T1 Carrier System

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T1 Carrier System

• ENGR 475 Telecommunications
• Harding University
• Jonathan White

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Outline

• AKA
• What it is and what it runs on.
• Why a digital network?
• Speeds
• How it operates (review).
• Framing.
• B8ZS
• Types of signaling
• How it gets synchronized
• ESF Extended SuperFrame

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T1 AKA

• T1 Transmission Level 1
• Includes the wires, jacks, hardware.
• DS1 Digital Signal 1
• The digital signal that goes over a T1
• Used interchangeably.
• E1
• What T1 is called everywhere else besides North
  America and Japan.
• It is at a slightly different speed.
• T carrier System
• An architecture

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What the T1 system is

• Standard to transmit voice and data.
• A T1 circuit consists of 24 8 bit channels
• Each channel is a pseudo 64 kbps voice channel.
• This is the standard voice channel that weve
  been talking about.
• Still sampled 8000 times a second.
• Called a DS0
• T1 speed 1.544 Mbps
• (Done on the board).
• Cost 70 to 500 a month, leased.

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T Carrier System Definition

• T1 is a high speed digital network (1.544 mbps)
  developed by ATT 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.

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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
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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?

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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?

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T1 History

• Developed by ATT (Bell Labs) in 1957.
• Not implemented until the early 1960s.
• PCM was invented in 1937.
• Originally, T1 was only between high level end
  offices.
• The trunks of the system.
• Why were these lines altered first?
• T1 speeds didnt come to users until the mid
  1970s.
• How fast were modems then?
• T1 architecture also defines the physical
  characteristics.

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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

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T1 Physical Characteristics

• Full Duplex
• Purely Digital
• Uses Time Division Multiplexing
• Is this efficient?
• Uses PCM
• Uses Bipolar/AMI representation of the digital
  1s and 0s.
• Voltages range from -3 to 3 volts provides
  power to repeaters.
• Clock is encoded in the signals
• Do you see a potential problem?
• Framed format
• Adds a 1 bit header to each frame of (8 x 24) bits

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Channelized vs NonChannelized

• You dont have to use your T1 connection for 24
  voice circuits.
• 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 isnt sending
  any data.
• This is the basis of time division multiplexing.
• This is wasteful.

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Framing

• After all of the 248bit samples, 1 framing bit
  is added.
• 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 1s and 0s.

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D4 Signal Framing

• The fourth generation of framing.
• Corrects several potential errors.
• Robs bits in the 6th and 12th frame.
• 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.

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Too many 0s

• (On board)

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Signaling

• What are some telephone signals?
• In Channel
• DTMF signals are part of the conversation.
• Do you really need much signaling once you start
  talking?
• Common channel
• 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 worlds first data network.
• The signaling is NOT passed with the data.

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Downsides of T1

• All your eggs are in one basket.
• Expensive at the beginning.
• You must get new equipment.

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Benefits

• The whole world isnt covered in cables.
• Allows better control via signaling channels.
• Digital is easier to transport.
• Has allowed incredible data rates.