Cisco Systems RJ-45-to-AUX manual Broadcast, ∙ Unicast ∙ Multicast, 138

∙Unicast

∙Multicast

Broadcast

In its simplest form, broadcast traffic consists of packets that reach every point of the network. In a typical network, broadcasts are stopped at the router. You can set the router to forward broadcasts, but doing so is not very efficient—it creates a lot of traffic on the network and slows the end users’ machines. Every host on the network must process the packet to see if it is destined for that host. Data broadcasts are typically small frames used in the local network—so, the performance effect is negligible, unless there is a broadcast storm.

Note In a broadcast storm, an incorrect packet is broadcast on the network. This causes most hosts to respond with incorrect answers, which in turn causes even more hosts to respond again. This process continues until the network can no longer carry any other traffic. A broadcast storm can also occur when there is more than one path through the network, allowing broadcasts to circle the network until there are so many that the network comes to a stop.

Multimedia broadcasts, in contrast, can be huge packets. Processing these types of broadcasts can quickly use up all the available bandwidth on the network and bring the end station to a crawl—particularly if you are in a shared 10BaseT environment.

Figure 7.1 illustrates broadcast traffic in the network.

Figure 7.1: : Broadcast traffic flow.

Unicast

In unicast, a single packet is sent from the source to the destination. It is a one−to−one relationship: For every packet that reaches the destination, one packet was sent by the source. This process is fine if the source is having different conversations with only a few hosts. Now, imagine that same source talking to hundreds of hosts on the same conversation—each identical packet must be generated by the source and must travel on the network.

Audio and video transmissions are so large that a high−bandwidth link is consumed very quickly. A 100Mbps link can support about 60 to 70 full−screen, full−motion video streams if each stream uses approximately 1.5Mbps of server−to−client bandwidth. You will need gigabit−per−second (Gbps) links between the server and the network in order to provide one audio/video broadcast to a couple hundred hosts. Unicast multimedia applications do not scale very well.

Figure 7.2 illustrates unicast traffic flow.

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