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CHAPTER 7
PACKET-SWITCHING
NETWORKS
Dept of computer science and engg
6th sem cse
SSE
VIRTUAL CIRCUIT PACKET SWITCHING
•Virtual circuit packet switching establishes a fixed path called
VIRTUAL CIRCUITS
VIRTUAL CIRCUIT SETUP PROCEDURE will take
place before any packets flow through the network.
 Virtual circuit resides in network layer.
Figure 2
 Delay is incurred when a message is broken into 3
packets and transmitter over the circuit.
 Same delay as datagram packet switching execpt for
additional delay used to set up the circuit.

Path is determined parameters are set in swiches by
exchanging Connect request and Connect confirm
messages
 If the switch does not have enough resources then it
responds with a connect reject message and the setup
procedure fails.
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Figure 3
•At the input of every switch ,the virtual circuit is identified by
VIRTUAL CIRCUIT IDENTIFIER(VCI)
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When packet arrives at the input port the VCI in the
header is used to access the table.
The table lookup provides the output port to which
the packet is fowarded and the VCI that is used at the
input port of the next switch.
Call setup procedure sets up chain of pointers across
the network that directs the flow of packets
Figure 4
STRUCTURE OF PACKET SWITCH
A packet switch will perform two main functions
 ROUTING
 FORWARDING

Routing function uses algorithm to find path to
destination
 Stores result in routing table
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Forwarding function processes incoming packet
from input port
 Forwards packet to appropriate output port
based on routing table information
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GENERIC PACKET SWITCH
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Input port, output port, interconnection fabric,
switch controller
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Input ports and output ports are paired
Line card
 Contain several input/output port.
 Capacity of link connecting the fabric is high speed, fully
utilized
 Implements physical, data link and network layer functions
 Symbolic timing, line coding, framing, physical layer
addressing ,error checking
ORGANIZATION OF LINE CARD
MADE UP OF VARIOUS CHIPSET
Figure 6->
Line card supports medium access control protocol to
handle broadcast network.
 Implemented by special purpose chipset
 Network layer routing table in line card
 Need to perform fast table lookup to find output port
 Contain buffers and associated scheduling algorithms
 Network processor performs table lookup and packet
scheduling
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CONTROLLER
Contain general purpose processor
 This manages control and management functions
 If controller is connectionless executes routing
protocol
 If connection oriented handles signally messages.
 Controller communicates with line card and
fabric to configure internal parameters.

INTER CONNECTION FABRIC
Transfers packet between line card
 Crossbar interconnection fabric transfer packets
parallel between input and output ports
 Buffers are added to crossbar to accommodate
packet contention.
 Buffer located at input or output port.
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OUTPUT BUFFERING
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Crossbar with output buffering should run N times
faster than port speed as N packets simultaneously
arrive at a particular output
If output is idle, one packet is transmitted rest are
buffered.
Only one packet is allowed to proceed to particular
output
INPUT BUFFERING
2 packets at input 2,first packet like to go to
output 3,second to output 8
 Packet from input buffer 1 also want to go to
output 3
 Frist packet from input buffer should wait ,2nd
should wait behind 1st even though output 8 is
idle
 Results in performance degradation .
 Problem called HEAD of Line Blocking

BANYAN SWITCH
Solution is building a large switch
 Composed of 2*2 switching elements interconnected in
certain fashion`
 Exactly 1 path will exist from input to each output
 Routing is done in distributed manner
 Ie appending binary the binary address of the output
number to each packet
 Switching element at stage I steers a packet based on ith
bit of the address.
 Bit is 0 the element steers packet to upper output.

