Multiplexing techniques such as time division multiplexing (TDM) and frequency division multiplexing (FDM) allow multiple users to share network links. TDM divides time into slots that are allocated to users on a fixed or dynamic basis. FDM assigns each user a unique frequency band to transmit in simultaneously. Switching networks move data packets through intermediate nodes using either circuit switching, which establishes a dedicated path, or packet switching, which breaks messages into packets that travel independently through the network.
4. Time Division Multiplexing
The Basic Idea:
As the name suggest, Divide time amongst the
users
Give each user some time to transmit his data
This process is periodic in a round robin fashion
The time given to a each user is referred to as
Time Slot or Time Quantum
16. Statistical TDM
Here Time slots are given on
demand……….rather than in round robin
fashion
Each User can get 2 or more consecutive
time slots
If time slot is not required, it is not
allocated
Hence, not wasted
19. FDM
Sharing is done by assigning each user a
specific frequency (Carrier Frequency)
Modulation equipment is used to move
each signal to the required frequency
band.
Multiplexing equipment is needed to
combine the modulated signal
25. Switching Networks
A network is made up of end hosts and
intermediate switching nodes
Data is usually passed through a
network of intermediate switching
nodes
The Switching nodes:
Not concerned with the contents of the data;
• Provide a switching facility that will move
data from node to node until they reach their
destination
27. Some Notes…
Some nodes only connect to other nodes
Some nodes connect to end hosts also
Usually the network is not fully connected; there
is not a direct link between each pair of nodes
If there are more than one paths between any
pair of nodes; this increases the reliability of the
network
Node-Node links are usually multiplexed
28. Two Technologies for Switching
Circuit Switching
Packet Switching
They differ in the way the nodes switch
information from one link to another on the
way from source to destination
31. Three Phases
Circuit Establishment
• Host-B send a connection request towards Host-D
• Intermediate nodes route the request to Host-D based
on measures of availability and cost
• If ready, D accepts the connection and a dedicated path
(generally full duplex) is established from B through the
intermediate nodes to D
Data Transfer
• The data (analog/digital) is carried on the dedicated
path
Circuit Disconnect
• Done by any one station
• Signals are propagated to intermediate nodes to de-
allocate the dedicated resources
32. Principles
• Circuit switching designed for voice
– Resources dedicated to a particular call
– Much of the time a data connection is idle
– Data rate is fixed
• Both ends must operate at the same rate
34. Problems in Circuit Switching
Circuit Switching approach is inefficient
Since data rate is constant, therefore the
devices interconnected must transmit and
receive at the same data rate, This limits the
interconnection of variety of hosts
Further calls are blocked when all the lines are
busy
35. Packet Switching…...A quick overview
Data is transmitted in short packets
If a source has larger message to send, the message is broken
up into a series of packets
Each packet contains user’s data plus some control
information (header)
The control information, at a minimum includes the
information that the network requires to be able to route the
packet through the network and deliver it to the intended
destination
At each node the packet is received, stored briefly and passed
on to the next node
36. Advantages
Line efficiency is greater
A packet switching network can perform data rate
conversion
In Circuit Switching, calls are blocked whereas in P.S.
packets are still accepted but delivery delay increases
Priorities can be used, thus a higher priority packet
experiences less delay
37. Types of Packet Switching
Datagram Approach
Virtual Circuit Approach
38. Datagram Packet Switching
Each packet is treated independently, with no reference to packets
that have gone before
Each packet contains the address of its destination
The packets with the same destination do not always follow the
same route
Some packets can get late and some can get destroyed in the
network
Therefore, packets can be received out of order at the destination
There must be some mechanism of re-ordering at the receiver
Each packet, treated independently, is referred to as datagram
40. Virtual Circuit Packet Switching
A preplanned route is established before
any packets are sent
All the packets follow that route
Just like circuit switching, the route
should be terminated after the
transmission of data