Table Of ContentSource: Sonet / SDH Demystified
11
CHAPTER
Beginnings
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The year was 1984,and Bill McGowan had a problem.Freshly bloodied and
scarred from the AT&T divestiture battlefield,where as the head of MCI,he
served as the attacking general who, in many people’s minds, single-
handedly drove the breakup of the Bell System.McGowan realized that the
toppling of the titan and subsequent shattering of AT&T into eight distinct
pieces (seven regional providers plus one long-distance provider),shown in
Figure 1-1,only resolved one of the challenges that would lead to the cre-
ation of a truly competitive marketplace.
Although the best-knownimpact of divestiture was the breakup of AT&T
(one result of which was the liberalization of the telecommunications mar-
ketplace in the U.S.),a second decision that was tightly intertwined with
the Bell System’s breakup was largely invisible to the public, yet was at
least as important to AT&T competitors,MCI and Sprint,as the breakup
itself.This decision,known asEqual Access,had one seminal goal:to make
it possible for end customers to take advantage of one of the products of
divestiture,the ability to select one’s long-distance provider from a pool of
available service providers—in this case AT&T, MCI, or Sprint. This, of
course,was the realization of a truly competitive marketplace in the long-
distance market segment.
To understand this evolution, it is helpful to have a high-level under-
standing of the overall architecture of the network.In the pre-divestiture
world,AT&T was the provider for local service,long-distance service,and
communications equipment. An AT&T central office (CO), therefore, was
awash in AT&T hardware,such as switches,cross-connect devices,multi-
plexers,amplifiers,repeaters,and myriad other devices.
Figure 1-2 shows a typical network layout in the pre-divestiture world.
A customer’s telephone is connected to the service provider’s network by a
local loop connection (so-called twisted pair wire).The local loop,in turn,
connects to the local switch in the central office.This switch is the point at
AT&T
Figure 1-1 Long Lines AT&T Lucent
Divestiture.
Western
Electric
NYNEX Bell Atlantic Ameritech
22 BOCs
Pacific
BellSouth USWest SBC
Telesis
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Beginnings 3
AT&T Long-Distance Switches
Figure 1-2
The pre-divestiture
switching hierarchy.
AT&T Local Switches
which customers first touch the telephone network,and it has the respon-
sibility to perform the initial call setup, maintain the call while it is in
progress,and tear it down when the call is complete.This switch is called a
local switch because its primary responsibility is to set up local calls that
originate and terminate within the same switch.It has one other responsi-
bility, though, and that is to provide the necessary interface between the
local switch and the long-distance switch, so that calls between adjacent
local switches (or between far-flung local switches) can be established.The
process goes something like this.When a customer lifts the handset and
goes off-hook, a switch in the telephone closes, completing a circuit that
enables current flow that in turn brings dial tone to the customer’s ear.
Upon hearing the dial tone,the customer enters the destination address of
the call (otherwise known as atelephone number).The switch receives the
telephone number and analyzes it,determining from the area code and pre-
fix information whether the call can be completed within the local switch or
must leave the local switch for another one. If the call is indeed local, it
merely burrows through the crust of the switch and then reemerges at the
receiving local loop.If the call is a toll or long-distance call,it must burrow
through the hard crunchy coating of the switch, pass through the soft,
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chewy center,and emerge again on the other crunchy side on its way to a
long-distance switch.Keep in mind that the local switch has no awareness
of the existence of customers or telephony capability beyond its own cabi-
nets.Thus,when it receives a telephone number that it is incapable of pro-
cessing,it hands it off to a higher-order switch,with the implied message,
“Here—I have no idea what to do with this,but I assume that you do.”
The long-distance switch receives the number from the local switch,
processes the call,establishes the necessary connection,and passes the call
on to the remote long-distance switch over a long-distance circuit. The
remote long-distance switch passes the call to the remote local switch,
which rings the destination telephone and ultimately, the call is estab-
lished.
Please note that in this pre-divestiture example, the originating local
loop, local switch, long-distance switch, remote local loop, and all of the
interconnect hardware and wiring belong to AT&T.They are all manufac-
tured by Western Electric,based on a set of internal manufacturing stan-
dards that, if other manufacturers in the industry were there, would be
considered proprietary.Because AT&T was the only game in town prior to
divestiture,AT&T created the standard for transmission interfaces.