Input 1 likes to send it to the output destines to
packet 5
 The switching element at stage 1 looks at first bit
Of the address and steers the packet to its lower
output.
At stage 2 it steers the packet to the upper output
since the second bit is 0.at stage 3 steers the
packet to lower output and sends to output 5
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ROUTING IN PACKET NETWORKS
Routing is concerned with determining feasible
paths for packet to follow from source to destination
1)Rapid and accurate delivery of packets

Routing algorithm should operate correctly
Find path to correct destination
Should not take long time to find path
2)Adaptablilty to changes in network topology
Network equipment and transmission line will fail
Routing algorithm should adapt and reconsider the path.
3)Adaptability to various source destination traffic
loads
Traffic loads change dynamically, hence should adjust path based
on the current traffic loads.
4) Ability to route packets away from congested links
Routing algorithm should avoid congested links and
balance the load.
5) Ability to determine connectivity of network
Routing system should know connectivity to find optimal
paths
6)Ablility to avoid loops
Inconsistent information will lead to routing loops.
Routing system should avoid the loops in presence of
distributed routing system.
7)Low Overhead
Should obtain connectivity information by exchanging control
messages with routing system. These messages create
overhead on bandwidth usage and hence should be
minimized.
ROUTING ALGORITHM CLASSIFICATION
Routing algorithm is classifies as
 1) Static routing
 2)Dynamic routing
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Static routing
Paths are precomputed based on the network
topology, link capacities and other information
Computation is completed the paths are loaded in
the routing table of each node.
This remains fixed for a long time.
Works when network size is small, and the traffic
doesn’t change in time
It becomes cumbersome, as network size increases.
Disadvantage is inability react to network failures
DYNAMIC ROUTING
Each node will continuously learn the state of the
network by communicating with the neighbors
 Change in network topology is eventually
propagated trough nodes
 Based on the information the node computes the
best path to the destination
 Disadvantage is the added complexity with the
nodes.

They can also be distinguished as
 Centralized routing
 Distributed routing

CENTRALIZED AND DISTRIBUTED ROUTING
In Centralized routing the network control
center computes all the path and then uploads
the information in the network
 In Distributed routing nodes cooperate by the
means of message exchanges and perform their
own routing computations.
 Distributed routing is better than centralized
routing but generate inconsistent results ,loop wil
develop
 If A thinks B is the best path to Z and If B thinks
A is the best path to B
 Then desired packet to Z have the misfortune of
arriving at A or B and stuck in a loop between A
and B

ROUTING TABLES
Once the routing algorithm defines the set of
path the path information is stored in the routing
table.
 In virtual circuit packet switching the routing
table translates each incoming VCI to outgoing
VCI an identifies the output port to which the
packet is to be forwarded.
 Datagram packet switching the routing table
identifies the next hop to forward the packet
based on destination address of the packet
 Virtual circuits are bidirectional.

A packet sent by node A with VCI 1 will eventually
reach node B and from node A with VCI 5 will
reach node D
VCI 1 from node A will be translated to 2 and to 7
and finally to 8 and reaches B
When node 1 will receive the header with VCI 1
that node replaces the incoming VCI with 2 ande
forwards the packet with node 3
If a packet with VCI 5 arrives at node 1 from
node A,the packets is forwarded to node 3 after
the VCI is replaced with 3
 After arriving at node 3 the packet receives the
outgoing VCI 4 and is forwarded to node 4
 Node 4 translates the VCI to 5 and forwards the
packet to node 5.
 Node 5 translates to VCI 2 and delivers the
packet to destination node D
 The routing table uses node numbers to identify
where the packet comes from and where the
packet is forwarded.
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ROUTING TABLE FOR PACKET SWITCHING
NETWORK
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Shows routing table for network topology
Minimum hoping routing objective
Packet destines to node 6 arrives at node 1, the
packet is fowarded to node 3 based on the
corresponding routing table entry
Node 3 forwards the packet to node 6
HIERARCHICAL ROUTING
The size of routing table is reduced
 Hosts that are near to each other should have
addresses that have common perfixes
 Routers will examine the part of address and
decide to route the packet
 Example of hierarchal address assignment and
flat address assignment

