Network Design

To estimate the necessary throughput you will need for your network , multiplay the expected number of users by the sort of application they will probably use . For example , 50 users who are cheifly browsing the web will likely consume 2.5 to 5Mbps or more of thouughtput at peak times , and will tolerate some latency . on the other hand , 50 sumultaneous VoIP users would requires 5 Mbps or more of thourghput in both directions with absolutely no ltency . Since 802.11g wireless equipment is half duplex ( tat is , it only transmits or receies , never both at once ) you should accordingly double the required throughtput , for a total of 10Mbps . Your wireless links must provide that capacity every second , or conversations will lag

Since all of your users are unlikely to use the connection at precisely the same moment , it is common practice to oversubscribe available throughtput by some factor ( that is , allow more users than the maximum available band width can support ) . Oversubscribing by a factor of 2 to 5 is quite common . In all likelihood , you willl oversubscribe by some amount when building your network infrastructure . By carefully monitoring throughput throuightout your network , you will be able to plan when to upgrade various parts of the network , and how much additional resources will be needed .

Expect that no matter how much capactiy you supply , your users will eventually find applications that will use it all . As we'll see at the end of this chapter , using bandwidth shaping technieuqs can help mititgate some latency problems . By using bandwidth shaping , web caching , and other techniques , you can signigicantly reduce latency and increase overall network throughput .

To get a feeling of for the lag felt on very slow connections , the ICTP has put together a bandwidth simulator . It will simultaneously downloa a web page at full speed and at a reduced rate that you choose . This demonstattion gives you an immediate understanding of how low throughtput and high latency redyce the usefulness of the Internet as a communications tool . It is avaiable at Http:// wireless.ictp . trisete .it / simulator /