Multiplexing refers to combining multiple information signals into one channel for transmission. Demultiplexing is separating the signals at the receiving end. There are four main types of multiplexing: frequency division, wavelength division, time division, and code division. Time division multiplexing transmits signals by assigning each a time slot, while frequency division divides the bandwidth into frequency channels. Packet switching uses statistical multiplexing without a fixed frame structure and may involve queuing delays, making it suitable for bursty traffic.

1. Transmission Format

2.  Multiplexing refers to the ‘combination’ of information streams from multiple sources
for transmission over a shared medium
 Transmitting two or more signals simultaneously can be accomplished by
setting up one transmitter-receiver pair for each channel, but this is an
expensive approach.
 Cost savings can be gained by using a single channel to send multiple
information signals over a single medium.
 Demultiplexing refers to the separation of the received signal back into separate
information streams
 Figure illustrates the basic concept:
 independent pairs of senders and receivers
• each sender communicates with a single receiver
 all pairs share a single transmission medium
 multiplexer combines information from the senders for transmission in such a
way that the demultiplexer can separate the information for receivers
Multiplexing

3. Types of Multiplexing
 There are four basic approaches to multiplexing that each have a set of variations and
implementations
 Frequency Division Multiplexing (FDM)
• Bandwidth of a single physical medium is divided into a number of smaller,
independent frequency channels.
 Wavelength Division Multiplexing (WDM)
• Transmitting multiple (optical) signals over a single (optical fibre) cable, each
with a different wavelength.
 Time Division Multiplexing (TDM)
• Time is shared; each connection occupies a portion of time in the link.
 Code Division Multiplexing (CDM)
• Each user is assigned a unique code that is used to modulate their signal.
The modulated signlas are then combined and transmitted over the same
channel.
 TDM and FDM are widely used
 WDM is a form of FDM used for optical fiber
 CDM is a mathematical approach used in cell phone mechanisms, as it allows multiple
users to share the same frequency band.

4. Frequency Division Multiplexing (FDM)
 Separation of spectrum into smaller frequency bands, assigned to different signals.
 Channel gets band of the spectrum for the whole time.
 Applications: Radio and Television broadcasting.
 Advantages:
 no dynamic coordination needed.
 works also for analog signals.
 Disadvantages:
 waste of bandwidth if traffic is distributed unevenly.
 Many filters and modulators are required.
 Cross-talk is high.
 guard spaces

5. Time Division Multiplexing (TDM)
Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several
sources so that the information from these sources can be transmitted serially over a single
communication channel.
At the Transmitter
 Simultaneous transmission of several signals on a time-sharing basis.
 Each signal occupies its own distinct time slot, using all frequencies, for the duration of
the transmission.
 Slots may be permanently assigned on demand.
At the Receiver
 Decommutator (sampler) has to be synchronized with the incoming waveform  Frame
Synchronization
 samples are recovered by the Low pass filter
 Feedthrough of one channel's signal into another channel -- Crosstalk
Applications of TDM: Digital Telephony, Data communications, Satellite Access,
Cellular radio.

6. TDM (contd.)
 Transmission or Switching Frames:
 Time on a transmission link is divided up into fixed-length intervals.
 Data from various sources are carried in repetitive frames.
 Each frame is further subdivided into time slots (example – M in the figure).
 Capable of carrying one byte of information.
 Sequentially numbered throughout the frame.
 TDM connection – synchronous TDM
 A certain portion of the bandwidth is reserved for a particular connection.
 Network Controller or Scheduler:
• Assign each newly requested connection to one particular time slot from
the switching frame.
• Different time slots within the frame carry information corresponding to
different connections.
• Return time slots to the unused pool (possible reassignment) upon
termination of a particular connection.
 The demultiplexer at the receiver identifies a particular connection by
determining the position of the time slot within the frame.
C
1
C
2
C
3
C
M
-
1
C
M
C
V
C
V
C
V
C
1
C
2
C
3
C
M
C
1
C
2
C
3
C
M
C
V
C
V
C
V
One Time Slot
1 Frame
Framing Pulse
Fig: Typical byte-interleaved transmission format

7. TDM (contd.)
 A demultiplexer cannot tell where a slot begins – a slight difference in the clocks used
to time bits can cause a demultiplexer to misinterpret the bit stream.
 Instead of taking a complete slot, framing inserts a single bit in the stream on
each round – Framing Pulse.
 Time-Slot Scheduling:
 Connections are assigned by the scheduler such that at no time are the time
slots on more than one inbound link destined for the same outbound link.
 Conversion occurs within the switch of an inbound time slot assignment to a
different outbound assignment.
Circuit
Switch
(N X N)
1
N
1
N
TDM
(M X 1)
1
M
I/S O/P O/S
I/P
5 N 6
1
Fig: Some Translation table formed at time of signaling.

