Numerous network devices, besides computers, are sometimes required to help you build networks especially when networks grow beyond the confines of a simple workgroup. Without dwelling too much on features and more sophisticated functions, here's a list of devices available for most major network technologies (or that incorporate interfaces based on such technologies) that you can use to extend or interconnect existing networks:
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Repeater: A repeater is a simple networking device that copies incoming signals from one connection and then sends them to one or more other connections (called ports ). The notion behind the name is that this device "repeats" exactly what and everything that it "hears." Repeaters can link only media segments that use the same network technology, but those segments can be different media types. Repeaters operate at the Physical layer of the Open Systems Interconnection (OSI) model. (See Chapter 3 for an explanation of the OSI model.) Use repeaters when you reach the legal limits for cable lengths that a network technology can support. Repeaters allow you to extend your networks farther than they could otherwise go. Note that network technologies also have rules about the maximum number of repeaters that occur between any sender and any receiver on a network. -
Bridge: A bridge is a network device that examines the addresses of incoming network traffic and copies incoming messages to other network segments only if their destinations can be reached through (or on) those segments. The notion behind the name is that these devices act as semi- intelligent links between networks and check low-level hardware addresses to decide what can pass from one network segment to another. Bridges operate at the Data Link layer (at the MAC sublayer, in fact) of the OSI model. We explain the OSI model in Chapter 3. Not only can bridges link media segments forthe same network technologies, but so-called translation bridges can interconnect network segments that use different network technologies. (Linking FDDI and Ethernet is a common example.) The protocols on both sides of the device must remain the same, however. Use bridges when your network includes nonroutable protocols, such as NetBEI or Data Link Control (DLC), and such traffic must be forwarded from one network segment to another. Note that some routers (next item) include bridging functions, and that hybrid devices called brouters (item after next ) can perform both bridging and routing functions. -
Router: A router is a sophisticated network device that reads and resolves network addresses from incoming traffic. Routers perform all kinds of interesting functions on such data, including filtering incoming data by address, managing multiple protocols, either blocking or allowing certain types of protocols as well as certain ranges of addresses associated with certain protocols, and more. Routers operate at the Network layer of the OSI model. (See Chapter 3 for details.) Routers can interconnect dissimilar network technologies and can even reformat data for transmission on an outgoing port whose technology differs from that of the incoming port. The most common example here is when 16-Mbps token ring and Ethernet are linked. Because token ring supports much larger message units than Ethernet, a router may have to break up a single token-ring message into as many as 44 equivalent Ethernet messages. Routers are what make it possible for two separate networks to function independently yet still exchange information when they must. Routers make phenomena such as the Internet possible and have played a critical role in its growth. Use routers when you seek to operate and control your own network(s) and connectivity to other networks is also required. -
Brouter: A brouter combines the functions of a bridge and a router. That is, it acts like a bridge for nonroutable protocols and like a router for routable protocols. Brouters are most commonly used on networks that have both kinds of protocols in use. They are also used when a local network requires bridging of nonroutable protocols, but access to the Internet or some other public network requires routing of routable protocols. Brouters operate at both the Data Link and Network layers of the OSI model. (We explain this model in Chapter 3.) -
Gateway: A gateway is a device that translates application information from one type of environment for some other type of environment. A typical example is an e-mail gateway that translates between Microsoft Exchange formats and the native Internet e-mail format known as the Simple Mail Transfer Protocol (SMTP), and vice versa. Other gateways can translate between dissimilar protocol suites, such as Systems Network Architecture (SNA) and Transmission Control Protocol/Internet Protocol (TCP/IP). Still other gateways support moving data between other dissimilar applications of the same type, such as database management systems or transaction processing systems. Gateways operate primarily at the upper layers of the OSI model (which we explain in Chapter 3) and are concentrated at the Session, Application, and Presentation layers. |
As you climb this set of ancillary network devices (beginning with the lowly repeaters and moving up to gateways), their sophistication and capabilities increase but their speed and overall handling capacity decline. That's because each step up this ladder involves increased processing and data-handling capabilities, which take time and programming smarts and therefore lowers their overall throughput.
You often find items at the lower end of this ladder such as repeaters and bridges sold as simple black boxes that are more or less ready to plug in and use. Routers and brouters, on the other hand, are usually special-purpose, high- powered , high-speed computers that accept two or more interface cards (one for each connection that you make to this device).
Windows 2003 includes powerful built-in routing capabilities, such as its Routing and Remote Access Server, or RRAS. Also, additional software, such as Internet Security and Administration Server (ISA Server), can be added to Windows Server 2003 to enhance its capabilities. Gateways, on the other hand, usually occupy a general-purpose computer, but they are also normally dedicated to performing only that job.