Fast forward now to January 1,1984,and put yourself into the mind of
Bill McGowan, whose company’s survival depended upon the successful
implementation ofEqual Access.Unfortunately,Equal Access had one very
serious flaw.Keep in mind that because the post-1984 network was emerg-
ing from the darkness of monopoly control,all of the equipment that com-
prised the network infrastructure was bought at the proverbial company
store and was,by the way,proprietary.
Consider the newly re-created post-divestiture network model shown in
Figure 1-3.At the local switch level,little has changed.At this point in time,
only a single local services provider is available.At the long-distance level,
however,a significant change has occurred.Instead of a single long-distance
service provider called AT&T, three are now available: AT&T, MCI, and
Sprint.The competitive mandate of Equal Access was designed to guaran-
tee that a customer could freely select his or her long-distance provider of
choice.If he or she wanted to use MCI’s service instead of AT&T’s,a simple
call to the local telephone company’s service representative would result in
the generation of a service order that would cause the customer’s local ser-
vice to be logically disconnected from AT&T and reconnected to MCI.This
way, long-distance calls placed by the subscriber would automatically be
handed off to MCI.The problem of Equal Access to customers for the three
long-distance providers was thus solved—almost.
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MCI Long-Distance Switch
Figure 1-3
The post-divestiture AT&T Long-Distance Switch Sprint Long-Distance Switch
switching hierarchy
and Equal Access.
RBOC Local Switches
The proprietary nature of the network infrastructure that MCI and
Sprint had to connect themselves to still existed and this was the root of
McGowan’s problem.The Equal Access amendment mandated that AT&T
create space in its now RBOC-owned central offices for the interconnect
equipment that MCI and Sprint required to establish points-of-presence
(POPs) so that they could interconnect with the local switching equipment
and therefore the customers. Unfortunately, because of the proprietary
nature of the single-vendor world into which they were inserting them-
selves,both MCI and Sprint were required to buy AT&T’s communications
equipment in order to connect to the incumbent network infrastructure.
Neither MCI nor Sprint were interested in pouring money into the pockets
of AT&T, which is why McGowan had a problem. He did not want to be
obligated to put jingle into AT&T’s pockets simply to satisfy the interoper-
ability requirement,so he took his case before a series of standards bodies,
including the Interexchange Carrier Compatibility Forum, Bellcore (now
Telcordia), ANSI, the CCITT (now the ITU-T), and a variety of other
regional and international bodies. He argued his case effectively before
them,claiming that requiring MCI and Sprint to purchase AT&T hardware
was unfair.They agreed and tasked themselves to create a standard that
would provide for true, open vendor interoperability.That standard, over
the course of the ensuing eight years,became the Synchronous Optical Net-
work (SONET) and ultimately,the Synchronous Digital Hierarchy (SDH).
It is synchronous because the send and receive devices for the most part
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dance to the same network timing tune,and optical because the standards
are designed to operate over fiber.
So what does this mean? At the time of divestiture,network providers
faced the following challenges. No equipment standards existed, so mid-
span meet, another term for interoperability, was virtually non-existent.
Network management, a singularly important capability, was primitive,
crude,and highly unreliable.Standards-based digital transmission for all
intents and purposes ended at DS3.Networking,such as it was,was frag-
mented, a patchwork quilt of technologies and network fabrics that
exchanged traffic by dint of brute force more than anything else.Standards
for optical multiplexing, a key component of high-speed, long-haul net-
works,were emergent and proprietary.
With the arrival of SONET and SDH, most of those problems evapo-
rated.Interoperability became a non-issue;seamless integration of legacy
and emerging optical technology solutions became the rule rather than the
exception;and network management’s capabilities,once rudimentary and
intermittent,expanded dramatically.
So with the arrival of SONET and SDH,networks took on an entire new
set of behaviors that added to the capabilities of service providers and end
customers alike.However,even in the pre-SONET/SDH environment,com-
munications networks were remarkably capable.So before we dive into the
details of SONET and SDH transmission,let’s first go back and look at the
networks that preceded them.
