INDEX
Introduction.
v Addressing.
v Three
phases of virtual circuit network.
v Advantages
and disadvantages.
v Application.
Introduction:
Virtual circuit
network:
·
A virtual circuit network is a cross
between a circuit switched network and a datagram
network.
It has some characteristics of both.
·
Circuit switched network: Dedicated path
between sender and receiver during communication. Example: Telephone net.
·
Datagram network: Data sent through net in discrete datagram /
packets.
·
As in datagram network, data are
packetized and each packet carries an address in the header. However, the
address in the header has local addresses.
·
As in circuit-switched network, all packets
follow the same path established during the connection.
·
A virtual-circuit network is normally
implemented in the data layer.
Addressing
In virtual circuit
network, two types of addressing are involved: local and global.
Global
addressing: A source or destination needs to have a global
address-a address that can be unique in the scope of a network or
internationally.
Local addressing: The
identifier that is actually used for data transfer is called the
virtual-circuit identifier (VCI).A VCI, unlike a global address, is a small
number that has only switch scope; it is used by a frame between two switches.
When a frame arrives at
a switch, it has a VCI; when it leaves, it has different VCI.
Virtual–circuit
identifier
·
Each link or packet has an identifier.
·
It is used for routing.
·
For example if end system A and End
system B are communicating, the link will be from End system A to End system B
is the virtual circuit.
Switch and tables in a
virtual-circuit network
In the above figure, there are two data
packets arriving at port 1.
Suppose the first data
packet with VCI 14 is arrived at port 1 then that data packet will exit from
port 3. Since in the table we have for port 1 and VCI 14, the outgoing data is
port 3 and VCI will changed to 22. So it will leave from port 3. And another
data packet follows the same.
Three phase :
As in
a circuit switched network, a source and destination need to go through 3
phases in virtual circuit network: setup, data transfer and tear down.
In the
setup phase, the source and destination use their global adrresses to help
switches make table entries foe the connection.
In the
tear down phase, the source and the destination inform the switches to delete
the corresponding entry.
Data
transfer occurs between these two phases.
Source to destination data transfer in virtual circuit
network
Setup request in a virtual circuit network
Setup Request: A setup request frame is sent from the source to the destination. The following figure shows the process.
a. Source A sends a setup frame
to switch 1.
b. Switch 1 receives the setup
request frame. It knows that a frame going from A to B goes out through port 3.
The switch, in the setup phase, acts as a packet switch and it has a routing
table which is different from the switching table. For the moment, assume that
it knows the output port. The switch creates an entry in its table for this
virtual circuit, but it is only able to fill three of the four columns. The
switch assigns the incoming port (1) and chooses an available incoming VCI (14)
and the outgoing port (3). It does not yet know the outgoing VCI, which will be
found during the acknowledgment step. The switch then forwards the frame
through port 3 to switch 2.
c. Switch 2 receives the setup
request frame. The same events happen here as at switch 1 and three columns of
the table are completed: in this case, incoming port (l), incoming
VCI (66), and outgoing port (2).
d. Switch 3 receives the setup request frame. Again, three columns are completed: incoming port (2), incoming VCI (22), and outgoing port (3).
d. Switch 3 receives the setup request frame. Again, three columns are completed: incoming port (2), incoming VCI (22), and outgoing port (3).
e. Destination B receives the
setup frame, and if it is ready to receive frames from A, it assigns a VCI to
the incoming frames that come from A, in this case 77. This VCI lets the
destination know that the frames come from A, and no other sources.
Setup acknowledgement phase in virtual
circuit network:
A special frame, called the
acknowledgment frame, completes the entries in the switching tables. The
following figure shows the process.
a. The destination sends an
acknowledgment to switch 3. The acknowledgment carries the global source and
destination addresses so the switch knows which entry in the table is to be
completed. The frame also carries VCI 77, chosen by the destination as the
incoming VCI for frames from A. Switch 3 uses this VCI to complete the outgoing
VCI column for this entry. Note that 77 is the incoming VCI for destination B,
but the outgoing VCI for switch 3.
b. Switch 3 sends an
acknowledgment to switch 2 that contains its incoming VCI in the table, chosen
in the previous step. Switch 2 uses this as the outgoing VCI in the table.
c. Switch 2 sends an
acknowledgment to switch 1 that contains its incoming VCI in the table, chosen
in the previous step. Switch 1 uses this as the outgoing VCI in the table.
d. Finally switch 1 sends an
acknowledgment to source A that contains its incoming VCI in the table, chosen
in the previous step.
e. The source uses this as the
outgoing VCI for the data frames to be sent to destination B.
Advantages
v
Uses connection orientated
services.
v
Can used different
physical links each time.
v There is one big advantage in a virtual-circuit network
even if resource allocation is on demand. The source can check the availability
of the resources, without actually reserving it.
Disadvantages
v Link setup time.
v Resilience to the loss of a trunk i.e. no dynamic
switching in case of link failure.
Applications
v It
is used in ATM (Asynchronous Transfer Mode)
network.
v It
is also used in telephone network.
