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==Phrack Inc.==
Volume Three, Issue 25, File 3 of 11
Bell Network Switching Systems
An Informational Definitive File
By Taran King
March 14, 1989
Throughout my many conversations with what many consider the "elite"
of the community, I have come to realize that even the highest up on the
hierarchical map do not know all of the little differences and specificities of
the switching systems that the BOCs use throughout the nation. This file was
written so that people could understand the differences between their switch
and those switches in areas where they have friends or that they pass through.
There are two broad categories that switches can be separated into:
local and tandem. Local offices connect customer lines to each other for
local calls and connect lines to trunks for interoffice calls. Tandem
switching is subdivided into two categories: local tandem offices and toll
offices. Local toll offices connect trunks to trunks within a metropolitan
area whereas toll offices connect trunks to trunks from the toll network
portion (class 1 to 4) of the hierarchical Public Switched Telephone Network
(PSTN).
Because of the convenience of having direct interface with customer
lines, local switching has built in functions needed to provide exchange
services such as local calling, custom calling features, Touch-Tone service,
E911 service, and exchange business services (like Centrex, ESSX-1, and
ESS-ACD. Centrex is a service for customers with many stations that is
provided out of the Central Office. ESSX-1 service limits the number of
simultaneous incoming and outgoing calls and the number of simultaneous
intragroup calls to software sizes specified by the customer. ESS-ACD is the
exchange service equivalent to Automatic Call Distribution except the call
distribution takes place in a Centrex-functioning portion of the electronic
switch.)
Geographic centralization of the tandem office allows efficiency in
providing centralized billing and network services.
Automatic switching was formally installed by the Bell System in 1919
and although there are many replacements that update old and less preferable
services, many older offices still exist in various parts of the country.
ELECTROMECHANICAL SWITCHING SYSTEMS
The Step By Step (SXS) switching system, also known as the Strowger
system, was the earliest switching system. Invented by A. B. Strowger in
1889, it is currently used in rural and suburban areas around the country as
well as some metropolitan areas which were small when the switch was
installed. The term "Step By Step" describes both the manner in which the
switching network path is established and the way in which each of the
switches in the path operates. They combine vertical stepping and a
horizontal rotary stepping motion to find the number dialed through pulse.
The drawbacks of the SXS system include not being able to have Touch Tone
calling or alternative routing without adding expensive equipment to the
office and also that the customer's telephone number is determined by the
physical termination/location of the line or connector on the system. The
line cannot be moved without changing the telephone number. The other
drawback is the high maintenance cost. These reasons, among others, have led
to a drop in the amount of SXS systems seen around the country.
The No. 1 Crossbar (XBAR) was developed for use in metropolitan
areas. The XBAR system uses horizontal and vertical bars to select the
contacts. There are five selecting bars mounted horizontally across the front
of each XBAR switch. Each selecting bar can choose either of two horizontal
rows of contacts. The five horizontal selecting bars can therefore select ten
horizontal rows of contacts. There are ten or twenty vertical units mounted
on the switch and each vertical unit forms one vertical path. Each switch has
either 100 or 200 sets of crosspoints/contacts depending on the number of
vertical units.
The No. 5 Crossbar was developed to fill the need for a switching
system that would be more productive in suburban residential areas or smaller
cities. The No. 5 XBAR also included automatic recording of call details for
billing purposes to allow for DDD (Direct Distance Dialing). The No. 5 XBAR
is separated into 2 parts: the switching network where all the talking paths
are established and the common-control equipment which sets up the talking
paths. Various improvements have been made on the No. 5 XBAR over the years
such as centralized automatic message accounting, line link pulsing to
facilitate DID (Direct Inward Dialing) to stations served by a dial PBX
(Private Branch Exchange), international DDD, Centrex service, and ACD
capability. The No. 5 Electronic Translator System (ETS) was also a
development which used software instead of wire cross-connections to provide
line, trunk, and routing translations as well as storing billing information
for transmissions via data link to a centralized billing collection system.
