Star Topology

Computers in a star topology are connected by cables to a hub. In this topology management of the network is made much easier (such as adding and removing devices), because of the central point. However because it is centralized more cable is required.

Because most star topologies use twisted-pair cables, the initial installation of star networks is also easier.
If one computer fails the network will continue to function, but if a hub fails all computers connected to it will also be affected. Star topologies are, or are becoming the topology of choice for networks.



Advantages:

• Cheaper than a mesh. (less cable, less hardware per node)

• Single links make the network easier to reconfigure.

• Low cable housing requirements

• Robustness. If one link is damaged the rest of the network can still function.

• Easy fault identification and isolation.

• Relatively low link traffic between hub and node compared to some other topologies.

Disadvantage:

• Cabling still excessive compared to some other topologies as everything must be linked to the one central hub.

Wireless horizontal

The horizontal market is characterised by the use of standard applications. The wireless network offers a standard protocol interface, for example TCP/IP. This means that users can buy off-the-shelf software, plug in their wireless modems and go. Because the network offers a standard interface, the software does not need to be specifically developed for the wireless network. This opens up a much broader market, as users are no longer constrained by the availability of specialist applications, so e-mail, database access, two-way paging and web browsing are all easily available.

In this picture, point-of-sales is shown as being a part of the horizontal market. In actuality point-of-sale lies in a a grey area, as it relies on a series of standard and non-standard protocols.

WDCS

There are lots of different wireless data communication systems. To help us understand them all we divide them into different categories, Local Area Networks, Metropolitan Area
Networks and Wide Area Networks. Some of the techniques used for wireless LANs and MANs are shown here, but we'll discuss those in a little more detail in the next slide.

Wide Area Networks have been divided up into two distinct groups here circuit switched and packet switched. On the circuit switched side are Satellite phones, the American mobile telephone standard AMPS, GSM and PDC. HSCSD is a new data oriented circuit switched service for GSM.


The oldest of the packet switched services is Mobitex; a dedicated packet switched network
that has been available since 1986.

The other three systems are conceptually very similar, they all build on existing circuit switched mobile telephony infrastructures and they all add a packet switched service. GPRS provides packet data services by building on the GSM infrastructure, PPDC provides packet data services by building on PDC and CDPD provides packet data services by building on the AMPS system.

Packet Circuit Switched

It is extremely important to understand the difference between packet switched and circuit
switched systems, a difference, which is illustrated here.

You can compare a circuit switched data to a train and the physical medium is the train line.
Before the train (data) can use the line it has to ask and be granted access. Once this is done
the train can then proceed on the line, but it has exclusive access to the line. No other trains can use the line while our train is there. In a data communications environment,
circuit switchings main drawbacks are the call setup overhead (that is, asking for and being
granted access to the line) and the low utilization of the physical medium (only one train at a time). One advantage of circuit switching is that the call setup procedure allows users to be allocated and guaranteed a certain bandwidth. A good example of circuit switching is the analogue telephone network. Before you can talk to anyone you have to dial their number (a call request). The call is physically switched through the network until you had an end-toend circuit between your telephone and the telephone you are calling. Only when the phone is answered at the other end (a call accept) can communication begin.

What about packet switching? Well, in this case our data can be imagined as cars and the physical medium as a motorway.

Instead of sending one big chunk of data, like we did with a circuit switched train, we send smaller chunks of data, with many users on the same channel simultaneously. The advantage with this is that we don't need any call set-up procedure, we just jump in the car and go. Packet switching also gives a much higher channel utilization, because many people can use the same motorway simultaneously.

The disadvantage is that because we have no way of knowing how much data other users are
going to send there is no guarantee of bandwidth if too many people try to use the motorway at the same time it will slow down and eventually stop. The best example of a packet switched network is the global Internet.