Packet switching networks, Exercises of Computer Networks

Download Packet switching networks and more Exercises Computer Networks in PDF only on Docsity!

CHAPTER 7

PACKET-SWITCHING

NETWORKS

Dept of computer science and engg 6 th^ sem cse SSE

VIRTUAL CIRCUIT PACKET SWITCHING

• Virtual circuit packet switching establishes a fixed path called VIRTUAL CIRCUITS

 (^) 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.  (^) Figure 3

• At the input of every switch ,the virtual circuit is identified by VIRTUAL CIRCUIT IDENTIFIER(VCI)

 (^) 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

GENERIC PACKET SWITCH

 (^) Input port, output port, interconnection fabric, switch controller

 (^) 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

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.

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.

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.

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