from a remote location. Capability of servicing 60,000 end users
Features
Introduced by AT&T just a few years later, the #lAESS switch had an additional attached processor and memory which allowed for significantly more sophisticated functionality such as: 100,000 line capacity Additional self-diagnostics and remote self-maintenance Software more "user-friendly" AMA provided by means of disk drive with magnetic tape backup In addition, the switch footprint of the lAESS was less than 1/3 the size of the #lESS. Issues associated with the implementation of Electronic Switching Systems were: Higher initial capital investment due to the electronic components and circuitry required for the switch Introduction of an abstract technology required a significant leap in skills for personnel maintaining the switch Fewer switching technicians were required
Introduction of the SCC
As illustrated in Figure 4.9, major hardware components of the ESS analog switch are: Line Scanner Network Portion of the Switch (Line cards, Line and Trunk translations) Central Processing Unit (CPU) Furthermore, there are additional requirements: Generic (or Operating System) Translations Line/Trunk Switch Temporary Memory Let's now process a call in the ESS switch.
Digital Switches
Many RBOCs use two primary vendors of digital switches: The #5ESS switch manufactured by AT&T The DMS100 switch manufactured by Northern Telecom Inc. (NTI). At the overview level these switches have similar components and operating characteristics. Figure 4.10 illustrates the major components of a 5ESS switch.
- Administration Module (AM)
- Communications Module (CM)
- Remote Switch Module (RSM)
- Switch Module (SM)
- Main Memory (MM)
- Input/Output (I/O)
- Time Multiplex Switch Unit (TMSU) Advantages of digital switches are: - Call processing is executed in nanoseconds (1/1,000,000,000 second)
- A/D & D/A conversion is performed in the SM.
- Digital switches with the appropriate generic (i.e., software or operating system) are required for providing ISDN or remote switch services.
- Digital switches with the proper upgrades are required for AIN
- Digital switches are more efficient in the way they allocate paths through the switch, virtual (time slots vs physical).
- Digital switches with appropriate hardware/software can reduce D-Banks in the Central Office.
Administration Module
The three basic pieces of the Administration Module are just like a computer. (Fig. 4.11)
1) Central Processor (CP) Process the stored program from Main Memory to complete call processing for administration functions. There are two 3B20 computers. This provides full duplication of the Central Processor.
2) Main Memory (MM) Storage of call processing programs (Generic program), transient information for administration functions (Parameters and Registers), Line, Trunk and Routing Translations. The Main Memory is fully duplicated.
3) Input/Output (VO) Provides input control to the switch and output to devices that store billing information, maintenance functions, and status information for all equipment within the switch. These are just some of the Input and Output functions. Communications Module (Fig. 4.12) The main function of the CM is to interface between the AM and the SMs. Some of the functions of the CM are:
- -Switches Subscriber Traffic (Data Time Slots) between SMs. -
- Switches Messages (Control Time Slots) between AM and SMs. -
- Provides System Synchronization. -
- Houses the TMSU. (Time Multiplex Switch Unit)
Switch Module, Remote Switch Module (Fig. 4.13)
The Switch Module (SM), or the Remote Switch Module (RSM) is the connection between the switch and the world. It contains the customer line cards which are connected (wired) to the MDF.
There are four types of connections:
Analog Lines - Customer or subscriber lines.
Digital Lines - Digital Data service for subscribers, Subscriber Loop Carriers and ISDN.
Analog Trunks - Communications to an Analog switch, or customer's analog switch/PBX.
Digital Trunks - Communications path to another Digital switch. Analog switch that has converted the analog signal to a digital signal. A customer's digital switch/PBX.
Switch Modules also have some general functions:
Metallic Access
Since a digital switch has eliminated the metallic paths, and only deals in time, there still is a need to test the subscriber's loop (a metallic path) for continuity. The Metallic Access service circuit enables metallic testing for lines or trunks.
