LAN Transmission

There are three categories of LAN transmission:

• Unicast

• Multicast

• Broadcast

Each of these is discussed in the following sections.

Unicast

In a unicast environment, data is sent from the source to a single destination on the network (such as LAN and WAN). The source host addresses the data with the address of the destination host, forwards the data to the network, where it is carried to its destination. Figure 5-11 illustrates a unicast environment.

Figure 5-11. Unicast Network

NOTE

Physical topology and transmission environments are independent of each other. For example, whether the LAN is hub-and-spoke or ring LANs has no bearing on whether the application environment is unicast, multicast, or broadcast.

Some examples of unicast applications are person-to-person e-mail, file transfer, or print services.

Multicast

In a multicast environment, as illustrated in the Figure 5-12, the data is forwarded to a specific group of hosts on the network. The source host addresses the packet by using a multicast address; for example, the TCP/IP Suite uses 224.0.0.0 239.255.255.255. The multicast packets then are sent to the network, which forwards copies to each registered host in the multicast address group.

Figure 5-12. Multicast Network

Some examples of multicast applications are videoconferencing or video broadcast applications, where some, but not all, users receive a streaming video feed from a source host.

Broadcast

Broadcasts often are found in LAN environments and will not traverse a WAN unless the router is configured with a helper-address to direct broadcasts to a specified network address often across a WAN.

In a broadcast environment, as shown in Figure 5-13, the data packets are sent to all hosts on the network. In broadcast environments, the source host addresses the data with the broadcast address, such as 255.255.255.255 (used in the TCP/IP suite). These data packets then are forwarded to every host on the network.

Figure 5-13. Broadcast Network

Some examples of broadcast applications are Novell's Get Nearest Server (GNS) requests or TCP/IP's Dynamic Host Configuration Protocol (DHCP).

NOTE

DHCP supports LAN hosts that are not statically (manually) configured with network (IP) addresses. When these hosts join a LAN, a DHCP request is sent to all hosts on the LAN in the hope that the DHCP server will receive the request and return the host configuration information.

Technical Note: Broadcast-based Networks

Broadcast-based networks are not preferred network implementations because every network device on the broadcast network must expend CPU cycles to process each data frame and packet to determine whether that device is the intended recipient. Data broadcasts are a necessary evil of LAN environments and usually have minimal impact because the data frames traversing the network typically are small. Broadcast storms can cripple a network in no time because the broadcasting device uses all available bandwidth on the network.