ENGR 475 – Telecommunications Harding University Jonathan White 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 T1 – Transmission Level 1 DS1 – Digital Signal 1 E1 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

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.

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. 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.

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 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

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.

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.

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.

Too many 0’s (On board)

Signaling What are some telephone signals? In Channel Common 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.

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.