FLOODING
Forwards an incoming packet to all the ports
except the one received from
 Each switch performs the flooding process and
eventually reach the destination
 Effective routing approach when the routing
information is not available
 Also effective when source need to send packet to
all the nodes
 Flooding may easily swamp network
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To reduce resource consumption in the network
Time to live field (TTL) in each packet
Source sends the packet TTL is set to some number
Each node decrements the TTL by 1 before flooding
the packet
If value reaches 0 the node discards the packet
Avoid wastage of bandwidth, minimum hop number
between two further nodes
2nd ,the node adds identifier to header of packet
When node receives the packet that contain the
identifier it discards it as it knows packet already
visited the node(prevents looping)
Identified with unique sequence number,node will
record the sequence number and source address
If already visited it will discard the packet
DEFLECTION ROUTING
Also called hot potato routing. eg: Manhatten street network
 They provide multiple path for each source –
destination
 Each path tries to forward the packet to the
preffered port
 If preferred port is busy or congested the packet
is deflected to another port
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Advantage is node are buffer less, packets do not have
to wait for a specific port to become available
If unavailable it will deflect the packet to another
port, reaches destination
Since they take alternate path they cannot guarantee
in-sequence delivery of packets
Used in optical network where optical buffers are
difficult to find
Used to implement high speed switches where
topology is regular and buffers are expensive
SHORTEST PATH ROUTING
Routing algorithm is based on
Shortest path algorithm
o Each link represents the cost of using the link
o Shortest path between node 2 to node 6 is trough
2-4-3-6 and the cost path is 4
Metrics used to assign the cost
1. Cost ~ 1/capacity : Higher cost to lower capacity links
2. Cost ~ packet delay: It includes queuing delay in switch
buffer and propagation delay in link
3 Cost ~ Congestion : Congestion measure is traffic
loading. Avoids Congested link
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THE BELLMAN – FORD ALGORITHM
If Neighbor of node A knows the shortest path to
node Z ,then node A can determine the shortest
path to Z
 Ie Node A will calculate the cost/Distance to node
Z trough its neighbors by picking the minimum

Dj – Current estimate to minimum cost to
destination node
 Cij – link cost from node I to node j (c13=c31=2)
(c11=0) (c15=c23= ∞)

Minimum cost from node 2 to node 6 trough node 1
,node 4 and node 5:
D2= min{C21 + D1 , C24 + D4 ,C23+ D5}
= min{3+3,1+3,4+2}
=4
1.Initialization: Di= ∞ for all i ≠ d
Dd=0
2. Updating: for each I ≠d
Di= Minj {Cij + Dj} for all j ≠ i
Initally all the nodes other than the destination node
are at infinite cost to node 6.
 (Iteration 1)Node 3 finds that it is connected to node
6 with cost of 1.Node 5 finds it is connected to node 6
with cost 2.Node 3 and 5 update their entries.
 (Iteration 2)Node 1 finds it can reach node 6 via node
3 with cost 3.Node 2 finds it can reach node 6 via
node 5 with cost 6. Node 4 finds it has path via 3 and
5 with costs 3 and 7 respectively.Node 4 selects the
path via node 3.Node 1,2 and 4 update their entries
 (Iteration 3)Node 2 finds it can reach node 6 via node
4 with distance 4.Node 2 changes the entry to (4,4)
and informs the neighbours
 Node 1,4,5 processes the new entry from node 2 but
donot find new shortest paths.The algorithm is
conservged
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Link connecting node 3 and node 6 breaks.
Compute minimum cost for each node to
destination node.
(update 1)As soon as node 3 detects that link(3,6)
breaks, node 3 recomputed the minimum cost to node
6.Node 3 looks for path to 6 trough neighbours, Node 1
and Node 4 and its calculation indicates that the new
shortest path is trough node 4 at a cost of 5.Node 3
then sends the new routing updates that is node 1 and
node 4
 (update 2)Node 1 and 4 recomputes their minimum
costs; node 1 findsits shortest path is still trough node
3 but the cost has increased to 7;node 4 finds the
shortest path is trough node 2 or node 5 with cost
5.node 4 chooses node 2.Node 1 transmits its routing
updates to node 2,node 3 and node 4
 Node 4 transmits its routing updates to node 1,2,3,5

