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Packet Switching Networks

Network services and internal network operation

Internal network operation

Packet Network Topology

Datagram and Virtual Circuits

Datagram or Connectionless Packet Switching

Structure of a Packet Switch

Routing in Packet Networks

ROUTING TABLES

Specialized Routing

Deflection Routing

Shortest Path Routing

Bellman-Ford Algorithm or Ford-Fulkerson algorithm

Source Routing

UNIT II- Packet Switching Networks-2


Datagram or Connectionless Packet Switching

Messages broken into smaller units (packets)

Source & destination addresses in packet header

Connectionless, packets routed independently (datagram)

When a message arrives at the packet switch, the destination address is examined to determine the next hop.

Packet may arrive out of order

Re-sequencing maybe required at destination.

Pipelining of packets across network can reduce delay, increase throughput

Lower delay that message switching, suitable for interactive traffic

Packet Switching Delay

Assume three packets corresponding to one message traverse same path

Minimum Delay = 3τ + 5(T/3) (single path assumed)

Additional queueing delays possible at each link

Packet pipelining enables message to arrive sooner
In general the delay incurred using a datagram switch involving L hops and consisting of k packets is L + LP+(k-1)P

Virtual – Circuit Packet Switching

l Before the transmission of packets , it involves establishment of virtual
circuit between source and destination
l All packets follow the same path
l Abbreviated header identifies connection on each link
l Packets queue for transmission
l Variable bit rates possible

Connection Setup

l Signaling messages propagate as route is selected
l Signaling messages identify connection and setup tables in switches
l Typically a connection is identified by a local tag, Virtual Circuit Identifier
(VCI)
l Each switch only needs to know how to relate an incoming tag in one input
to an outgoing tag in the corresponding output
l Once tables are setup, packets can flow along path

Connection Setup Delay

l Connection setup delay is incurred before any packet can be transferred
l Delay is acceptable for sustained transfer of large number of packets
l This delay may be unacceptably high if only a few packets are being
transferred
l The minimum delay in virtual circuit packet switching is similar to that in
datagram packet switching, except for an additional delay required to setup
the virtual circuit.

Cut-Through switching

It is the modified form of virtual circuit packet switching

Some networks perform error checking on header only, so packet can be forwarded as soon as header is received & processed

Delays reduced further with cut-through switching