The Voice Network
The original voice network, including access, transmission facilities, and
switching components,was exclusively analog until 1962,when T-Carrier
emerged as an intra-office trunking scheme.The technology was originally
introduced as a short-haul, four-wire facility to serve metropolitan areas.
Over the years, it evolved to include coaxial cable facilities, digital
microwave systems,fiber,and satellite.
As the network topology improved,so did the switching infrastructure.
In 1976,AT&T introduced the 4ESS switch primarily for toll applications
and followed it up with the 5ESS in 1981 for local switching access,as well
as a variety of remote switching capabilities.Nortel,Siemens,and Ericsson
all followed suit with equally capable hardware.
The goal of digitizing the human voice for transport across an all-digital
network grew out of work performed at Bell Laboratories shortly after the
turn of the century.That work led to a discrete understanding of not only
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Beginnings 7
the biological nature and spectral makeup of the human voice,but also to a
better understanding of language, sound patterns, and the sounded
emphases that comprise spoken language.
The Nature of Voice
A typicalvoice signal comprises frequencies that range from approximately
30 Hz to 10 KHz.Most of the speech energy,however,lies between 300 Hz
and 3,300 Hz,the so-called voice band.Experiments have shown that the
frequencies below 1 KHz provide the bulk of recognizability and intelligi-
bility, whereas the higher frequencies provide richness, articulation, and
natural sound to the transmitted signal.
The human voice comprises a remarkably rich mix of frequencies, but
this richness comes at a considerable price.In order for telephone networks
to transmit voice’s entire spectrum of frequencies, significant network
bandwidth must be made available to every ongoing conversation.A sub-
stantial price tag is attached to bandwidth;it is a finite commodity within
the network,and the more of it that is consumed,the more it costs.
The Network
Thankfully, work performed at Bell Laboratories at the beginning of the
20thcentury helped network designers confront this challenge head-on.To
understand it,let’stake a tour of the telephone network.
The typical network, as shown in Figure 1-4, is divided into several
regions: the access plant, the switching, multiplexing, and circuit connec-
tivity equipment (the central office),and the long-distance transport plant.
The access and transport domains are often referred to as the outside plant;
the central office is,conversely,the inside plant.The outside plant has the
responsibility to aggregate inbound traffic for switching and transport
across the long-haul network,as well as to terminate traffic at a particular
destination.The inside plant,on the other hand,has the responsibility to
multiplex incoming traffic streams,switch the streams,and select an out-
bound path for ultimate delivery to the next central office in the chain or
the final destination.
Let’s examine each region of the network.
Outside Plant The most common form of network access is via a single
pair of twisted wire that connects the customer’s telephone to the central
office.The pairs of wire that run to each home or business are aggregated
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Long-distance transport
Figure 1-4
Modern network
functionality.
CO
Access
Switching,
multiplexing,
circuit connectivity
into a cable that runs from the neighborhood or downtown business district
back to the central office that serves that area.The cables may be deployed
as either underground plant,in which case,they are buried in conduit (see
Figure 1-5),oraerial,in which case,they are suspended on telephone poles
(see Figure 1-6).
Ultimately,the wire pairs,bundled as cables,make their way back to the
central office where they are once again broken out and connected to the
local switch so that they can be individually switched as required.
Inside Plant Whenthe cables first enter the central office,they pass into
a subterranean chamber in the basement of the CO called the cable vault
(see Figure 1-7).At this point,the large cables are broken down into smaller
cables (see Figure 1-8),after which they leave the cable vault and climb into
the technological rafters of the office.
On a higher floor,the cables are dissected into their composite pairs,and
each pair is then attached to electrical appearances on a large iron rack
known as the main distribution frame (MDF) (see Figure 1-9). From the
MDF,the pairs are interconnected to the switch,giving it the capability to
establish demand connections from any pair in the office to any other pair
in the office,as well as to a long-distance trunk should the need arise.This
is shown in Figure 1-10.
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Beginnings 9
Figure 1-5
Buried outside plant.
Figure 1-6
Aerial outside plant.
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Figure 1-7
The cable vault.
Figure 1-8
Cable differentiation
in the cable vault.
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