Computer network switching

Computer Network Switching
by:
Vishal Sharma
Lecturer, NIT Hamirpur
vishalfzd@gmail.com

Switched network
• Switched network: Series of interlinked nodes, called
switches.
• Switches: Devices capable of creating temporary
connections between two or more devices linked to the
switches . Some of these switches are connected to the
end systems ( computers or telephones) . Others used only
for routing

1. CIRCUIT-SWITCHED NETWORKS
• A circuit switched network consists of a set of
switches connected by physical links.
• A connection between 2 stations is a dedicated path
made of one or more links.
• However each connection uses only one dedicated
channel on each link.
• Each link is divided into n channels using FDM or
TDM.

Three phases are need to communicate two
parties or multiple parties in a conference call):
• Connection setup
• data transfer
• Connection teardown

• The setup phase: means creating dedicated channels
between the switches.
• Data Transfer Phase: After the establishment of the
dedicated circuit (channels), the two parties can
transfer data.
• Teardown Phase: When one of the parties needs to
disconnect, a signal is sent to each switch to release
the resources.

A trivial circuit-switched network

• Example : when system A needs to connect to
system M:
• A sends a setup request that includes the address of system
M, to switch I. Switch I finds a channel between itself and
switch IV. Switch I then sends the request to switch IV, which
finds a dedicated channel between itself and switch III. Switch
III informs system M of system A's intention at this time.
• An acknowledgment from system M needs to be sent in the
opposite direction to system A.
• After system A receives this acknowledgment the connection
established.

Circuit-switched network
• Circuit switching takes place at the physical layer
• Data transferred between the two stations are not
packetized. The data are a continuous flow sent by the source
station and received by the destination station
• There is no addressing involved during data transfer. Of
course, there is end-to-end addressing used during the setup
phase.

Efficiency
• circuit-switched networks are not so efficient as
the resources are allocated during the entire
duration of the connection.
• These resources are unavailable to other
connections.

2. PACKET SWITCHED NETWORK
• Messages need to be divided into packets.
• Size of the packet is determined by the network and
the governing protocol.
• no resource reservation, but allocated on demand.
• The allocation is done first come, first served based
• When a switch receives a packets , the packet must
wait if there are other packets being processed, this
lack of reservation may create delay

2.1 DATAGRAM NETWORKS
• Each packet (called as datagrams in this approach) is treated
independently of all others
• All packets (or datagrams) belong to the same message may
travel different paths to reach their destination .

• Datagram Switching is done at the network layer
• This approach can cause the datagrams of a transmission
to arrive at their destination out of order with different
delays between the packets.
• Packets may also be lost or dropped because of a lack of
resources.
• In most protocols, it is the responsibility of an upper-layer
protocol to reorder the datagrams or ask for lost
datagrams before passing them on to the application.
• The datagram networks are referred to connectionless
networks. There are no setup or teardown phases.
• How are the packets routed to their
destination??

Routing table in a datagram network
• Each packet switch has a routing table
which is based on the destination address.
• The routing tables are dynamic and are
updated periodically.
• The destination addresses and the
corresponding forwarding output ports are
recorded in the tables.
• The destination address in the header of a
packet in a datagram network remains the
same during the entire journey of the
packet.
• When the switch receives the packet, this
destination address is examined; the
routing table is consulted to find the
corresponding port through which the
packet should be forwarded.

Efficiency
• Better than that of a circuit-switched network.
• Resources are allocated only when there are packets
to be transferred. If a source sends a packet and
there is a delay of a few minutes before another
packet can be sent, the resources can be reallocated
during these minutes for other packets from other
sources.
• Switching in the Internet is done by using the
datagram approach to packet switching at
the network layer

Delay in a datagram network
• Total delay =3T + 3t + WI + W2
T: transmission times
t: propagation delays
(WI + W2) : waiting times

2.2 VIRTUAL-CIRCUIT NETWORKS
• It’s a cross between circuit switched network and datagram
network, and has some characteristics of both.
 Characteristics:
 Packets from a single message travel along the same path.
 Three phases to transfer data (set up, data transfer and tear
down)
 Resources can be allocated during setup phase
 Data are packetized and each packet carries an address in the
header
 Implemented in data link layer

Virtual-circuit network Addressing
 Global addressing:
- Source and destination needs unique addresses (used by the
switches only to create a virtual-circuit identifier ) during the set up
phase
• Local addressing (virtual-circuit identifier –VCI):
- Actually used for data transfer
- A small address used by a frame between two switches.

Setup request
• The switch, in the setup phase acts as a packet switch ; it has
a routing table used to know the output port number

Setup acknowledgement
• When Destination B receives the up frame , and it is ready to
receive frames from A, it assign a VCI (in this case :77). This
VCI lets the destination know that the frames comes from A
not other sources.

Data Transfer and Tear down Phase
• After sending all frames, a special frame is send to end the connection
• Destination B responds with a teardown confirmation frame

Delay in Virtual Circuit switching
Total delay = 3T+ 3t +setup delay + teardown delay
• In datagram network,
each packet may
experience a wait at a
switch before it is
forwarded. In addition,
the delay is not uniform
for the packets of a
message.
• In a virtual-circuit
network, there is a one-time
delay for setup and
a one-time delay for
teardown. If resources
are allocated during the
setup phase, there is no
wait time for individual
packets.

Computer network switching

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