8. TDM (contd.)
 Data rate of the transmission line and the data rate of each connection is determined
by the number of time slots per frame, and period of the switching frame.
 data rate for each user/connection is fixed.
 Example:
 DS1 transmission system allows 24 (1 byte) time slots = 192 bits/frame.
 8000 frames can be transmitted per second = frame length is 125 µsec.
 Each connection consists of one byte every 125 µsec = 64 kbps connection data
rate.
 Frame-mark bit (or the framing pulse) = 1bit/frame = 8 kbps signalling channel.
 Data rate of the transmission link –
• 24 time slots/frame, each slot supports 64 kbps connection = 1.536 Mbps
• signalling channel of 8 kbps
• Total Data rate of DS1 link is 1.544 Mbps.
 Disadvantage of this form of switching –
 inflexible – the user can’t transmit more than, say, 64 kbps because the slot
duration (= 8 bits) is fixed frame duration (= 125 µsec) is also fixed.
 What happens when different users have different data rates to transmit?
 Multi rate circuit switching – allocate more than 1 slot to same user!
 BUT if we have a spectrum of users with different data rates (e.g.: from 1 kbps to 1
Mbps), how to choose the basic channel rate?
 What happens if all slots are already allocated, and another user dials in?

9. Another Transmission Format
S
2
S
3
S
4
S
N
S
1
S
1
S
2
S
3
S
N
One Time Slot 1 Frame
Fig: Packet-interleaved transmission format
C
V
C
V
C
V
C
V
C
V
C
V
Data Field Header
 Again using repetitive switching frames
 time slots contain multiple bytes.
 for example in ATM, each time slot may contain 53 bytes.
 Using network scheduler, each requested connection would be assigned to a particular
time slot of successive frames.
 Now, 53 byte segments/cells/packets corresponding to each connection can be
multiplexed.
 53-byte cell:
 data field (48 bytes): user-supplied information
 header field (5 bytes): network-supplied control information
 3 bytes used to identify the connection
• includes a number unique to each connection carried over the transmission
link.
 The switch/demultiplexer would
 “read” the connection identifier contained within the header
 make appropriate internal routing decisions dynamically for each cell.

10. Packet Switching
 No need for a synchronous time-multiplexed frame!
 Use Statistical Multiplexing!
 In this scheme:
 Transmission time slots occur regularly but may not have any framing structure.
 Time slots are not allocated to any connection.
• No particular admission control.
• Payloads/cells/packets associated with a particular connection may occur
asynchronously!
 All users are allowed to transmit – if the combined data rate of all the users
 exceed the output capacity, bits would have to be either dropped or
queued/buffered.
 is less than the output capacity, no queuing.
Stat Mux
(buffer)
(K X 1)
1
K
 The terminating switch must contain smoothing buffers to temporarily store cells that
arrive on 2 or more inbound transmission links destined for the same outbound link
simultaneously.
 A ‘scheduler’ will schedule these packets on to the output link.
 Only one cell can be present on any outbound link at any given time.
 Such competing cells are sequentially read onto the time slots of the outbound
links.

11. Switching

12. Packet Switching (contd.)
 Circuit switching – Blocking
 If all slots are occupied, new connection is blocked.
 performance characteristic – probability of call blocking.
• The only delays are the switching latencies that occur in space division
switching
 suitable for real time services
 Statistical Multiplexing – Queuing
 no concept of admission control.
 performance characteristic – queuing delay.
 suitable for bursty traffic.
• ratio of peak rate to average rate is very large for bursty traffic;
permanent allocation leads to under-utilization of the channel.
• these bursty sources must be capable of tolerating some delays.
 Packet Switch network is ‘a best effort’ network.
 network will make every attempt to transmit the packet
 but offers no quality of service guarantees, in terms of delays.
 In any practical statistical multiplexer, the buffer size is finite/limited.
 In case buffer gets full, anymore incoming packet gets dropped.
 Real time services require some guarantee on delays and/or packet loss.