T1 Multiplexing
2
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
Time Division Multiplexing
•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, and
2. Statistical, or asynchronous 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
• Frames
Time 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
10
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.
Synchronization
6.12
• 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
Synchronous Time Division Multiplexing
(continued)
15
•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
16
Synchronous Time Division Multiplexing
(continued)
T-1 Multiplexing
17
•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.