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COMMUNICATIONS NETWORK

Network Devices
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Functions of network devices
• Separating (connecting) networks or
expanding network
• e.g. repeaters, hubs, bridges, routers,
brouters, switches, gateways

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switches or gateways 3 . brouters. routers.Expanding Network • Networks cannot be made larger by simply adding new computers and more cables • Less efficient !! • Can install components to • segment (divide) large LAN to form smaller LANs • connect LANs • Required components • Repeaters. bridges.

Repeaters And Hubs • Repeaters or hubs work at the OSI physical layer to regenerate the network’s signal and resend them to other segments • Primitive hub can be viewed as a multiport repeater • It regenerates data and broadcasts them to all ports 4 .

Limitation And Feature • Cannot link unlike segments • Cannot join segments with different access methods (e. CSMA/CD and token passing) • Do not isolate and filter packets • Can connect different types of media • The most economic way of expanding networks 5 .g.

Bridges • Has one input and one output • Used to isolate network traffic and computers • Has the intelligent to examine incoming packet source and destination addresses • But cannot interpret higherlevel information • Hence cannot filter packet according to its protocol 6 .

of address • If destination address is in the same segment as the source address.How Bridges Work • Bridges work at the Media Access Control Sublayer of the OSI model • Routing table is built to record the segment no. stop transmit • Otherwise. forward to 7 the other segment .

Creating a Switching Table • Based on the addresses of the sending computers • New addresses are added if they are not in the table Add02 S 02 D 01 Switching Table Seg 1 Seg 2 01 Add01 Stop 8 Add03 S 01 D 02 02 .

Remote Bridges • Bridges are often used in large networks that have widely dispersed segments • Remote bridges can be used to connect remote segments via data-grade telephone line 9 .

Differences Between Bridges and Repeaters Repeaters Bridges OSI layer Physical layer Data link layer Data regeneration Regenerate data at the signal level Regenerate data at the packet level Yes Reduce network No traffic 10 .

Switches • Switches operate at the Data Link layer (layer 2) of the OSI model • Can interpret address information • Switches resemble bridges and can be considered as multiport bridges • By having multiports. can better use limited bandwidth and prove more cost-effective than bridge 11 .

3Mbps 10Mbps 10Mbps 10Mbps .3Mbps Switch 3.3Mbps 10Mbps 3.Switches • Switches divide a network into several isolated channels • Packets sending from 1 channel will not go to another if not specify • Each channel has its own capacity and need not be shared with other channels Hub 3.

but not others 13 .g. e.Advantages of Switches • Switches divide a network into several isolated channels (or collision domains) • Reduce the possibility of collision • Collision only occurs when two devices try to get access to one channel • Can be solved by buffering one of them for later access • Each channel has its own network capacity • Suitable for real-time applications. video conferencing • Since isolated. hence secure • Data will only go to the destination.

not the case as in Ethernet • Some higher level protocols do not detect error • E. UDP • Those data packets are continuously pumped to the switch and introduce more problems 14 .g.Limitations of Switches • Although contains buffers to accommodate bursts of traffic. can become overwhelmed by heavy traffic • Device cannot detect collision when buffer full • CSMA/CD scheme will not work since the data channels are isolated.

1500 Bytes 4 Bytes Read the first 14 bytes of each packet. Add 1 Byte 2/6 Bytes Sour. Add Length 2/6 Bytes 2 Bytes Data FCS 46 . then transmit Much faster Cannot detect corrupt packets Can propagate the corrupt packets to the network Best suited to small workgroups .Cut Through Mode Preamble 7 Bytes • • • • • Des.Method of Switching .

Store and Forward Mode • Read the whole packet before transmit • Slower than the cut-through mode • More accurate since corrupt packets can be detected using the FCS • More suit to large LAN since they will not propagate error packets • Facilitate data transfer between DB segments of different speed 16 .Method of Switching .

Using Switches to Create VLANs • Switches can logically group together some ports to form a virtual local area network (VLAN) SW1 SW2 SW3 17 VLAN1 VLAN2 Hub Hub Switches can be configured to communicate only within the devices in the group Hub .

Routers • Layer 2 Switches cannot take advantage of multiple paths • 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 18 .

Data Link layer source and destination addresses are stripped off and then recreated • Enables a router to route a packet from a TCP/IP Ethernet network to a TCP/IP token ring network • Only packets with known network addresses will be passed hence reduce traffic • Routers can listen to a network and identify its busiest part • Will select the most cost effective path for transmitting packets .How Routers Work • As packets are passed from routers to routers.

How Routing Table is formed • Routing table is formed based on communications between routers using “Routing Protocols” • Routing Protocols  Routable Protocol • Routing Protocols Routers communicate within themselves collect data about current network status and contribute to selection of the best path .

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

can coexist with RIP In general case.Routing Protocol Example .OSPF for IP • • • • • • OSPF . best path refers to the shortest path In case of traffic congestion. can go a longer path Each router maintains a database of other router’s links If link failure notice is received.Open Shortest Path First Make up the limitations of RIP . router can rapidly compute an alternate path • Require more memory and CPU power .

Automatic discovery of new routes Always use the same Can select the best route route More secure Need manual configuration to improve security .Static and Dynamic Routers Static Routers Dynamic Routers Manual configuration Manual configuration of the first of routes route.

Distinguishing Between Bridges and Routers • Bridges forward everything they don’t recognize • Routers select the best path • Routers are layer 3 devices which recognize network address • Bridges are layer 2 devices which look at the MAC sublayer node address .

however.Layer-3 Switches • Layer-3 switches operate in both layer 2 (data link layer) and 3 (network layer) • Can perform both MAC switching and IP routing • A combination of switch and router but much faster and easier to configure than router Why Layer-3 switches? • Traffic of LAN is no longer local • Speed of LAN is much faster • Need a much faster router. very expensive .

can divide a network into some logical channels • Routers interconnect networks and provide filtering functions. but can understand the node addresses • Switches can be considered as multiport bridges.Summary • Repeaters are the least expensive way to expand a network. but they are limited to connecting two segments • Bridges function similar to repeaters. They can determine the best route .

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