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T1 Carrier System

T1 Carrier System. ENGR 475 – Telecommunications Harding University Jonathan White. 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. T1 AKA.

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T1 Carrier System

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  1. T1 Carrier System ENGR 475 – Telecommunications Harding University Jonathan White

  2. 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

  3. 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

  4. 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 we’ve 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.

  5. 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.

  6. T Carrier System Layers

  7. 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?

  8. 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?

  9. T1 History • Developed by AT&T (Bell Labs) in 1957. • Not implemented until the early 1960’s. • 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 didn’t come to users until the mid 1970’s. • How fast were modems then? • T1 architecture also defines the physical characteristics.

  10. 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

  11. T1 Physical Characteristics • Full Duplex • Purely Digital • Uses Time Division Multiplexing • Is this efficient? • 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 • Do you see a potential problem? • Framed format • Adds a 1 bit header to each frame of (8 x 24) bits

  12. Channelized vs NonChannelized • You don’t 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 isn’t sending any data. • This is the basis of time division multiplexing. • This is wasteful.

  13. Framing • After all of the 24*8bit 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 1’s and 0’s.

  14. 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.

  15. Too many 0’s • (On board)

  16. 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 world’s first data network. • The signaling is NOT passed with the data.

  17. Downsides of T1 • All your eggs are in one basket. • Expensive at the beginning. • You must get new equipment.

  18. 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.

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