(Update 3)Node 1 finds its shortest path is still trough
node 3.Node 2 finds its shortest path is still trough
node 4 but the cost has increased to 6.Node 3 finds its
shortest path is still through node 4 but the cost has
increased to 7.Node 4 and Node 5 finds the shortest
path is not changed. Node 2 transmits its updates to
node1,node4 and node 5 and transmits its updates to
node 1 and 4.
 (update 4) Node 1 finds its shortest path is through
either node 2 or node 3 with cost 9.Suppose that node 1
chooses node 2.Node 4 now finds the shortest path
through node 5 with cost 5 since the cost through 2 has
incresed to 7.Node 5 does not change its shortest
path.Node 1 transmits its updates to node 2,node
3,node 4 and node 4 transmit its updates to node 1,2,3
and 5
 (update 5) None of the nodes find short paths

Update
node1
Before break (2,3)
After break (2,3)
1
(2,3)
2
(2,5)
3
(2,5)
4
(2,7)
5
(2,7)

node2
(3,2)
(3,2)
(3,4)
(3,4)
(3,6)
(3,6)
(3,8)
node3
(4,1)
(2,3)
(2,3)
(2,5)
(2,5)
(2,7)
(2,7)
SPLIT HORIZON WITH POISON REVERSE
Update
Node 1
Before break (2,3)
After Break (2,3)
1
(2,3)
2
(-1, )

Node 2
(3,2)
(3,2)
(-1, )
(-1, )
Node3
(4,1)
(-1, )
(-1, )
(-1, )
After the link breaks node 3 sets the cost of the
destination to infinity,since the minimum cost
node 3 has received from node 2 is also
infinity.When node 2 received the update
message it also sets to infinity.Node 1 learns the
destination is unreachable
DIJKSTRA ‘S ALGORTIHM
Alternate algorithm for finding the shortest paths to
source to all other nodes.
 To identify the closest nodes from the source node in
the order of their increasing path cost.
 1 Initialalization

N={s}
Dj=Csj,for all j ≠ s
2.Finding the next closest node
Di=min Dj
Add i to N
3.Update minimum cost from node I to node N
Dj=min{Dj,Di+Cij}
SOURCE ROUTING VERSUS HOP BY HOP
ROUTING
Datagram network each node determines the
next hop along the shotest path ,packet travels
from source as it follows HOP BY HOP routing to
destination
 SOURCE ROUTING the path to the destination
is determined by source.Explicit routing allow a
perticular packet node to determine the path.
 Source includes path information in the packet
header that contain sequence of nodes to traverse
and sufficient information to nodes till packet is
fowarded to destination.

Each node examines the header,removes address
identifying the node,forwards packet to next node
 If we reserve the path information we can
traverse the node back to the source by reversing
the path.

LINK STATE ROUTING VERSUS DISTANCE VECTOR
ROUTING
Distance Vector routing
Neighboring routes exchange routing tables that sets the
vector of known distance to destination. Bellman ford
algorithm is used to find the best path though
information from the neighbors. With new path the
router wil send new vector to neighbors.
This adapts to change in network topology
Link State Routing
Each router will flood information to the state of the
links that connects to neighbors
Allows routers to construct map of entire network and
then uses Djikstra s algorithm. If state is changed the
router detects the change and floods the new
information to network

ATM NETWORKS
Asynchronous transfer mode (ATM) is a method for
multiplexing and switching that supports broad range of
services
 Connection oriented packet switching technique that
provides quality of service(QoS)
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Variable bit rate delay bursty traffic processing
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TDM Multirate only
low,fixed Inefficient
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Packet
variable
Easily handled
efficient
Minimal ,high
speed
header and
packet processing
ATM is packet based,can easily handle services and
generate information in bursty fashion on variable
bit rates.
 The abbreviated header of ATM facilitates
implementations that result in low delay and high
speed

Information flow generated from various users is
converted into cell and sent to ATM multiplexer
 It arranges the cell into one or more queues and
implements some scheduling strategy to determine
which order it is transmitted
