Chapter 26. Telecommunication Multiplexing
Telecommunication multiplexing is used between switching offices and it runs on Interoffice trunks and Intertoll trunks. The Telcos (telecommunication companies such as Bell Canada, AGT, BC-Tel, etc.) share communication facilities which can be either FDM or TDM. A communication path can change in mid-stream from FDM to TDM, and back again, depending on whose communication facility is being used.
FDM is analog, and is currently being updated to TDM throughout the world. However, there are locations where FDM is still being used.
31. Telecommunication Multiplexing
FDM - Channel Groups
FDM is based on channel groups. The basic channel is called the Voice Channel and it has a bandwidth of 0-4 kHz. The channel groups are based on multiples of the voice channel, as shown below:
| Multiplex Level | Voice Circuits | Freq Band(kHz) | BW(kHz) |
| Voice Channel | 1 | 0 - 4 | 4 |
| Group | 12 | 60 - 108 | 48 |
| Supergroup | 60 | 312 - 552 | 240 |
| Mastergroup | 600 | 564 - 3,084 | 2520 |
| Jumbogroup | 3600 | 564 - 17,548 | 16984 |
The Mastergroup and Jumbogroup have guard-bands added to the bandwidth.
A Group is made of 12 Voice Channels.
A Supergroup (60 Voice channels) is made of 5 Groups (12 Voice Channels).
A Mastergroup (600 Voice Channels) is made of 10 Supergroups (60 Voice Channels).
A Jumbogroup (3600 Voice Channels) is made of 6 Mastergroups (600 Voice Channels).
TDM - T1 Carrier System
TDM is based on the T1 Carrier System. It is a digital system that digitizes the analog Voice Channel into 8 bit data: there are 28 or 256 levels that can be represented by this data.
It samples the analog signal 8000 times a second (2x 4 kHz - makes Nyquist happy!). It is a serial data stream, so we transmit the 8 bit data 1 bit at a time. This means that for a digitized Voice Channel, the data rate is as follows:
8 bits x 8000 samples = 64 Kbps
The basic Carrier used in the T1 Carrier System is called the T1 (sometimes called DS-1) and it carries 24 Voice Channels. The Bit Rate for the T1 Carrier is 1.544 Mbps. If we multiply:
24 Voice Channels x 64 Kbps per Voice Channel = 1.536 Mbps
The missing 8 K is used to "frame" the data. It is information used for the Start Frame bytes, End Frame, Error Checking, Routing information, etc.
| Digital Circuit | Voice Channels | Bit Rate | # of T1 Circuits |
| T1 (DS-1) | 24 | 1.544 Mbps | 1 |
| T2 (DS-2) | 96 | 6.312 Mbps | 4 |
| T3 (DS-3) | 672 | 44.736 Mbps | 28 |
| T4 (DS-4) | 4032 | 274.176 Mbps | 168 |
Typically, information is run over the following:
T1 - Twisted Pair or Coax Cable
T2 - Coax Cable
T3 - Coax, fiber Optics or Light Route Radio
T4 - Coax or Fiber Optics
You can rent just a portion of a T1 line; you don't have to rent the complete circuit. You basically rent a time-slot on the line that is based on individual 64 kbps channels (this is called Fractional T-1).
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