Mr. Mark Welton. Most telecom infrastructure that exists today is based on standards more than 100 years old Telecom infrastructure is often so reliable

CSIS 4823Data Communications

Networking - Telecommunications

Mr. Mark Welton

Most telecom infrastructure that exists today is based on standards more than 100 years old

Telecom infrastructure is often so reliable that we expect reliable phone service more than reliable Internet service or power

Telecom Infrastructure

Telecommunications refers to the traditional circuit-based switching technologies primary used by voice services

data communications often refers to packet-based switching of the modern Internet

What is telecom?

ACD – automatic call distribution◦ An ACD is usually found in a call center,

where calls may come in from anywhere and need to be directed to the next available operator or queued until one is available

Telecommunications Terms

Add/Drop – used to describe the capability of peeling off channels from a circuit for another use

An add/drop CSU/DSU can separate ranges of channels, thus allowing a T1 to be split for both voice and data use or as two partial T1s


analog in telecom refers to a signal that is continuous in amplitude and time, any small fluctuation of the signal is important◦ Radio waves, power waves, and sound waves are

analog◦ When you speak, you create waves of air that hit

people’s eardrums


Digital refers to a signal that has discrete values

If you analyze a sound wave, and then assign a value to each sample of the wave at specific time intervals, you will create a digital representation of the analog wave


Bandwidth vs. throughput A range of frequencies is called a band The width of the band is referred to as

bandwidth US FM channels run from 87.8 MHz to 107.9

MHz The bandwidth is 20MHz


On a digital link what is referred to as bandwidth is really throughput

Throughput is the number of possible state transitions per second

This can also be referred to as the data rate The signal on the right has more bits per

second (bps) then the left (19 possible state changes vs. 6)


BERT – Bit Error Rate Test BERT are disruptive tests run on a T1 to

validate the integrity of the circuit (looks for errors)

CO – central office◦ the first hop where phone lines go

Channel bank – a device that separates a T1 into 24 individual analog phone lines


CSU/DSU – Channel Service Unit and Data Service Unit

The CSU is responsible for interfacing with the WAN service

The DSU is responsible fore interfacing with the data equipment, such as the router

The CSU typically has an RJ-45 connection from the demark point and the DSU has a V.35 connection to a router

In a CSU/DSU router card, the RJ-45 connection from the demark point goes to the router card and the connection on the card interfaces it to the router


CPE – customer premises equipment Traditionally, the term was used to describe

equipment owned by a telephone service provider that resided at customer premises, but it has evolved to include equipment owned by anyone


DACCS (pronounced dacks) – Digital Access Cross-Connect System, allows changes to the way voice channels are connected between trunks


Demarc (pronounced dee-mark) – demarcation point where the telecom provider’s responsibilities end and the customer’s begins

What is an extended demark? Provider installed location somewhere

beyond the real demarc


DS hierarchy – Digital Signal, describes the signaling rates of links


Designator Carrier Transmission rate

Voice channels

DSO N/A 64 Kbps 1

DS1 T1 1.544 Mbps 24

DS2 T2 6.312 Mbps 96

DS3 T3 44.736 Mbps 672

DS4 T4 274.176 Mbps 4,032

T-carrier – the generic name of digital multiplexed carrier systems in North America◦ T stands for trunk, since they originally designed

to trunk multiple phone lines between central offices

E-carrier - The European version of digital multiplexed carrier systems

J-carrier – The Japanese version of digital multiplexed carrier systems


SONET – synchronous optical network, fiber optic system makes use of the optical carrier levels ◦ OC1 – 51 Mbps ◦ OC3 – 155 Mbps ◦ OC12 – 622 Mbps ◦ OC48 – 2,488 Mbps


ISDN – Integrated Services Digital Network allows for the simultaneous transmission of voice

and data Uses a separate channel for signaling (Data

channel or D-channel) and two remaining channels for content (Bearer channel or B-Channel)

Each B-Channel is capable of 64kbps or a total of 128kbps and the D-channel capable of 16kbps on a Basic Rate Interface (BRI) and 64kbps on a Primary Rate Interface (PRI)

A PRI is an ISDN T1 composed of 23 B-channels and 1 D-channel


LATA – local access and transport area, government defined areas in which a telecom provider can provide local services

IXC – interexchange carrier providers services to local exchanges by connecting in between LATAs

LEC – local exchange carrier, provides local service within a LATA

Local loop – the last mile connection for a circuit from a telecom facility to it destination


Latency – term used to describe the amount of time it takes for data to be processed or moved along a network

Not related to throughput, bandwidth, or speed of a link

Related to distance, speed of light, and amount of time for hardware processing

Two parts of latency – propagation delay and processing delay.


