1. © 2001 by Prentice Hall14-1 Local Area Networks, 3rd Edition David A. Stamper Part 5: Connecting to Other Systems and Networks Chapter 14 Making Network Connections

2. © 2001 by Prentice Hall 14-2 Chapter Preview LAN-to-LAN connections Repeaters, bridges, routers, and gateways Switches and virtual LANs Remote access Common carrier services In this chapter you will study:

3. © 2001 by Prentice Hall 14-3 LAN-to-LAN Connections Companies that have LANs in separate geographical locations or LANs that cover distances greater than the maximum medium distance allowed, or companies with more nodes that can be accommodated by one LAN, must segment their network into two or more LANs. Having separate LANs allows a company to split functions and gives an additional level of security. LANs may also be connected in order to consolidate independent LANs that may have been formed in an ad-hoc manner. Responsiveness of the system can be maintained while the number of users increases by adding more resources to an existing LAN—memory, disks, another server, etc. —or by splitting the LAN into two or more smaller LANs.

4. © 2001 by Prentice Hall 14-4 Repeaters Every LAN has a distance restriction. IEEE standards specify a maximum segment length of 500 meters. If you want to span longer distances, you can use a repeater to connect two segments. As signals travel along the medium, they lose strength through attenuation. Weak signals can cause transmission errors. A repeater accepts a signal, regenerates it, and passes it along at full strength. A repeater does not separate one segment of the network from another.

5. © 2001 by Prentice Hall 14-5 A Repeater, Bridge, and Router and the OSI Reference Model Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Processor 1 Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Processor 2 Router Bridge Repeater

6. © 2001 by Prentice Hall 14-6 Bridges Early bridges were used to connect two networks, each of which used the same MAC protocol. Today, bridges also connect LANs having different MAC protocols. This device may be called a ‘brouter’ or multiprotocol bridge. Most bridges being sold today are called learning bridges, or transparent bridges. A learning bridge builds its routing table from messages it receives and does not need to be loaded with a predefined routing table.

7. © 2001 by Prentice Hall 14-7 Basic Bridge Functions 1. Accept packet from LAN A. 2. Examine address of packet. 3. If packet address is a LAN A address, allow the packet to continue on LAN A. 4. If packet address is a LAN B address, transmit the packet onto the LAN B medium. 5. Do the equivalent for LAN B packets. Media conversion Learning Remote connection Signal conversion Speed conversion Packet statistics Token ring to ethernet conversion Packet Routing Function Additional Functions

8. © 2001 by Prentice Hall 14-8 Token Rings Connected by a Bridge Bridge LAN A Token-Ring Network LAN B Token-Ring Network

9. © 2001 by Prentice Hall 14-9 Spanning Tree Algorithms Spanning tree algorithms, in which bridges exchange routing information with each other, can be used on any type of LAN. The advantages of the spanning tree algorithm are that it is MAC-layer-independent, bridges can learn the topology of the network without manual intervention, and paths can change if an existing path becomes inoperable or if a better path is introduced. The algorithm overhead is the size of the routing table for networks with many communicating nodes, and the extra network traffic resulting from status messages and flooding.

10. © 2001 by Prentice Hall 14-10 Source Routing Source routing is an IEEE standard used as a routing algorithm for token-passing networks. The advantage of the source routing algorithm is that bridges are not responsible for maintaining large routing tables for extensive networks. Each node is responsible for maintaining routing information only for the nodes with which it communicates. The disadvantages are the overhead of sending numerous packets during discovery and the extra routing data that must be appended to each message.

11. © 2001 by Prentice Hall 14-11 Remote Bridge Connection Alternatives RS-232 serial lines Synchronous transmission at 56 Kbps or 64 Kbps Fractional T-1 at multiples of 64 Kbps Integrated services digital network (ISDN) Digital subscriber lines (DSL) RS-422 serial lines at 19.2 Kbps to 2 Mbps T-1 Line at 1.5 Mbps X.25 packet-switching network Frame relay

12. © 2001 by Prentice Hall 14-12 Routers Routers operate at the network layer of the OSI Reference Model. The network layer is responsible for packet routing and for collecting accounting information. Some networks use a static routing algorithm, meaning that packet routing between two nodes never changes. In a CSMA/CD bus LAN, a packet is broadcast to every node; in a token ring, a packet is transmitted from one node to the next node in the ring. A Novell network uses a protocol called sequenced packet exchange/internetwork packet exchange (SPX/IPX) to transfer packets between nodes. SPX operates at the transport layer and IPX at the network layer. Another protocol used by many networks is the transmission control protocol/internet protocol (TCP/IP). TCP/IP is the protocol used on the Internet.

13. © 2001 by Prentice Hall 14-13 Gateways The interface between two dissimilar networks is called a gateway. A gateway is basically a protocol converter. It reconciles the differences between the networks it connects. The components of a gateway are the network interfaces and the logic that carries out the conversion necessary when moving messages between networks. The conversion must change the header and trailer of the packet to make it consistent with the protocol of the network or data link to which the message is being transferred.

