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2 Repeaters, Bridges, Routers and Gateways LAN Interconnection

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2 Repeaters, Bridges, Routers and Gateways LAN Interconnection LAN Interconnection 2 Repeaters, Bridges, Routers and Gateways g LAN interconnection refers to the ability to inter- network LANs, MANs, and WANs, through relays. b A layer n relay is a device that interconnects two systems not directly connected to each other (OSI Ref. Model) ` Layer n relay shares a common layer n with other systems, but does not participate in layer n+1 protocol g Terminology is non uniformly used in the litera- ture Prof. A. Bruce McDonald Spring’01 LAN Interconnection 3 Interconnecting LANs g There are four generic devices for LAN intercon- nection b Repeater : Physical layer relay b Bridge : Data link layer relay b Router : Network layer relay b Gateway : Any relay at higher layer than net- work layer g One final category combines bridges and routers functionalities b Brouters Prof. A. Bruce McDonald Spring’01 LAN Interconnection 4 Relays, Bridges, Routers and Gateways End System End System Relays Network Relays Bridge Repeater Physical Media Prof. A. Bruce McDonald Spring’01 LAN Interconnection 5 Shared-Bandwidth LANs Thick Ethernet Trunk Cable Maximum Trunk Link, 500 Meters Transceiver Terminating Resistor at each end Transceiver Cable Bridge Prof. A. Bruce McDonald Spring’01 LAN Interconnection 6 Dedicated-Bandwidth LANs Structured Wiring Work Location Wall Plate Wiring Closest LAN Hub Patch Panel Horizental Distribution V (UTP Cat 5) e r t i c B a u l i l d i n g UTP B a 100 m distance D c i k VD s b µ t o Fiber (62.5/125 m) r n 2 Km distance i e b u t i o n Prof. A. Bruce McDonald Spring’01 LAN Interconnection 7 Repeaters g Repeaters are used to interconnect homogene- ous networks, and to extend their range of opera- tions g Repeaters pass signals in both directions g Their main functions include b Amplifying signals b Regenerating signal b Signals are not changed except for ‘‘cleaning’’ g Repeaters have no effect on protocols Prof. A. Bruce McDonald Spring’01 LAN Interconnection 8 Standalone Repeaters g Repeaters forward packets to all ports b Shared Network g All segments interconnected by means of repeaters are in one Electrical Collision Domain Prof. A. Bruce McDonald Spring’01 LAN Interconnection 9 Electrical Collision Domain g All collisions occurring in a single collision domain must be detected by the nodes causing the collisions before they terminate their transmissions b This has to hold for all network nodes, includ- ing the ones at the farthest ends of the net- work g As a result, the network diameter is directly related to the minimum packet size, which is 512 bits Prof. A. Bruce McDonald Spring’01 LAN Interconnection 10 Electrical Collision Domain DTE Delay Cable Delay Repeater Delay Cable Delay DTE Delay g To handle collision appropriately, the following rule must hold true 2 × ( Total repeater delays ) + 2 × ( Total cable delays ) + 2 × ( Total DTE delays ) < 512 bit times Prof. A. Bruce McDonald Spring’01 LAN Interconnection 11 Bridges For Shared LANs g Bridges allow for the simplest and most common form of internetworking, interconnection of homogeneous networks g IEEE 802 committees developed standards for bridges to interconnect different types of LANs b Rather a complex task Prof. A. Bruce McDonald Spring’01 LAN Interconnection 12 Typical Bridge Configuration Host Host LAN Bridge Bridge LAN Host Host Bridge-to-Bridge MAC Protocol Protocol Prof. A. Bruce McDonald Spring’01 LAN Interconnection 13 Why Bridges? g Several reasons for the use of bridges to connect LANs b Performance, by building ‘‘firewalls’’, b Reliability, network can be partitioned into self contained units b Security, control and monitor traffic Prof. A. Bruce McDonald Spring’01 LAN Interconnection 14 Bridges Characteristics g Bridges operate at the data link layer b Usually the MAC layer of LANs ` The basic bridge, with no special features, does not look any further than the address fields Prof. A. Bruce McDonald Spring’01 LAN Interconnection 15 Bridges g Bridges do not interfere with higher level proto- cols b Transparently pass traffic between upper- layer running different protocols (XNS, TCP/IP, and OSI) b Only devices running the same protocol can communicate with one another Prof. A. Bruce McDonald Spring’01 LAN Interconnection 16 Bridges Main Advantages g Bridges can connect LANs that use different phy- sical media such as coax cable, fiber optics, or twisted pairs. g Bridges can connect LANs that use different media access control protocols media such as CSMA/CD and Token Ring. b The possible difference in maximum frame size must be handled by upper layers protocol ` Passing through the bridge, the packet does not violate the