1. UNIT 4

2.  Functions of network devices Separating (connecting) networks or expanding network e.g. repeaters, hubs, bridges, routers, switches, gateways

4.  Repeaters work at the OSI physical layer to regenerate the network’s signal and resend them to other segments  A repeater operates only in the physical layers  A repeater connects segments of a LAN  A repeater forwards every frame; it has no filtering capability  The repeater is a two-port device that extends the LANs’ physical length

6. A repeater is a regenerator, not an amplifier

7.  An active hub is actually a multiport repeater  It is normally used to create connections between stations in a star topology  Hubs can also be used to create multiple levels of hierarchy; removing the length limitation of 10Base-T (100m)

8.  Hub Advantages:  simple, inexpensive device  Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions  Extends maximum distance between node pairs (100m per Hub)

9.  A bridge is a computer that has its own processor, memory and two NIC cards to connect to two portions of a network.  A bridge operates in both physical and data link layers Has one input and one output The bridge can check the physical(MAC)address (source & Destination)contained in the frame.

11.  Transparent /Learning bridges  A bridge in which the stations are completely unaware of the bridge’s existence  Three criteria for a transparent bridge ▪ Frames must forward from one station to another ▪ The forwarding table is automatically made by learning frame movements in the network ▪ Loops in the system must be prevented  Source routing bridges  A sending station defines the bridges that the frame must visit.  The address of these bridges are included in the frame.

13.  Routers work at the OSI layer 3 (network layer)  They use the “logical address” of packets and routing tables to determine the best path for data delivery  Only packets with known network addresses will be passed - hence reduce traffic  Will select the most cost effective path for transmitting packets

14. Routing table is formed based on communications between routers using “Routing Protocols” Routing Protocols collect data about current network status and contribute to selection of the best path

15.  It is normally a computer that operates in all five layers of internet or seven layers of OSI model.  A gateway takes an application message, reads it and interprets it.  It is used to connect two networks that uses two different protocols.  A gateway is a network point that acts as an entrance to another network.network

17. DevicePurposePresent in which OSI layer RepeatersElectrical Specifications of a signalPhysical BridgesAddressing ProtocolsData link RoutersInternetworking between compatible networks Network GatewaysTranslation services between incompatible networks All

18.  Routing is the process that a router uses to forward packets toward the destination network.  A router makes decisions based upon the destination IP address of a packet.  To make the correct decisions, routers must learn how to reach remote networks.  There exists a routing table which contain route information.

19. Two Types  Static/Non-Adaptive routing: a)Information entered manually b) when table is created it can not be updated when there is a change in internet.  Dynamic/Adaptive routing: a) constantly changing networks. b) Update all the tables in the routers automatically.

20.  A routing protocol is the communication used between routers.  A routing protocol allows routers to share information about networks and their proximity to each other.  Routers use this information to build and maintain routing tables.  Examples of routing protocols are as follows:  Routing Information Protocol (RIP)  Interior Gateway Routing Protocol (IGRP)  Enhanced Interior Gateway Routing Protocol (EIGRP)  Open Shortest Path First (OSPF)

21.  Performance Criteria: Number of hops, Cost, Delay, Throughput, etc  Decision Time: Per packet basis (Datagram) or per session (Virtual-circuit) basis  Decision Place: Each node (distributed), Central node (centralized), Originated node (source)  Network Information Source: None, Local, Adjacent node, Nodes along route, All nodes

22. Routing Protocols Intradomain Distance VectorLink State Interdomain Path Vector RIP OSPF BGP

24.  Neighboring routers periodically exchange information from their routing tables.  Routers replace routes in their own routing tables anytime that neighbors have found better routes.  These regular updates between routers communicate topology changes.  The distance vector routing algorithm is also known as the Bellman-Ford algorithm.

27.  Network is a directed graph; routers are vertices  Find “best” path between every pair of vertices  In the simplest case, best path is the shortest path

39. RIP (Routing Information Protocol) ― the oldest one Use no. of hops between nodes to determine best path Does not consider the network congestion condition Broadcast every 30 sec the routing table to neighboring routers to convey routing information RIP is limited to interpreting a maximum of 16 hops Not suitable for large network (e.g. Internet) Can create excessive network traffic due to broadcasting May take a long time to reach at the far stations.

40.  Topology and link costs known to all nodes.  All nodes have same information.  computes least cost paths from one node (source) to all other nodes gives forwarding table for that node.  After k iterations, know least cost path to k destination nodes. Analogous to a city map.

41.  In link state routing, four sets of actions are required.  1. Creation of the states of the links by each node, called the link state packet (LSP).  2. Dissemination of LSPs to every other router, called flooding, in an efficient and reliable way.  3. Formation of a shortest path tree for each node.  4. Calculation of a routing table based on the shortest path tree.

43. Strategy  send to all nodes (not just neighbours)  ‰Information about directly connected links (not entire routing table) Link State Packet (LSP)  Id of the node that created the LSP  cost of link to each directly connected neighbour.  sequence number.

44. Dijkstra algorithm  ‰Calculates the shortest path between two points on a network, using a graph made up of nodes and edges.  Algorithm divides the nodes into two sets: tentative and permanent  It chooses nodes, makes them tentative,  examines them, and if they pass the criteria, makes them permanent.

49.  Congestion - the situation in which too many packets are present in the subnet.

50.  Congestion occurs when a router receives data faster than it can send it  Insufficient bandwidth  Slow hosts  Data simultaneously arriving from multiple lines destined for the same outgoing line.  The system is not balanced  Correcting the problem at one router will probably just move the bottleneck to another router.

51.  Flow control  controls point-to-point traffic between sender and receiver  e.g., a fast host sending to a slow host  Congestion Control  controls the traffic throughout the network

52.  Another method of congestion control is to “shape” the traffic before it enters the network.  Traffic shaping controls the rate at which packets are sent.  At connection set-up time, the sender and carrier negotiate a traffic pattern (shape).  Two traffic shaping algorithms are:  Leaky Bucket  Token Bucket

53.  The Leaky Bucket Algorithm used to control rate in a network.  It is implemented as a single-server queue with constant service time.  If the bucket (buffer) overflows then packets are discarded.

55. Leaky bucket (pg.no.778)

56. Leaky bucket implementation

57. (a) A leaky bucket with water. (b) a leaky bucket with packets.

58. A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full. Note

59.  In contrast to the LB, the Token Bucket Algorithm, allows the output rate to vary, depending on the size of the burst.  In the TB algorithm, the bucket holds tokens. To transmit a packet, the host must capture and destroy one token.  Tokens are generated by a clock at the rate of one token every  t sec.  Idle hosts can capture and save up tokens (up to the max. size of the bucket) in order to send larger bursts later.

60. 60 (a) Before. (b) After. 5-34

61. Token bucket

62. 24.62 The token bucket allows bursty traffic at a regulated maximum rate. Note

63. 63 Leaky Bucket V/S Token Bucket  LB discards packets; TB does not. TB discards tokens.  With TB, a packet can only be transmitted if there are enough tokens to cover its length in bytes.  LB sends packets at an average rate. TB allows for large bursts to be sent faster by speeding up the output.  TB allows saving up tokens (permissions) to send large bursts. LB does not allow saving.