The No. 4 Crossbar is a common-control system designed for toll
service with crossbar switches making up its switching network. The No. 4A
XBAR system was designed for metropolitan areas and added the ability to have
CAMA (Centralized Automatic Message Accounting) as well as foreign-area
translation, automatic alternate routing, and address digit manipulation
capabilities (which is converting the incoming address to a different address
for route control in subsequent offices, deleting digits, and prefixing new
digits if needed). The No. 4A ETS replaced the card translator (which was
used for translation via phototransistors) and allowed billing and route
translation functions to be changed by teletypewriter input as it was a
stored-program control processor. CCIS (Common Channel Interoffice
Signaling) was added to the No. 4A XBAR in 1976 for more efficient signaling
between toll offices among other things.
ELECTRONIC SWITCHING SYSTEMS
The Electronic Switching Systems were made possible by the invention
of the transistor. They apply the basic concepts of an electronic data
processor, operating under the direction of a stored-program control, and
high-speed switching networks. The stored-program control allows system
designs the necessary flexibility to design new features and install them
easily. The SPC controls the sequencing of operations required to establish a
call. It can control a line or trunk circuit according to its application.
The first electronic switching trial took place in Morris, IL in
1960. The first application of electronic local switching in the Bell System
took place in May of 1965 with the cutover of the first 1ESS switch in
Succasunna, NJ.
The 1ESS switching system was designed for areas where large numbers
of lines and lines with heavy traffic are served. It generally serves between
10,000 and 65,000 lines. The memory of the 1ESS is generally read only memory
(ROM) so that neither software or hardware malfunctions can alter the
information content.
The 1A Processor was introduced in 1976 in the first 1AESS switch.
It was designed for local switching applications to be implemented into a
working 1ESS switch. It allowed the switching capacity to be doubled from
the old 1ESS switches also. The 1A Processor uses both ROM and RAM (Random
Access Memory). Magnetic tape units in the 1A Processor allow for system
reinitialization as well as detailed call billing functions.
Both the 1ESS and the 1AESS switches use the same peripheral
equipment which allows for easy transition. Programs in both switches control
routine tests, diagnose troubles, detect and report faults and troubles, and
control emergency actions to ensure satisfactory operation. Both switches
offer the standard custom calling features as well as business features like
Centrex, ESS-ACD, Enhanced Private Switched Communications Service or ETS
(Electronic Tandem Switching).
The 2ESS was designed to extend electronic switching into suburban
regions but doing so economically, meeting the need for 2,000 to 10,000 line
offices. It has a call capacity of 19,000 with a maximum of 24,000 terminals
per system. One of the differences between the 1ESS and the 2ESS is that in
the 2ESS, lines and trunks terminate on the same side of the network, which is
called a folded network. There is no need for separate line and trunk link
networks as in the 1ESS. Also, the network architecture was designed to
interface with customer lines carrying lighter traffic, the features were
oriented toward residential rather than business lines, and the processor was
smaller and less expensive.
In 1976, the first 2BESS switch was introduced in Acworth, GA. The
2BESS switch is similar to the 1AESS in that it has something added into the
switch. In this case, though, it is the 3ACC (3A Central Control), which is
in the place of the processor. The 3ACC doubles the call capacity originally
available in the 2ESS switch by combining integrated circuit design with
semiconductor memory stores. It also requires one-fifth of the floor space
and one-sixth of the power and air conditioning that the 2ESS central
processor requires. The 3ACC is a self-checking, microprogram-controlled
processor capable of high-speed serial communication. Resident programs in
the 3ACC are hardware write-protected, but non-resident programs like
maintenance, recent change (RC), and back-up for translations or residential
programs are stored on a tape cartridge.
Also in 1976, the need for switching in rural areas serving fewer
than 4500 lines resulted in the introduction of the 3ESS switch. The 3ESS
switching equipment is the smallest Western Electric space-division,
centralized electronic switching system which serves 2,000 to 4,500 lines.
The 3ACC is used as the processor in the 3ESS, which was designed to meet the
needs of a typical Community Dial Office (CDO). It, too, is a folded network
like the 2ESS and 2BESS. The switch was designed for unattended operation,
implementing extensive maintenance programs as well as remote SCCS (Switching
Control Center System) maintenance capabilities.