PBX – private branch exchange, used by large organizations as a localized phone company for its ability to share a limited number of public phone lines and the ability to call any internal extension number

POTS – plain old telephone service, your traditional analog phone line

Smart jack – a device that terminates a digital circuit, the service provider can perform remote testing of a circuit with the smart jack installed


RBOC – Regional Bell Operating Company Result of the 1984 breakup of AT&T Bell System,

into seven regional local only carriers The Telecom Deregulation Act of 1996 allowed LECs

(RBOCs) to sell local and long distance services. ◦ Bell Atlantic ◦ Southwestern Bell ◦ Nynex ◦ Pacific Bell ◦ Bell South ◦ Ameritech ◦ US West


T1 circuits are full-duplex asynchronous circuits

Two Common types◦ Channelized T1 – voice circuit with 24 voice

channels, each channel contains is own signaling information inserted into the data stream (voice) via in-band signaling

◦ PRI – voice circuit with 24 channels, one of which is dedicated to signaling (out of band signaling), leaving 23 available voice channels

T1 Circuits

two types of encoding Alternate Mark Inversion (AMI) – used for

voice only Binary Eight Zero Substitution (B8ZS) – used

for data but can be used for voice

T1 Circuits

There are two “states” (zero and one) for the signal on the line

The mark state representing a one (+/-5 volts) and the space representing a zero (zero volts)

The next mark is always the opposite of the preceding mark

Robbed-bit signaling – every eighth bit is used to keep signal (always set to 1 regardless of original value)

AMI Encoding

introduced to resolve the shortcomings of AMI

In voice the eighth bit 1 is not an issue because you can not hear the change

In data this would change the information Need to deal with the synchronization issue

of too many zeros in a row without changing the information

B8ZS Encoding

if eight zeros in a row are detected in a signal, those eight zeros are converted to a pattern including intentional BPVs

B8ZS Encoding

Sample audio 8,000 times per second Each sample is converted in to an eight-bit

value 8th bit is used for signaling (robbed-bit

signaling in AMI)

Framing

In standard voice framing each eight-bit sample is relayed from each channel in order

each eight-bit sample is relayed from each channel in order

8 bits X 24 channels = 192 bits The 193rd bit is used as a framing bit It will change this bit over 12 frames using

the pattern 110111001000 12 frames are a superframe Used for voice circuits

D4/Superframe

D4/Superframe

D4/superframe standard is not practical for data transmission (lack error detection)

Extended Super frame (ESF) was developed to deal with this

Uses a pattern of 24 frames instead of 12◦ Frames 4, 8, 12, 16, 20, and 24 (every fourth frame)

These frames’ framing bits are filled with the pattern 001011◦ Frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23 (every

odd-numbered frame) These frames’ framing bits are used for a new, 4,000 bps virtual

data channel. This channel is used for out-of-band communications between networking devices on the link

◦ Frames 2, 6, 10, 14, 18, and 22 (the remaining even-numbered frames) These frames’ framing bits are used to store a six-bit CRC value for

each superframe.

Extended Super Frame

Loss of Signal (LOS) – no electrical pulses detected (line is dead)

Out of Frame (OOF) or Loss of Frame (LOF) – a number of frames have been received with errors (synchronization invalid between two sides)

BiPolar Violation (BPV) – two mark signals occur in sequence at the same polarity (usually caused by voltage spike on the line)

Performance Monitoring

CRC6 - Cyclic Redundancy Check (six-bit) mechanism for error checking in ESF (check sum did not match)

Errored Seconds – shows the number of seconds in a 15-minute window in which errors on the line have occurred

Red Alarm – a local failure on the circuit or continuous OOF error

Yellow Alarm – remote alarm indication of problem down the line

Loopback test – loopback state allowing the sending of data over the link, data should return to sender exactly as sent

Performance Monitoring

Two types of DS3◦ Channelized

672 DS0s (28 DS1s), each capable of supporting a single POTS-line phone call

Same times referred to as a “channeled T3” DS3 is not the same as a T3

◦ Clear-channel Has no channels and is used for pure data

DS3

DS3 is actually a group of seven DS2s multiplexed together

Originally designed to handle phone calls Each DS1multiplied into a DS3 has its own

clocking, framing, and encoding Multiplexing them into a DS3 can not affect

this The DS3 must also have its own clocking,

framing, and encoding

DS3 Framing

M13 (pronounced M-one-three, not M-thirteen) is short for Multiplexed DS1 to DS3

Requires two stages of multiplexing◦ M12 – multiplexed DS1 to DS2◦ M23 - multiplexed DS2 to DS3

DS3s were originally used to aggregate T1s

M13

M13

Documents

Mr. Mark Welton. Most telecom infrastructure that exists today is based on standards more than 100 years old Telecom infrastructure is often so reliable