Computer Networks. Definition of LAN. Connection of Network. Key Points of LAN. Lecture 06 Connecting Networks

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1 Computer Networks Lecture 06 Connecting Networks Kuang-hua Chen Department of Library and Information Science National Taiwan University Local Area Networks (LAN) 5 kilometer IEEE Ethernet IEEE Token-Bus IEEE Token-Ring Metropolitan Area Networks (MAN) IEEE DQDB Wide Area Networks (WAN) Language & Information Processing System, LIS, NTU 6-2 Classification of Interconnected Processors Interprocessor Processors Example Distance located in same 0.1m Circuit board Data flow machine 1m System Multicomputer 10m Room 100m Building Local area network 1km Campus 10km City Metropolitan area network 100km Country Wide are network 1,000km Continent 10,000km planet The Internet Definition of LAN A LAN is a data communication system allowing a number of independent devices to communicate directly with each other within a moderately sized geographic area over a physical communications channel of moderate data rates. ~ by IEEE Language & Information Processing System, LIS, NTU 6-3 Language & Information Processing System, LIS, NTU 6-4 Key Points of LAN Independent devices allow different systems connected together Communicate directly any two computers in LAN communicate directly Moderately sized geographic area 5km Moderate data rates 1Mbps to 10Mbps, now 100Mbps Connection of Network Direct connection Language & Information Processing System, LIS, NTU 6-5 Language & Information Processing System, LIS, NTU 6-6 1

2 Connection of Network Connection of Network Connect to the WAN 1 2 WAN 3 Two LAN connected via WAN WAN Language & Information Processing System, LIS, NTU 6-7 Language & Information Processing System, LIS, NTU 6-8 Network Interconnection LAN Connectivity Devices Eventually your LAN may reach its limit on distance or number of nodes that you can have on a segment. When this happens, you may turn to hardware devices such as bridges or repeaters to extend your network and allow for expansion. LAN Connectivity Devices include: Repeater Bridges Ethernet Hubs Language & Information Processing System, LIS, NTU 6-9 Language & Information Processing System, LIS, NTU 6-10 Names for Connecting two Networks Layer 1: Repeaters copy individual bits between cable segments Layer 2: Bridges store and forward data link frames between LANs Layer 3: Multiprotocol routers forward packets between dissimilar networks Layer 4: Transport gateways connect byte streams in the transport layer Above 4: Application gateways allow internetworking above layer 4 Internetworking Communication Protocol Physical Data Link Layer Network Layer MAC LLC Layer Repeater same same same same Bridge different different same same Router different different different same Gateway different different different different Language & Information Processing System, LIS, NTU 6-11 Language & Information Processing System, LIS, NTU

3 Repeaters Repeaters (Continued) One of the easiest devices you can use to extend the distance of a network Repeaters work at the Physical layer to regenerate the electrical signal on the network media. Repeaters simply understand the electrical signal of 1 s and 0 s. Repeaters are normally two-port boxes that connect two segments. Usually the repeater does more than just amplify the electrical signal. The signal is read as the binary 1 s and 0 s and retransmitted as the 1 s and 0 s so the noise in the signal can be cleaned out. Repeaters send and receive data at the speed of the network, but signal regeneration does take a small amount of time. Language & Information Processing System, LIS, NTU 6-13 Language & Information Processing System, LIS, NTU 6-14 Repeaters (Continued) Rule Repeaters do not have a lot of built-in intelligence, so they can t connect dissimilar network types. One thing that repeaters can do, however, is convert the type of cable being used on a similar network. Most network types have a limit to the number of repeaters that can be used to connect segments. In Ethernet, this rule is called the rule. Language & Information Processing System, LIS, NTU 6-15 Language & Information Processing System, LIS, NTU 6-16 Repeaters (Continued) Advantages of repeaters: Repeaters easily extend the length of a network. They require no processing overhead, so very little performance degradation occurs. You can connect segments from the same network type that use different types of cable. Disadvantages of repeaters: Repeaters cannot be used to connect segments of different network types. They cannot be used to segment traffic on a network to reduce congestion. Many types of networks have limits on the number of repeaters that can be used at once. Language & Information Processing System, LIS, NTU 6-17 Bridges Repeaters do not do any type of filtering of traffic that they pass, but bridges do. Bridges work at the Data Link layer of the OSI model and, like a repeater, attach two different network segments and pass data. What a bridge does that a repeater does not is filter the data on whether it needs to be passed. Place the bridge for maximum effectiveness. The optimum situation would be to block about seventy percent of the traffic from coming to the segmented LAN. Language & Information Processing System, LIS, NTU

