![]() ![]() One of the most common uses of gateways is to enable LANs that use TCP/IP and Ethernet to communicate with mainframes that use other protocols. A gateway may be a stand-alone computer with several NICs and special software or a front-end processor connected to a mainframe computer. ![]() Gateways translate one network layer protocol into another, translate data link layer protocols, and open sessions between application programs, thus overcoming both hardware and software incompatibilities. Gateways process only those messages explicitly addressed to them (i.e., using their data link layer address) and route those messages that need to go to other networks (Figure 8.4). Some gateways operate at the application layer as well. They may connect the same or different types of cable. Gateways connect two or more networks that use the same or different (usually different) data link and network protocols.įigure s.4 Use of gateways to connect local area networks and a mainframe Gateways are more complex than switches or routers because they are the interface between two or more dissimilar networks. Gateways operate at the network layer and use network layer addresses in processing messages. (As noted previously, it creates a new data link layer packet.) Sometimes, however, changes are needed, such as when the maximum data link layer packet size on one network is different from another, which forces the router to split a message into several smaller messages for transmission. The router attempts to make no changes to the network layer packet and user data it receives. The router will then process the message by building an entirely new data link layer packet, then transmit it to the other network. In contrast, because routers operate at the network layer, the router’s data link layer must first recognize that the incoming message is specifically addressed to the router at the data link layer level before the message is passed to the network layer for processing. Switches simply forward the message unchanged to the other network. Switches process all messages that appear on the network and forward them to the appropriate network on the basis of their data link layer address. ![]() One other important difference between a router and a switch is that a router processes only those messages that are specifically addressed to it. Because a router knows its own location, as well as the packet’s final destination, it looks in a routing table to identify the best route or path.įigure s.3 Use of routers to connect local area networks One major feature of a router is that it can choose the "best" route between networks when there are several possible routes between them. In general, they perform more processing on each message than switches and therefore operate more slowly. Routers may be special purpose devices or special network modules in other devices (e.g., wireless access points for home use often include a built-in router). Routers strip off the data link layer packet, process the network layer packet, and forward only those messages that need to go to other networks on the basis of their network layer address (Figure 8.3).įigure s.2 Use of switches to connect local area network segments Routers are the "TCP/IP gateways" that we first introduced in next topic. Routers connect two or more network segments that use the same or different data link protocols but the same network protocol. They learn addresses by reading the source and destination addresses. These are the same layer-2 switches discussed in next topic in that they use the data link layer address to forward packets between network segments (Figure 8.2). They understand only data link layer protocols and addresses. They connect two or more network segments that use the same data link and network protocol. Most switches operate at the data link layer. These include switches, routers, and gateways (Figure 8.1). The hardware devices can be computers or special-purpose devices that just transfer messages from one network to another. The cable is essentially the same as that used in LANs, except that it is often fiber optic to provide higher data rates. There are two basic components to a BN: the network cable and the hardware devices that connect other networks to the BN. ![]()
0 Comments
Leave a Reply. |