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Transcript
Introduction to Computer Networks Elements of Network Connectivity This lesson explores the devices and technologies available to expand networks beyond the scope of simple local area networks. Connectivity Devices • Devices used to expand LANs include modems, repeaters, bridges, routers, brouters, and gateways. • Modem Technology: Modems convert digital signals to analog waves, and vice versa. Modem Hardware • Modems are known as data communications equipment (DCE) and share the following characteristics: • A serial (RS-232) communications interface • An RJ-11 telephone-line interface (a fourwire telephone plug) Modem Standards • Some of the common industry standards for modems include: • Hayes-Compatible: manufacturers currently offer modems with speeds of 56,600 bps or more. • International Standards: the International Telecommunications Union (ITU) has developed standards for modems. • These specifications, known as the V series, include a number that indicates the standard. ITU Modem Standards • V.22bis V.32 14,400 28,800 57,600 56,600 2400 9600 1991 1994 1995 1998 1984 1984 V.34 V.42 V.90 V.32b • Several of the newer modems feature industry standards, such as V.42bis/MNP5 data compression, and have transmission speeds of 57,600 bps; and some modems go up to 76,800 bps. Types of Modems • There are different types of modems because different types of communication environments require different methods of sending data. • These environments can be divided roughly into two areas related to the timing of communications: • Asynchronous • Synchronous Asynchronous Communication (Async) • Async was developed in order to make use of common telephone lines. • Communication is not synchronized: There is no clocking method to coordinate the transmission between the sender and the receiver. • Speed can be up to 28.8K bps. However, the data compression can boost the rate to 115.2K. Synchronous Communication • Synchronous communication relies on a timing scheme to transmit bits in block known as frames. • Special characters are used to synchronize and check its accuracy periodically. Synchronous protocols • Synchronous protocols perform a number of jobs that asynchronous protocols do not. • Format data into blocks. • Add control information. • Check the information to provide error control. • The primary protocols in synchronous communication are: • Synchronous Data Link Control (SDLC), Highlevel Data Link Control (HDLC), Binary Synchronous Communications Protocol (bisync). Asymmetric Digital Subscriber Line (ADSL) • This technology converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. • These new connections can transmit more than 8 Mbps to the subscriber and up to 1 Mbps from the subscriber. • The technology requires special hardware, including an ADSL modem on each end of the connection. • It also requires broadband cabling, which is not available in every locations, and there is a limit to the connection length. Expanding a Network Using Components • There are components that can be installed to increase the size of the network within its existing environment. These components include: • Hubs. • Repeaters. • Bridges. • Routers. • Gateways. Hubs • Connecting or adding hubs to a LAN can effectively increase the number of workstations. • This method of growing a LAN is popular, but does come with many design limitations. • Uplink crossover cables are wired differently than standard patch cables. Repeaters • Repeaters regenerate weakened signals • Repeaters do not translate or filter signals. • A repeater cannot connect segments using different access methods. Implementing a repeater • Use a repeater to: • Connect segments of similar or dissimilar media, • To increase the distance transmitted. • Pass all traffic in both directions. • Connect in the most cost-effective manner. • Do not use a repeater when: • There is heavy network traffic. • Segments are using different access methods. • Data filtering is needed. Bridges • Bridges use routing table to segment network traffics, and make each segment more efficient. Remote Bridges • Because bridges can expand and segment networks, they are often used in large networks tha have widely dispersed segments. Spanning tree algorithm (STA) • Because remote LAN segments can be joined over telephone lines, it is possible for multiple LANs to be joined by more than one path. • In this situation, it is possible that data might get into a continuous loop. To handle this possibility, the 802.1 Network Management Committee of IEEE has implemented the spanning tree algorithm (STA). • STA can sense the existence of more than one route, determine which would be the most efficient, and then configure the bridge to use that one. Routers • Router works in an environment that consists of several network segments with differing protocols and architectures • Router is a device that not only knows the address of each segment, but can also determine the best path for sending data and filtering broadcast traffic. • Routers work at the network layer of the OSI reference model, it can exchange protocol-specific information between separate networks. Routing Algorithms • OSPF ("open shortest path first") is a link-state routing algorithm. Link-state algorithms control the routing process and allow routers to respond quickly to changes in the network. NetWare Link Services Protocol (NLSP) is a link-state algorithm to be used with IPX. • RIP (Routing Information Protocol) uses distance-vector algorithms to determine routes. Transmission Control Protocol/Internet Protocol (TCP/IP) and IPX support RIP. Types of Routers • Static Routers. • Static routers require an administrator to manually set up and configure the routing table and to specify each route. • Dynamic Routers. • Dynamic routers discover routes automatically and examine information from other routers to make decisions about how to send data across the network. Distinguishing Between Bridges and Routers • Bridges work at the data-link layer MAC sublayer, and routers work at the network layer Brouters • A brouter can act as a router for one protocol and as a bridge for all the others. • Brouters can: • Route selected routable protocols. • Bridge nonroutable protocols. • Deliver more cost-effective and more manageable internetworking than separate bridges and routers. Gateways • Gateways enable communication between different architectures and environments. • A gateway links two systems that do not use the same: • Communication protocols. • Data-formatting structures. • Languages. • Architecture. How Gateways Work • Gateways are taskspecific, typically perform protocol conversion at the application layer.