4 Bridges (Continued) Bridges (Continued) Advantages of bridges: Bridges extend network segments by connecting them together to make one logical network. They segment traffic between networks by filtering data if it does not need to pass. Like repeaters, they can connect similar network types with different cabling. Special translational bridges can connect different network types together. Language & Information Processing System, LIS, NTU 6-19 Language & Information Processing System, LIS, NTU 6-20 Bridges (Continued) Disadvantages of bridges: Bridges process information about the data they receive, which can slow performance. They cost more than repeaters due to extra intelligence. Types of bridges transparent bridge Ethernet source-route bridge is used in Token-Ring translational bridge is used to connect dissimilar network types. Transparent bridges Transparent bridges use hardware network card addresses to know which data to pass and which should be filtered. Bridges use unique address assigned to NIC at manufacture to decide which frames are passed and which are not. MAC addresses are stored in a table, one for each port. When data is received, the destination address is checked and compared against this table. Language & Information Processing System, LIS, NTU 6-21 Language & Information Processing System, LIS, NTU 6-22 Broadcast Storm Broadcast Storm (Continued) The broadcast is sent out, and the two bridges both receive the same message at the same time. Each bridge then passes the broadcast message to the other segment. It is not passed once, but once by each bridge which creates two broadcast messages. Now Bridge A receives Bridge B s broadcast, and Bridge B receives Bridge A s broadcast. They each resend the broadcast they receive repeatedly. Language & Information Processing System, LIS, NTU 6-23 Language & Information Processing System, LIS, NTU

5 Spanning Tree Protocol A protocol has been developed to keep network nightmares like this from happening. It is called the Spanning Tree Protocol. This protocol checks for network loops each time a bridge is initialized and closes these loops. No more than one bridge may connect two network segments together. At a set interval the bridges will send out a packet checking to make sure the one working bridge is still active. If not, one of the disabled bridges takes over and connects the two segments. Source-route bridges Token Ring networks use a different type of bridge called a source-route bridge. Source-route bridges use information in the token ring frame to determine whether to pass the data or not. Before a connection is made to a remote computer, the source computer sends out an explorer packet. Language & Information Processing System, LIS, NTU 6-25 Language & Information Processing System, LIS, NTU 6-26 Source-route bridges (Continued) When the destination computer receives an explorer frame, it flips the list of bridge IDs in the packet around so the path back is reversed and sends the explorer frame back. The frame then takes the exact route back that it took to get to the destination. Then the bridge looks for the one with the shortest bridge ID list. This is the list that it attaches to the rest of the packets destined for that computer Translational bridges Translational bridge allows you to connect dissimilar networks together. Translational bridges have ports for the two different network types. Translational bridges handle the conversion of the frames from one type to another and take into account the media access method. Language & Information Processing System, LIS, NTU 6-27 Language & Information Processing System, LIS, NTU 6-28 Ethernet Hubs Ethernet hubs are basically just multiport repeaters for UTP cable. Hubs just repeat the signal given to them, and no intelligence is built into the system. You can only have up to four hubs between any two points on a network to follow the rule. There are two types: passive and active. There are some hubs that have the intelligence of a bridge built-in, called switches. Ethernet Hubs (Continued) Advantages of hubs: Hubs need almost no configuration. Active hubs can extend maximum network media distance. No processing is done at the hub to slow down performance. Disadvantages of hubs: Passive hubs can greatly limit maximum media distance. Hubs have no intelligence to filter traffic, so all data is sent out all ports whether it is needed or not. Since hubs can act as repeaters, networks using them must follow the same rules as repeaters. Language & Information Processing System, LIS, NTU 6-29 Language & Information Processing System, LIS, NTU

