5ESS Switching System
5ESS Switching System is a Class 5 telephone electronic switching
system developed by
Western Electric for the American Telephone and
Telegraph Company (AT&T) and the
Bell System in the United States.
2.1 Switching Module
2.2 Administrative Module
2.3 Communication Module
2.4 Very Compact Digital Exchange
4 See also
6 External links
The 5ESS came to market as the
Western Electric No. 5 ESS. It first
commenced service in
Seneca, Illinois on 25 March 1982, and was
destined to replace the
Number One Electronic Switching System
Number One Electronic Switching System (1ESS
and 1AESS) and other electromechanical systems in the 1980s and 1990s.
The 5ESS was also used as a
Class 4 telephone switch or as a hybrid
Class 4/Class 5 switch in markets too small for the 4ESS.
Approximately half of all US central offices are served by 5ESS
switches. The 5ESS is also exported internationally, and manufactured
outside the US under license.
The development effort for 5ESS required five thousand employees,
producing 100 million lines of system source code, with 100 million
lines of header files and makefiles. Evolution of the system took
place over 20 years, while three releases were often being developed
simultaneously, each taking about three years to develop.[citation
The 5ESS-2000 version, introduced in the 1990s, increased the capacity
of the switching module (SM), with more peripheral modules and more
optical links per SM to the communications module (CM). A follow-on
version, the 5ESS-R/E, was in development during the late 1990s but
did not reach market. Another version was the 5E-XC.
The 5ESS technology was transferred to the AT&T Network Systems
division upon the breakup of the Bell System. The division was
divested by AT&T as Lucent Technologies, and after becoming
Alcatel-Lucent, it was acquired by Nokia.
5ESS switch has three main types of modules: the Administrative
Module (AM) contains the central computers; the Communications Module
(CM) is the central time-divided switch of the system; and the
Switching Module (SM) makes up the majority of the equipment in most
exchanges. The SM performs multiplexing, analog and digital coding,
and other work to interface with external equipment. Each has a
controller, a small computer with duplicated CPUs and memories, like
most common equipment of the exchange, for redundancy. Distributed
systems lessen the load on the Central Administrative Module (AM) or
Power for all circuitry is distributed as –48 VDC (nominal), and
converted locally to logic levels or telephone signals.
Each Switching Module (SM) handles several hundred to a few thousand
telephone lines or several hundred trunks or combination thereof. Each
has its own processors, also called Module Controllers, which perform
most call handling processes, using their own memory boards.
Originally the peripheral processors were to be Intel 8086, but those
proved inadequate and the system was introduced with Motorola 68000
series processors. The name of the cabinet that houses this equipment
was changed at the same time from Interface Module to Switching
Peripheral units are on shelves in the SM. In most exchanges the
majority are Line Units (LU) and Digital Line Trunk Units (DLTU). Each
SM has Local Digital Service Units (LDSU) to provide various services
to lines and trunks in the SM, including tone generation and
detection. Global Digital Service Units (GDSU) provide less-frequently
used services to the entire exchange. The Time Slot Interchanger (TSI)
in the SM uses random-access memory to delay each speech sample to fit
into a time slot which will carry its call through the exchange to
another or, in some cases, the same SM.
T-carrier spans are terminated, originally one per card but in later
models usually two, in Digital Line Trunk Units (DLTU) which
DS0 channels into the TSI. These may serve either
interoffice trunks or, using Integrated Subscriber Loop Carrier,
subscriber lines. Higher-capacity DS3 signals can also have their DS0
signals switched in Digital Network Unit
SONET (DNUS) units, without
demultiplexing them into DS1. Newer SM's have DNUS (DS3) and Optical
OIU interfaces (OC12) with a large amount of capacity.
SMs have Dual Link Interface (DLI) cards to connect them by multi-mode
optical fibers to the Communications Modules for time-divided
switching to other SMs. These links may be short, for example within
the same building, or may connect to SMs in remote locations. Calls
among the lines and trunks of a particular SM needn't go through CM,
and an SM located remotely can act as distributed switching,
administered from the central AM. Each SM has two Module
Time Slot Interchange (MCTSI) circuits for redundancy.
In contrast to Nortel's
DMS-100 which uses individual line cards with
a codec, most lines are on two-stage analog space-division
concentrators or Line Units, which connect as many as 512 lines, as
needed, to the 8 Channel cards that each contain 8 codecs, and to
high-level service circuits for ringing and testing. Both stages of
concentration are included on the same GDX (Gated Diode Access) board.
Each GDX board serves 32 lines, 16 A links and 32 B links. Limited
availability saves money with incompletely filled matrixes. The Line
Unit can have up to 16 GDX boards connecting to the channel boards by
shared B links, but in offices with heavier traffic for lines a lesser
number of GDX boards are equipped.