maximum length on any network segment Prof. A. Bruce McDonald Spring’01 LAN Interconnection 17 Bridges Basic Functionalities g A Bridge, between network 1 and 2, performs the following functions b Reads all frames on network 1, and accepts those addressed to network 2 b Retransmits accepted frames onto network 2, using the MAC protocol for network 2 b Performs equivalent functions on network 2 to network 1 traffic Prof. A. Bruce McDonald Spring’01 LAN Interconnection 18 Bridges Basic Operations Host A Host B Layers 4-7 Network Layer PKT PKT Bridge LLC PKT PKT PKT MAC 802.3PKT 802.3PKT 802.4PKT 802.4PKT Physical 802.3PKT 802.3PKT 802.4PKT 802.4PKT 802.3PKT 802.4PKT CSMA/CD LAN Token Bus LAN Prof. A. Bruce McDonald Spring’01 LAN Interconnection 19 Bridges Over Point-To-Point Links Station Station User User ( 1 ) ( 1 ) LLC LLC ( 2) ( 2) Bridge Bridge MAC MAC ( 3) MAC Link Link MAC ( 3) PHY ()3 ()4 (4) ( 3 ) PHY PHY PHY PHY PHY LAN LAN Packet Data Unit 1 : User Data Packet Data Unit 2 : LLC-H User Data Packet Data Unit 3: MAC-H LLC-H User Data MAC-T Packet Data Unit 4 : Link-HMAC-H LLC-H User Data MAC-T Link-T Prof. A. Bruce McDonald Spring’01 LAN Interconnection 21 Routing with Bridges g Most basic form of bridges, ‘‘No Frills’’ bridges perform the following operations : b Promiscuously, listen to every packet transmitted b Store each packet until it can be transmitted b When ready transmit the packet on all adja- cent LANs, except the one on which it was received b Preferably, operates transparently not to cause protocols on stations to fail Prof. A. Bruce McDonald Spring’01 LAN Interconnection 22 Basic Bridges g ‘‘No Frills’’ bridges extend the capabilities of the LANs b Limits collisions in CSMA/CD b Extend the maximum number of stations in 802.5 Prof. A. Bruce McDonald Spring’01 LAN Interconnection 23 Basic Bridges g ‘‘No Frills’’ may waste total LAN bandwidth b Manual insertion of addresses in the database ` May involve difficult management g If it were for them to know which stations were on which LANs, bridges may become more efficient b Bridges must be equipped with greater rout- ing capabilities Prof. A. Bruce McDonald Spring’01 LAN Interconnection 24 Routing with Bridges g Three basic routing strategies : b Fixed routing ` Only suitable for small LANs ` Mostly proprietary protocols b Spanning tree, developed by IEEE 802.1 ` Intended for use in interconnecting LANs with similar and dissimilar MAC Standards, (802.3, 802.4, 802.5, FDDI) ` Routing technique is used is referred to as Spanning Tree b Source Routing, developed by IEEE 802.5 ` Intended for use in interconnecting Token Rings Prof. A. Bruce McDonald Spring’01 LAN Interconnection 25 Interconnection of Different Types of LANs To CSMA/CD Token Bus Token Ring From 802.3 802.4 802.5 802.3 1,4 1,2,4,8 802.4 1,5,8,9,10 9 1,2,3,8,9,10 802.5 1,2,5,6,7,10 1,2,3,6,7 6,7 1 Reformat the frame, compute new checksum 2 Reverse the bit order 3 Copy priority, even if not meaningful 4 Generate fictitious priority 5 Discard priority 6 Drain the ring 7 Fake A and C bits 8 Deal with congestion 9 Deal with temporary token handoff 10 Deal with long frames for receiving LAN Prof. A. Bruce McDonald Spring’01 LAN Interconnection 26 Bridging with Different LANs g Two techniques are widely used : b Encapsulation, b Translation (Mapping) Prof. A. Bruce McDonald Spring’01 LAN Interconnection 27 Encapsulation g The frame of the in-bound LAN is included in the information field of the frame of the out-bound LAN g The receiving bridge performs de-encapsulation Prof. A. Bruce McDonald Spring’01 LAN Interconnection 28 Encapsulation g Major limitations : b No standards for encapsulation are defined b Requires that both bridges subscribe to the same encapsulation proprietary scheme ` Limits interoperability ` Limits transparency Prof. A. Bruce McDonald Spring’01 LAN Interconnection 29 Translation Concept g Bridges map the content of incoming incoming frame into the outbound frame that conforms to the outbound LAN g Translation may require higher processing rate than encapsulation Prof. A. Bruce McDonald Spring’01 LAN Interconnection 30 Translation g To achieve translation, bridge internal sublayer performs : b Frame disassembly at the in-bound LAN b Altering of certain field of the in-bound LAN frame b Reconstruction of the out-bound LAN frame g These services are defined within the bridge by a set of primitives and parameters Prof. A. Bruce McDonald Spring’01 LAN Interconnection 31 Bridge Architecture Service Relationship Higher Layer Entities (Bridge Protocol Entity, Bridge Management,...) LLC LLC MAC Relay Entity Media Access Method Independent MAC Services MAC Services ISS ISS MAC Entity MAC Entity Media Access Media Access Method Dependent Method Dependent ISS : Internal Sublayer Service Prof.
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