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  Networks: Access Management Class Notes # 6 Internetworking September 29, 2003 It is impossible to connect all networks into one big single network. Each network has there own features and protocols. However, there is a need to interconnect various networks so that any station on any network can communicate. Although a variety of ways have been taken to provide internetworking services, the overall requirement can be stated in general. These include: 1.   Provide a link between networks. At minimum, a physical and link control connection is needed. 2.   Provide for the routing and delivery of data between processes on different networks. 3.   Provide an accounting service that keeps track of the use to the various networks and gateways and maintains status information. 4.   The internetworking facilities must accommodate a number of differences among networks. These include: ã   Different addressing schemes ã   Different maximum packet size ã   Different timeouts ã   Error recovery ã   Status reporting ã   Routing techniques ã   User access control ã   Connection, connectionless service. Single segment networks provide good service. However, as the network grows, two main factors force the network structure to change: ã   Too many devices competing for the same physical transmission medium.  Network performance start to degrade and become sluggish. For example, too many users on the school’s network. ã   A new facility is added; the new facility’s network (i.e. LAN) must be somehow connected to the previous networks (i.e. LAN). To solve these problems, network designers break a single broadcast network into separate segments. Internetworking devices connect the individual network segments. These internetworking devices are: repeaters, hubs, switches, bridges, routers, and gateways. Each internetworking device operates at a particular layer of the OSI model. Repeaters Repeaters are physical layer devices. These devices act as a piece of wire and do not evaluate the incoming data. Also, they regenerate the incoming signal and retransmit the signal. Basically, repeaters are used to boost the signal and extend the physical length of the network segment. 1  Repeaters are typically very fast and cause very little delay in the signal. Repeaters can also connect different types of physical media. For example, repeaters can connect one network that uses twisted pair wires to another that uses coaxial cable. Repeaters cannot  be used to connect two different LAN protocols, such as Ethernet bus and token ring. Hubs A hub, also known as wiring concentrator, function as a shared bus or multi-port repeater. Hubs are also physical layer devices. Any computer that is connected to a particular hub will receive all frames transmitted by any of the computers that are connected to that hub. The token ring uses a special type of hub (known as the multi-station access unit or MAU). Each MAU connects to adjacent MAUs through their designated ports. You can also extend the distance between MAUs using a lobe repeater (token ring repeater). As the network grows, more hubs can be added to the network to increase the number of stations attached to the network. See figure one below. However, there is still the same limitation in where only one station can talk at a time. Therefore, the increase traffic will reduce the performance of the expanded network. To correct this problem, other types of internetworking devices can be used. Hub 1Station1Station 2Station 3Hub 2Station 4Station 5Station 6   Figure 1: A network consisting of two hubs. Bridge Bridges operate at the data link layer of the OSI model. The bridge is used to increase the performance of the network by isolating traffic within network segments. Bridges are losing their popularity due to switching. Bridges are composed of hardware and software. A bridge listens to all traffic and examines the NIC addresses of each incoming frame. The bridge routes the frames based on its internal table. The internal table contains port numbers and the MAC address of the station connect to that port. A bridge provides three important functions: 1.   Forwarding – If the frame’s destination address is on a different segment than its source address, then the bridge sends the frame only to the port connected to that segment. 2  2.   Filtering – If the frame’s destination address is on the same segment as its source address, then the bridge does not forward the frame to any ports. All stations on the srcinating segment have already received the frame. 3.   Learning – the bridge automatically builds and maintains its own bridge table, by listening to incoming frames and noting their source address. If a frame is addressed to a destination station not yet recorded in the bridge table, then the  bridge broadcasts the frame to all ports. The bridge table is used to determine where the bridge is to forward the frame. Table one is an example of a typical bridge table. For example, when an incoming frame arrives with a destination address of F81.667.AB1.DDF, that frame is sent out on port 3of the  bridge. Table one: Typically Bridge table Frame address Port number 2F3.48E.223.3FF 1 445.EEF.231.667 2 981.667.AB1.DDF 3 FFA.BC2.C22.D88 4 The bridge looks at the frame header and has no regard for the contents of the frame. A  bridge can link LANs that use different upper layer protocols. The bridge is completely transparent to the upper layers (i.e. 3 through 7) of the OSI model, see figure two. Transport Layer Network Layer Data Link Layer Physical Layer  Application Layer Presentation Layer Data Link Layer  Source Data Link Layer Physical Layer Transport Layer Network Layer Data Link Layer Physical Layer  Application Layer Presentation Layer Data Link Layer  DestinationBridge   Figure 2: Bridge and OSI model Switches Switches, also operates at the data link layer, is used to increase performance in LANs. The switch consists of high-speed ports connecting either LAN segments or individual stations on a port-by-port basis. Also, like a bridge, a switch isolates traffic to their segment. The switch evaluates the destination MAC address in each frame, and switches individual frames to the correct port. When a switch is first powered on, it broadcasts individual frames. Over time, the switch  builds a table that associates frame address with port numbers by watching incoming frames for new source addresses and adding those addresses to the switch memory table. 3  The switch performs most of its functions in hardware instead of software and therefore, the performance is greater than the bridge. Also, the switch dedicates the entire LAN media bandwidth to each port-to-port frame transmission. Where as, a bridge shares the LAN bandwidth among all of its ports. Therefore, the switch effectively multiplies the amount of network bandwidth. Station1Station 2Station 3Station 4Station 5Station 6Switch Segment 1Segment 2Segment 3 Station7 Segment 4   Figure 3: Switching network When a frame is sent from station 1 to station 5, the switch connects segment 1 to segment 4. Simultaneously, station 7 can send data to station 3; the switch will connect segment 2 and segment 3. Effectively, this doubles the bandwidth. When a switch receives a frame destined for station with a destination address that is not in the switch’s memory, then the switch will send the broadcast the frame on all ports. Routers A router operates at the network layer of the OSI model. The router forwards the packets  based on each of the packet’s network address. Routers are protocol dependent because they can interconnect networks that have the same network communication architecture and possible different lower level architectures. Routers are typically more complex and software intensive than repeaters, bridges, and switches. Routers segment a network into separate logical sub-networks, and provide security, control, and redundancy between individual sub-networks. Each port on a router connects to a different network or subnet. Routers provide physical and logical separation of networks. The router evaluates the destination address of the packet, which indicates where the destination node is on the network. A network layer address (packet address) identifies both the destination network and the destination station. If the packet address indicates the destination station is in the same network as the source station, the router will isolate traffic within that network or subnet. If the packet address indicates the destination station is not in the 4
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