T1 Carrier System
ENGR 475 – Telecommunications
Harding University
Jonathan White
Outline



AKA
What it is and what it runs on.
Why a digital network?

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



Speeds
How it operates (review).
Framing.
B8ZS
Types of signaling
How it gets synchronized
ESF – Extended SuperFrame
T1 AKA

T1 – Transmission Level 1

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DS1 – Digital Signal 1



The digital signal that goes over a T1
Used interchangeably.
E1



Includes the wires, jacks, hardware.
What T1 is called everywhere else besides North
America and Japan.
It is at a slightly different speed.
T carrier System

An architecture
What the T1 system is

Standard to transmit voice and data.
 A T1 circuit consists of 24 8 bit channels
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T1 speed: 1.544 Mbps
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Each channel is a pseudo 64 kbps voice channel.
This is the standard voice channel that we’ve been
talking about.
Still sampled 8000 times a second.
Called a DS0
(Done on the board).
Cost: $70 to $500 a month, leased.
T Carrier System Definition
 T1
is a high speed digital network (1.544
mbps) developed by AT&T in 1957.

The carrier system is just multiplies of T1s.
 Designed
to support long-haul pulse-code
modulation (PCM) voice transmission.
 The primary innovation of the T carrier
system was to introduce "digitized" voice.
T Carrier System Layers
DS0
64Kbps
1/24 of T-1
1 Channel
T1 / DS1
1.544Mbps
1 T-1
24 Channels
T3 / DS3
44.736 Mbps
28 T-1
672 Channels
T4 / DS4
274.176 Mbps
168 T-1
4032 Channels
T1 is a Standard
 Specified
by ANSI
 A standard that telephone vendors agree
to comply with.

Specifies the data rate, the sampling rate, and
the electrical properties.
• What is IEEE 802.3?
• How about 802.11g?
 Can
you cheat on the standard?
Why a Digital Network?
 How
many wires did the first voice
systems use?
 What happens to noise in a purely analog
system?

How about in a digital system?
 What

is attenuation?
How is this handled in analog and digital
systems?
 How
much does cabling cost?
T1 History

Developed by AT&T (Bell Labs) in 1957.

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Originally, T1 was only between high level end
offices.

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The trunks of the system.
Why were these lines altered first?
T1 speeds didn’t come to users until the mid
1970’s.


Not implemented until the early 1960’s.
PCM was invented in 1937.
How fast were modems then?
T1 architecture also defines the physical
characteristics.
T1 Physical Characteristics
4
wire circuit using 22 AWG unshielded
twisted pair copper wires.


For every 1000 feet, 7 db of attention occurs.
This is why repeaters must be spaced much
more closely (2000 – 6000 feet).
• Remember, humans need an SQR of at least 30
db.
2
wires are used to transmit
 2 wires are used to receive
T1 Physical Characteristics



Full Duplex
Purely Digital
Uses Time Division Multiplexing



Uses PCM
Uses Bipolar/AMI representation of the digital 1’s and
0’s.


Voltages range from -3 to +3 volts – provides power to repeaters.
Clock is encoded in the signals

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Is this efficient?
Do you see a potential problem?
Framed format

Adds a 1 bit header to each frame of (8 x 24) bits
Channelized vs NonChannelized
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You don’t have to use your T1 connection for 24
voice circuits.
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Some examples: 12 voice circuits and 1 high speed
Internet connection.
However, the maximum bandwidth is still 1.544 Mbps.
In a channeled environment, the time slices are
always present, even if the line isn’t sending any
data.

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This is the basis of time division multiplexing.
This is wasteful.
Framing
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After all of the 24*8bit samples, 1 framing bit is
added.
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Every second, 8000 samples are taken.
So these added bits add up to 8 kilobit of addressing
information.

Framing: Provides a way for a receiver to better
synch up with the sender. It is a repeatable
pattern.
 There have been several generations:

Generation 1: Alternating 1’s and 0’s.
D4 Signal Framing
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The fourth generation of framing.
Corrects several potential errors.
“Robs” bits in the 6th and 12th frame.

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In these frames, the least significant digit in every sample is
altered to provide 48 bits of signaling.
24 of these bits make up a CRC code.
Still repeats a pattern in the other framing bits: 1000
1101 1100
This came out before the Internet.



No one notices if your voice is altered by 1 – 48 bits.
Would you care if your e-mail was altered by 48 bits.
This is why we have 56k modems.
Too many 0’s
 (On
board)
Signaling


What are some telephone signals?
In Channel

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
DTMF – signals are part of the conversation.
Do you really need much signaling once you start
talking?
Common channel

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One of the 24 channels is dedicated to signaling.
Now, the other 23 channels can be used to their
fullest extent to send data.
This is called SS7, and it lead to the creation of the
world’s first data network.
The signaling is NOT passed with the data.
Downsides of T1
 All
your eggs are in one basket.
 Expensive at the beginning.

You must get new equipment.
Benefits
 The
whole world isn’t covered in cables.
 Allows better control via signaling
channels.
 Digital is easier to transport.
 Has allowed incredible data rates.