6 Passive hubs and Active hubs Passive hubs provide no signal regeneration. Passive hubs are simply cables connected together so that a signal is broken out to other nodes without regeneration. Active hubs act as repeaters and regenerate the data signal to all ports. Active hubs have no real intelligence to tell whether the signal needs to go to all ports, it is blindly repeated. Switches Switches are multiport bridges. Switches filter traffic between the ports on the switch by using the MAC address of computers transmitting through them. Switches can be used when greater performance is needed or when collisions need to be reduced. Switches are the key to a large and fast Ethernet network. Language & Information Processing System, LIS, NTU 6-31 Language & Information Processing System, LIS, NTU 6-32 Internetworking Devices Several pieces of hardware that enable you to connect LANs include Modems Multiplexers Routers Brouters CSU/DSUs Gateways Internetworking Devices And OSI model Language & Information Processing System, LIS, NTU 6-33 Language & Information Processing System, LIS, NTU 6-34 Modems Modems are small devices that connect networks, though at a slow speed, over normal telephone lines. Modems handle the conversion of signals between computers and telephone lines. Modems convert signals through a process called modulation and demodulation (which is where the name comes from, MODulation/ DEModulation). Modulation is converting the digital computer signals to analog telephone signals. Demodulation is the opposite conversion, analog to digital. Modems (Continued) Most analog modems operate at speeds from 14.4Kbps up to a theoretical limit of 56Kbps, and have the capability of compression. Common modem protocols as defined by ITU: V.32: 9600bps Communications speed V.32bis: 14.4Kbps Communications speed V.34: 28.8Kbps Communications speed, 33.6Kbps V.90: 56Kbps Communications speed V.42: Modem error control V.42bis: Compression protocol V.17: Faxing capability Language & Information Processing System, LIS, NTU 6-35 Language & Information Processing System, LIS, NTU

7 Modems (Continued) Advantages of modems: Inexpensive hardware and telephone lines Easy to set up and maintain Mature standards and multiple vendors Disadvantages of modems: Very slow performance Modems (Continued) Language & Information Processing System, LIS, NTU 6-37 Language & Information Processing System, LIS, NTU 6-38 Multiplexers Multiplexers (or MUXes) are what you use to send multiple signals across one transmission media. Multiplexing (or MUXing) takes the different signals and combines them to form one single signal. Multiplexers have no real intelligence in them as to what data to send across the link they are connecting. They are popular with central networks where remote terminals connect to a large mainframe, since no real intelligence is needed. Routers Routers are used to connect complicated networks with many segments. Routers do more than just filter traffic; they make intelligent decisions on the path of the data. Routers can use either MAC addresses or administratively assigned logical addresses (such as IP addresses) to handle data routing. This allows you to segment your network into what are called subnets. A subnet is a network connected to another network via a router. Routers operate at the Network layer of the OSI model. Language & Information Processing System, LIS, NTU 6-39 Language & Information Processing System, LIS, NTU 6-40 Routers (Continued) Bridging routers, or brouters, offer the best of both worlds between bridges and routers. Advantages of routers: They use the highest level of intelligence to route data accordingly. Routers can also act as a bridge to handle nonroutable protocols such as NetBEUI. Disadvantages of routers: Higher level of intelligence takes more processing time, which can affect performance. Routers are very complicated, which makes installation and maintenance difficult. Routers (Continued) Language & Information Processing System, LIS, NTU 6-41 Language & Information Processing System, LIS, NTU

8 The Architecture of Routers Higher Layer Protocol Network Layer Router Network Layer Higher Layer Protocol Network Layer Data Link Layer 1 DLL1 DLL 2 Data Link Layer 2 Physical Layer 1 PL1 PL1 Physical Layer 2 LAN 1 LAN 2 Multiprotocol Routers Routers take incoming packets from one line and forward them on another The line may belong to different networks and use different protocols Each router has a routing table used to resolve the selection of route Fixed routing table Dynamic routing table Language & Information Processing System, LIS, NTU 6-43 Language & Information Processing System, LIS, NTU 6-44 CSU/DSU CSU/DSU is used to connect to a digital line such as a 56Kbps or T1 The CSU portion of the CSU/DSU handles line management and loop-back testing. The DSU portion handles the interface to your hardware, as well as the data formatting into frames to be sent out. Most CSU/DSUs are leased from the telephone company you are getting service from. Gateways Gateways can operate at all seven layers of the OSI model. Gateways function is to do any necessary conversion of protocols between networks. Gateways are customized and designed to perform a specific function and are used on a case-by-case basis. Gateways may do anything from converting protocols to converting application data. Language & Information Processing System, LIS, NTU 6-45 Language & Information Processing System, LIS, NTU 6-46 Gateway Gateway between WANs Gateway make a connection between two totally different networks transform the packet format transform the address format transform the protocol Language & Information Processing System, LIS, NTU 6-47 Language & Information Processing System, LIS, NTU

9 Backbone Network All networks are connected the backbone The backbone network has high bandwidth and capability to propagating long-distance signal Backbone usually is composed of fiber optical or microwave FDDI Higher efficiency Higher reliability Better flow control LAN Hardware Language & Information Processing System, LIS, NTU 6-49 Language & Information Processing System, LIS, NTU 6-50 Functionality of Internetworking Devices Device # of Physical Networks # of Logical Networks Repeater 1 1 Bridge N 1 Router N N Gateway N N Language & Information Processing System, LIS, NTU