T1 Multiplexing

T1 Multiplexing

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Under the simplest conditions, a medium can carry only one signal at any moment in time.

For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.

The current techniques that can accomplish this include

•frequency division multiplexing (FDM)

•time division multiplexing (TDM)

•Synchronous vs. statistical

•wavelength division multiplexing (WDM)

•code division multiplexing (CDM)

Multiplexing

• Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.

• A Multiplexer (MUX) is a device that combines several signals into a single signal.

• A Demultiplexer (DEMUX) is a device that performs the inverse operation.

Categories of Multiplexing

Frequency Wave-length

•Sharing of the signal is accomplished by dividing available transmission time on a medium among users.

•Digital signaling is used exclusively.

•Time division multiplexing comes in two basic forms:1. Synchronous time division multiplexing, and2. Statistical, or asynchronous time division multiplexing.

Time Division Multiplexing

Time-division Multiplexing (TDM)

• Time-division multiplexing (TDM) is a digital process used when the data rate capacity of the transmission medium > data rate required by the sending and receiving devices.

TDM

TDM is a digital multiplexing technique to combine data.

Time-division Multiplexing (TDM)

• Syncrhonous = Multiplexer allocates exactly the same time slot to each device at all times▫ Regardless if they don’t want to transmit

• FramesTime slots are grouped into frames. A frame

consists of a one complete cycle of time slots, including one or more slots dedicated to each sending device.

TDM frames

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Synchronous Time Division Multiplexing(continued)

Time-division Multiplexing (TDM)

Time slot order in a synchronous TDM system does not vary from frame to frame▫ Very little overhead information needed. ▫ However, one or more synchronization bits are

usually added to the beginning of each frame. These bits, called framing bits, allows the

demultiplexer to synchronize with the incoming stream so that it can separate the time slot accurately.

6.12Synchronization

• The receiver looks for the anticipated bit and starts counting bits till the end of the frame.

• Then it starts over again with the reception of another known bit.

• These bits (or bit patterns) are called synchronization bit(s).

• They are part of the overhead of transmission.

• Flow control, error control, etc. will be handled before and after the multiplexers.

Framing bits

DS hierarchy

15 Synchronous Time Division Multiplexing(continued)

•If one device generates data at a faster rate than other devices, then the multiplexor must either

•Sample incoming data stream from that device more often than it samples other devices

•OR•Buffer faster incoming stream

•If a device has nothing to transmit, •Multiplexor must still insert a piece of data from that device into the multiplexed stream

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Synchronous Time Division Multiplexing(continued)

17 T-1 Multiplexing

•T-1 multiplexor stream is a continuous series of frames

6.18

Time Division Multiplexing

• DS-1 Transmission Format

▫Frame Structure (193 bits) 8 bits/channel 24 channels 1 framing bit.

▫Data Rate 193 bits/frame x 8 k frames/sec =1.544

Mbps.

Time Division Multiplexing

• DS-1 Transmission Format

▫Voice Uses bit robbing.

Every sixth frame has one bit "robbed" for control signaling from each channel.

▫Data Bit 8 is used for control signaling (8,000

bps.) Bit 1-7 used for 56 kbps service. Bit 2-7 used for 9.6, 4.8, and 2.4 kbps

service.

Inefficient use of Bandwidth

•Sometimes an input link may have no data to transmit.

•When that happens, one or more slots on the output link will go unused.

•That is wasteful of bandwidth.