In digital transmission era, analog signal from phone is sampled into digital symbols. The sample rate is 8000kHz and the depth is 8bit, resulting a 64kbps signal stream. This stream is then packed into E1/T1 channels and get exchanged in switching and transmission system.
The E1 ISDN primary rate line is a bundled trunk link of 30 bearer channels and two signal channels. The traffic carrying bearer channels are numbered 0 to 14 and 16 to 30 with channels 15 and 31 carrying signaling traffic. Each bearer channel is 64Kbps and the 30 channels provides for 2.048 Mbps bandwidth.
E1 ISDN can be used for both voice traffic and be connected to a PBX to provide 30 incoming/outgoing external calls each call being assigned the standard PSTN 64Kbps TDM standard format. Alternatively an E1 can be provisioned as an un-channeled single 2Mbps data link for packet switching networks.
But in North America and Japan, they use T1 channels to envelope the signal.
The North American digital telephone hierarchy defines how the low-data-rate telephone signals are multiplexed together onto higher-speed lines. The system uses pulse code modulation (PCM) in conjunction with time-division multiplexing to achieve this.
The basic digital multiplexing standard established in the United States is called the Bell System Level 1 PCM Standard or the Bell T1 Standard. This is the standard used for multiplexing 24 separate 64-kbps (8 bits/sample ×8000 samples/s) voice channels together.
Each 64-kbps voice channel is designated as digital signaling level 0 or DS-0. Each frame in the 24-channel multiplexer consists of
8 bits/channel × 24 channels + 1 framing bit = 193 bits
The total data rate when transmitting 24 channels is determined by:
193 bits/frame × 8000 frames/s = 1.544 Mbps = T1 designation
If four T1 lines are multiplexed together, we get
4 × 24 channels = 96 channels = T2 designation
Multiplexing seven T2 lines together we get
7 × 96 = 672 channels = T3 designation
