Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

T1 Digital Carrier system

• T1 digital carrier system is a North American digital multiplexing standard since 1963.
• T1 stands for transmission one and specifies a digital carrier system using PCM encoded analog signal.
• A T1 carrier system is time division multiplexes PCM encoded samples from 24 voice band channels for transmission over a single metallic wire pair or optical fiber transmission line.
• Each voice band channel has BW around 300Hz to 3000KHz.

Fig 1: T1 digital system

• A multiplexer is simply a digital switch with 24 independent inputs and one time division multiplexed output.
• The PCM output signals from 24 voice band channels are sequentially selected and connected through the multiplexer to the transmission line.
• With T1 carrier system, there is sampling, encoding and multiplexing of 24 voice band channels.
• Each channel contains an 8-bit PCM code and sampled 8000 times a second.
• Each channel is sampled at same rate but not at same time.
• The figure shows that, each channel is sampled once in each frame, but not at same time.
• Each channel’s sample is offset from previous channel’s sample by 1/24 of total frame time.
• Therefore one 64Kbps PCM encoded sample is transmitted for each voice band channel during each frame. The line Speed is calculated as:

Fig 2: T1 frame structure

• An additional bit (called framing bit) is added to each frame.
• The framing bit occurs once per frame (8000bps rate) and recovered in receiver, where it is used to maintain frame and sample synchronization between TDM transmitter and receiver.
• So each frame contains 193 bits and line speed for T1 digital carrier system is

• AMI line coding is used for T1 digital Systems