14. © 2001 by Prentice Hall 14-14 Ways to Increase LAN Media Throughput Faster hardware Higher network speeds Lower bandwidth demands LAN segmentation Full duplex Switching hubs

15. © 2001 by Prentice Hall 14-15 LAN Switches LAN switches work similarly to the way in which a telephone switch works. In the idle state, you are not connected to anything except the telephone switch and cannot communicate with anyone until a circuit is set up. When you dial someone’s number, the complete transmission capacity of the telephone network is not dedicated to your call; instead, a connection is made between your telephone and the telephone of the person you are calling and a single circuit is used, leaving other circuits available to other subscribers. LAN switches, also called switching hubs, look much like standard wiring hubs. The switching hub examines the data link header of the packet and obtains the destination address. The switch then establishes a dedicated connection between the sender’s port and the recipient’s port, and the two communicate.

16. © 2001 by Prentice Hall 14-16 A LAN Switch Common Server Switching Hub 10/100 Mbps Conventional Hub-10/100 Mbps To Workstations or Segment Servers To Workstations or Segment Servers To/From Other Hubs

17. © 2001 by Prentice Hall 14-17 Some Benefits of Switching Hubs Efficient network segmentation to balance traffic. Good price/performance. Availability of shared and dedicated bandwidth. Support of new technologies such as asynchronous transfer mode and other network protocols. Preservation of investment in cabling and LAN adapters. Devices can communicate at NIC speeds as long as necessary. Provide bridging and routing as well as integration with faster technologies, such as FDDI, 100 Mbps ethernet. Multiple connections can be established concurrently. Ability to define virtual LANs (VLANs)

18. © 2001 by Prentice Hall 14-18 Virtual LANs The benefit of VLANs is the ability to collect existing computers into a VLAN and to move workstations while maintaining their VLAN connection. Because VLANs are identified by addresses or switch ports and not by physical connections, LAN nodes can be moved from one location to another and remain on the same VLAN without having to change connections at the wiring hubs. A variety of methods are used to determine how users are grouped into a VLAN: – Port – MAC Address – Packet Tagging – Network layer addresses

19. © 2001 by Prentice Hall 14-19 LAN-to-Host Connections The Host as a LAN Node – Some hosts have the ability to connect to the LAN as a node. This is the most effective way to establish the connection. Asynchronous Connections – Virtually every computer has the ability to send and receive by using an asynchronous data link protocol. Because most computers support the asynchronous data link protocol, it is sometimes used to link a microcomputer to a host. Usually, a microcomputer attached to a host asynchronously operates in one of two modes: file transfer or terminal emulation. Dedicated Connection Per Microcomputer – One way to connect a LAN node to a host is to provide a dedicated connection between a port on the host and each microcomputer needing a host connection. A dedicated connection provides direct host access, and the microcomputer does not use LAN resources for communicating with the host.

20. © 2001 by Prentice Hall 14-20 LAN-to-Host Connections (cont.) – A dedicated connection has several disadvantages. First, as with all asynchronous connections, the sped of the link is slow. These connection speeds can be over 100,000 bps, but typically for microcomputer connections they are 33.6 Kbps or less. If many LAN nodes must communicate with the host, many host ports are required. Multiplexing – A multiplexer is a hardware device that allows several devices to share one communication channel. Multiplexing is typically used to consolidate the message traffic between a computer and several remotely located terminals. This technique can also be used to allow several microcomputers to share a communication link to a host processor. Shared Asynchronous Connections – In some applications, each LAN node needs occasional access to the host, but the number of concurrent connections is far fewer than the number of LAN nodes. In such situations, a dedicated line per node is excessive. A better solution is to share asynchronous connections.

21. © 2001 by Prentice Hall 14-21 Common Features of Asynchronous Communications Software Scripts Mouse support File transfers (CompuServe, Xmodem, Ymodem, Kermit) Terminal emulation (ANSI, DEC VT220, IBM 3101, TTY) Electronic mail Phone directory Capture of data to a disk Text editor Password security

22. © 2001 by Prentice Hall 14-22 Interconnection Utilities File Transfer Utilities – File transfer utilities allow you to move files between network nodes. File transfer capabilities are an intrinsic part of many routers; part of the TCP/IP protocol suite is a file transfer capability. Remote Logon – A remote logon essentially establishes a remote user as a local user on the remote node. Once a user has successfully logged onto the remote node, commands issued by that user are processed and acted on by the remote node rather than by the local node. Remote Access – Through remote connections, LAN administrators can resolve problems from home or other work locations; users can perform some of their work at home and telecommute; and travelers can conduct work while away form the office.

23. © 2001 by Prentice Hall 14-23 Interconnection Utilities (cont.) Integrated Services Digital Networks (ISDNs) – One objective of ISDNs is to allow international data exchange. This requires interfaces between a number of national and regional providers of such services. The first mission of the ISDN program has been to define the functions and characteristics of the network and to establish implementation standards. – ISDN was the first high-speed alternative to switched, analog connections for Internet access. ISDN when used for Internet access usually provides speeds of 128 Kbps. Digital Subscriber Lines (DSL) – Digital subscriber lines are an emerging service that provides much faster transmission rates than analog modems and ISDN.