ISDN lines are served by individual line cards in an ISLU (Integrated
Services Line Unit).
The Administrative Module (AM) is a dual-processor mini main frame
computer of the AT&T 3B series, running UNIX-RTR. AM contains the
hard drives and tape drives used to load and backup the central and
peripheral processor software and translations. Disk drives were
originally several 300 megabyte SMD multiplatter units in a separate
frame. Now they consist of several redundant multi-gigabyte SCSI
drives that each reside on a card. Tape drives were originally half
inch open reel at 6250 bits per inch, which were replaced in the early
1990s with 4 mm
Digital Audio Tape
Digital Audio Tape cassettes.
The Administrative Module is built on the
3B21D platform and is used
to load software to the many microprocessors throughout the switch and
to provide high speed control functions. It provides messaging and
interface to control terminals. The AM of a 5ESS consists of the 3B20x
3B21D processor unit, including I/O, disks, and tape drive units.
3B21D has loaded the software into the 5ESS and the switch is
activated, packet switching takes place without further action by the
3B21D, except for billing functions requiring records to be
transferred to disk for storage. Because the processor has duplex
hardware, one active side, and one standby side, a failure of one side
of the processor will not necessarily result in a loss of switching.
The Communications Module (CM) forms the central time switch of the
exchange. 5ESS uses a time-space-time (TST) topology in which the
Time-Slot-Interchangers (TSI) in the Switching Modules assign each
phone call to a time slot for routing through the CM.
CMs perform time-divided switching and are provided in pairs; each
module (cabinet) belonging to Office Network and Timing Complex (ONTC)
0 or 1, roughly corresponding to the switch planes of other designs.
Each SM has four optical fiber links, two connecting to a CM belonging
to ONTC 0 and two to ONTC 1. Each optical link consists of two
multimode optical fibers with ST connectors to plug into transceivers
plugged into backplane wiring at each end. CMs receive
time-multiplexed signals on the receive fiber and send them to the
appropriate destination SM on the send fiber.
Very Compact Digital Exchange
The Very Compact Digital Exchange (VCDX) was developed with the
5ESS-2000, and marketed to mostly non-Bell telephone companies as an
inexpensive, effective way to offer
ISDN and other digital services in
an analog switching center. This avoided the capital expense of
retrofitting the entire analog switch into a digital one to serve all
of the switch's lines when many wouldn't require it and would remain
An example would be the (former) GTE/Verizon Class-5 telephone switch,
the GTD-5 EAX. Like the
Western Electric 1ESS/1AESS, it served mostly
medium to large wire centers.
The standalone VCDX was also capable of serving as a switch for very
small wire centers (a CDX- Community dial office) of fewer than ~400
lines. However, for small wire centers, 400-4000 lines, that function
was usually served by RSM's, a 5ESS "Remote SM". The RSM is controlled
over redundant and diverse fiber optic "umbilicals". An RSM can have
more than one SM, and also many other peripheral units that are part
of a full 5ESS switch. An RSM can serve parts of a larger metro area,
but is more often used as a Class-5 wire center for small to medium
towns hosted from a 5ESS located in a larger city. The VCDX was also
used as a large private branch exchange (PBX).
The standalone VCDX has a single Switching Module, and no
Communications Module. Its
SPARC workstation runs the
Solaris (operating system)
Solaris (operating system) that executes a 3B20/21D
MERT OS emulation system, acting as the VCDX's
Administrative Module. The VCDX uses the CO's normal telephone power
sources (which are very large uninterruptible power supplies), and has
connections to the CO
Digital cross connect system for T1 access, etc.
The 5ESS has two different signaling architectures: Common Network
Interface (CNI) Ring and Packet Switching Unit (PSU)-based SS7
Main article: Operations, Administration, Maintenance and Provisioning
The system is administered through an assortment of teletypewriter
"Channels", also called the system console, such as the TEST channel
and Maintenance channel. Typically provisioning is done either through
a command line interface (CLI) called RCV:APPTEXT, or through the
menu-driven RCV:MENU,APPRC program. RCV stands for Recent
Change/Verification, and can be accessed through Switching Control
Center System. Most service orders, however, are administered through
Recent Change Memory Administration Center (RCMAC). In the
international market, this terminal interface has localization to
provide locale specific language and command name variations on the
screen and printer output.
5ESS Switching System
5ESS Switching System (The AT&T Technical Journal,
July–August 1985, Vol. 64, No. 6, Part 2)
Evolution of Switching Architecture to Support Voice Telephony over
ATM by Judith R. McGoogan, Joseph E. Merritt, and Yogesh J. Dave.
Extending 5ESS-2000. Bell Labs Technical Journal, April–May 2000
Switch Basics 5ESS Scribd.com
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