The 4ESS switching equipment is a large-capacity tandem system for
trunk-to-trunk interconnection. It forms the heart of the Stored-Program
Control (SPC) network that uses CCIS (Common-Channel Interoffice Signaling)
yet still supports Multi-Frequency (MF) and Dial-Pulse (DP) signaling. The
SPC network allows for features such as the Mass Announcement System (MAS)
(which is where we find all of our entertaining 900 Dial-It numbers) and
WATS (Wide-Area Telecommunications Services) screening/routing. The 4ESS also
provides international gateway functions. It uses a 1A Processor as its main
processor, which, along with its use of core memories and higher speed logic,
is about five times as fast as the 1ESS processor. The 4ESS software
structure is based on a centralized development process using three languages:
a low-level assembly language, the intermediate language called EPL (ESS
Programming Language), and a high level language called EPLX. The assembly
language takes care of real-time functions like call processing while
measurements and administrative functions frequently are programmed in EPL.
Some maintenance programs and audits which are not as frequently run are in
EPLX. Up to six 4ESS switches can be remotely administered and maintained
from centralized work centers which means that very few functions need to be
performed at the site of the switch itself.
In March of 1982, the 5ESS switch first went into operation. It is a
digital time-division electronic switching system designed for modular growth
to accommodate local offices ranging from 1,000 to 100,000 lines. It was
designed to replace remaining electromechanical switching systems in rural,
suburban, and urban areas economically. Features of new generic versions of
the program allowed multimodule configuration and local/toll features for
combined class 4 and class 5 operation. The 5ESS administrative module
processor consists of two 3B20s. The communications module consists of a
message switch and a TMS (Time-Multiplexed Switch), which is used to connect
voice channels in one interface module to voice channels in another interface
module as well as for data messages between the administrative modules and
interface modules and also is used for data messages between interface
modules. The interface module can host analog line/trunk units, digital
line/trunk units, digital carrier line units, digital service circuit units,
or metallic service units in addition to miscellaneous test and access units.
There are 2 software divisions in the 5ESS. The portion in the administrative
module processor is responsible for officewide functions such as the human
interfaces, routing, charging, feature translations, switch maintenance, and
data storage and backup. The portion in the interface module is responsible
for the standard call-processing functions associated with the lines and
trunks terminating on that interface module. Most software is written in C
and has a modular structure to afford easy expansion and maintenance.
The last thing to mention here are Remote Switching Systems (RSS) and
Remote Switching Modules (RSM). The No. 10A RSS is designed to act as an
extension of a 1ESS, 1AESS, or 2BESS switching equipment host and is
controlled remotely by the host over a pair of dedicated data links. It
shares the processor capabilities of these nearby ESS switches and uses a
microprocessor for certain control functions under the direction of the host
central processor. The RSS is capable of stand-alone functioning if the links
between it and the host are severed somehow. If this occurs, though, custom
calling, billing, traffic measurements, etc. are unavailable -- only basic
service on intra-RSS calls is allowed. The No. 5A RSM can be located up to
100 miles from the 5ESS host and can terminate a maximum of 4000 lines with a
single interface module. Several RSMs can be interconnected to serve remote
offices as large as 16,000 lines. It is a standard 5ESS system interface
module with the capability for stand-alone switching capability if the
host-remote link fails. One difference from the RSS of the RSM is the ability
to use direct trunking, whereas the RSS requires that all interoffice calls
pass through the host switch.
Of course, there are many other switches out there, but these are the
basic Western Electric switches provided for the Bell System. The following
is a time-table to summarize the occurrences of SPC switching systems that have
been used by BOCs and AT&T:
1965 The 1ESS used for local metropolitan allows 65,000 lines and 16,000
trunks.
1968 The 1ESS expands for local metropolitan and local tandem.
1970 The 2ESS used for local suburban has 30,000 lines and trunks together.
1974 The 1ESS allows 2-wire toll switching.
1976 The 4ESS uses large 4-wire toll for use of 100,000 trunks.
1976 The 1AESS for large metropolitan local use has 90,000 lines and 32,000
trunks
1976 The 2BESS for local suburban use has 30,000 lines and trunks together.
1976 The 3ESS for local rural use has 5,800 lines and trunks together.
1977 The 1AESS using 4-wire toll.
1979 The 1AESS has local, tandem, and toll capability.
1979 The 10A RSS is for local small rural areas with 2,000 lines.
1982 The 5ESS for local rural to large metropolitan areas with tandem and
toll capabilities has from 150,000 lines and 50,000 trunks to 0 lines
and 60,000 trunks.
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