Transmission Test
Transmission tests are performed on all interoffice trunk groups. These tests ensure that the trunks are at acceptable transmission service levels.
Scanner
Device used to monitor lines and trunks to detect OFF-HOOK conditions. Automatic Line Insulation Testing
Automatically tests every line in an office from 2 AM until 6 AM.
These tests are accomplished with the aid of the Metallic Access circuit. Tone Decoder
Detects Multi-Frequency tones on subscriber's lines and trunks.
How the Digital Switch Works
Click here for a diagramatic representation of remote modules and the 5ESS As we saw in the Analog switch, once a call was in a stable condition, that path through the switch could only handle that one call. In a Digital switch, a path in the switch can handle many calls on that one path (fiber link), in fact it can handle about 512 calls on one path. This Fiber Link is called a Network, Control and Timing Links (NCT). The NCT runs from each SM to the CM. The NCT transports the PCM words generated by our A-D or D-A conversion process between the SMs and CM. In addition to the PCM words, it transports Control information needed by either the SM, or the CM. The PCM words are allocated a Time Slot (TS) on the NCT link. So now our path, at least part of our path, has become a time slot on this NCT link. In fact, to complete a call we will need two time slots, one TS for our originating, and one for the terminating.
Major Components of a Digital Switch Vendors use different terminology when referring to switch components. Unless stated otherwise, the AT&T terminology will be italicized and the NTI (Northern Telecom Incorporated) terminology will be underlined.
Administrative Module (AM) or Central Control Complex (CCC) Communications Module (CM) or Communications Processor (CP) Time Multiplexed Switch Unit (TMSU)
Provides the control functions for the switch; where the software resides along with the decision-making responsibility for the switch; Central Processing Unit (CPU) or main processor are synonyms. Directs traffic through the switch; interfaces with the CPU to collect information required for call processing. Tracks the time slots (moments of time); provides virtual connections between time slots; switches the call. Switch Modules (SM)* or Peripheral Module (PM) Switch interface with the end user; performs the analog to digital and digital to analog conversion. 512 time slots are available between each SM and the TMSU (Time Multiplex Switch Unit)
(Line/Trunk translations) Generic or Bulk Change Supplement (BCS) Software that provides control functions for the switch. * Various types of switch modules are: ISDN Switch Modules and Remote Switch Modules.
Remote Switches
Remote switches are a way of providing digital seIvice to customers who would not normally be served from a digital switch. It requires that a digital host (i.e., 5ESS or DMS100) be located within a specified distance of the remote switch. The central processing unit for the remote switch is located at the host.
The SM and CM in the remote location are 95% self-sustainable (in case the host fails) and to process all local calls without the host. The remote switch is connected to a host switch at the remote switch module (RSM). Figure 4.15 illustrates use of a remote switch. Main reasons for using remote switches are:
Traffic Measurements
Additional switch capabilities include the ability to collect network performance data which result in traffic measurements. There are three main traffic measurements used in forecasting network demand.
1. Busy Hour (BH) is the one hour during a 24-hour time frame that has the greatest number of calls. It tends to be a very predictable indicator of traffic demand because it occurs every day. Busy hour may changed based on: Type of customer (business versus residential) Geography (certain types of businesses will predominate in certain areas) Type of business (stockbrokers have a different busy hour than retail customers or hotels)
2. Busy Day(s) (BD) is the day (or days) when traficc can be predicted to be the highest. Busy day tends to be less predictable than busy hour. Some customers have very predictable busy days; others do not. Busy day is an important measurement to take into account with busy hour, because: The busy hour which occurs on the busy day will be far busier than other busy hour measurements. The same variables that affect busy hour also affect busy day.
3. Busy Month/Season (BM/S) is a month or season which has the greatest volume of calls. Busy month/season tends to be the least predictable because: Many businesses don't have predictable patterns. Those that do usually must be studied over time to be able to predict them accurately. As with busy day, the busy hour during a busy month or season may predict a far greater volume of calls than busy hour alone would do.
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