Communications network

Every communications environment can be perceived differently at different levels of abstraction. We believe that the basic characteristics of a communications environment are the same though, regardless of the level of abstraction that we are referring to. We claim that a communications environment of any level can be perceived as a mechanism of message exchange among entities via some media.

The first step in describing a communications environment is to identify the entities and the media of this environment. The entities and media that we identify depend on the viewpoint that we use to look at the communications environment.

Although this statement looks very similar to the concept of the OSI-RM, there is not necessarily a direct mapping between the OSI-RM layers and our levels of perception of a communications environment. For example, if we were investigating mobility on the network layer of the OSI-RM, we could model a number of network layer peers as network entities, and their connections over a network layer protocol as the media. On the other hand, we could model a person and a terminal as network entities, where the person is attached to the terminal via the input/output interfaces that the terminal offers; these interfaces would be the media.

For example, let us assume a home user, who is accessing a specific web page on the Internet. The user has Internet access via an Internet Service Provider (ISP), to whom he connects over a dial-up connection, as in Figure 3-1(a). From the user’s view, the communications environment includes his home terminal, the host of the ISP that he is connecting to and a number of web-sites that he can access via his ISP; these are the

entities o f his perception o f a communications environment. Also, this environm ent includes the dial-up connection to the ISP and the connection from the ISP to the w eb­ sites. If any o f these connections is down connectivity is impossible; these connections are the media o f the user’s perception o f a communications environment.

U s e r ’s te r m in a l U s e r Dial-Up connection Internet IS P h o st (a)

mm

Figure 3-1: Three different com m unications networks as perceived in the sam e com m unications environment: (a) home user’s view, (b) Internet Service Provider’s view, (c) Telecom provider’s

view.

In the same example, the ISP view o f the communications environm ent differs from that o f the home user. The environm ent has a num ber o f home users whose terminals are additional entities that gain connectivity to some o f the hosts that the ISP owns. The users’ hosts use the dial-up m edia to connect to the ISP ones. Also, the ISP connects to an Internet backbone via another host, which is another entity, and the connection to this entity is an additional medium. The ISP’s perception o f the comm unications environm ent is illustrated in Figure 3 - 1(b).

We can also consider the view of the telecom provider who offers the links between the home users’ terminals and the ISP host. According to this view, which is illustrated in Figure 3-1(c), the communications environment consists of a number of terminal points to which the home users’ terminals and the ISP hosts are attached and a number of switching elements; these are the entities. The media are the connections between these entities that allow message exchanges between terminal points. Figure 3-1 illustrates these three different levels of perception of a communications environment. Each perception of this communications environment features different entities and media. We understand that in real systems, the view of the ISP or the Telecom provider can be more complex and involve many more entities than those of our example. We believe, however, that this simplified view serves as well for the purpose of our example.

As this example illustrates, a communications environment consists of a number of entities that are interconnected by media. Also, there can be more than one view or perception of a communications environment. Each of these views or perceptions may feature different entities or media.

Communications Environment: an environment that features a number

o f entities and media. The entities and the media can be perceived at different levels o f abstraction, which offer different views o f the environment. Each view may feature a different set o f entities and media.

Each view of a communications environment we call a communications network. A communications network is composed of a number of elements. We distinguish between two kinds of elements: network entities (or entities) and media. We also assume that a message exchange between network entities of a communications network signifies a service offered by one entity to another.

The smallest communications network, in terms of number of entities and media involved, is one that features one medium, to which two entities are attached. The case where there is no medium in a communications network is invalid in our framework, since it implies that if there are network entities, they cannot exchange information between them, and therefore, there is no network. The case where only one entity is involved in a communications network is also invalid in our framework, since our concept

of a communications network provides for exchange of information between different entities.

Communications Network or Network: a particular view o f a

communications environment. It is composed o f two kinds o f "elements’*: "network entities” (or entities) and "media”. The entities can exchange

"messages ” between them via the media. Each message exchange signifies a "service ” that one entity can offer to another. A minimum communications network involves one medium and two entities attached

to it.

We can consider numerous other examples to which this concept of a communications network can map. Apart from data communications networks, where the network entities are physically attached to each other via wireless or wireline media, we could also describe an organisation as a communications environment. From the viewpoint of a manager for example, there is a communications network in this organisation, where the entities are the employees and the media that they are interconnected with are the telephone service, the e-mail, or the contact between them when they are in the same office.

Similarly, the radio link that the taxis use in some big cities can be perceived as part of a communications network from the taxi drivers’ viewpoint. This network has a number of network entities, which are the radio-equipped taxis and the centre that broadcasts the taxi requests over the radio link. The medium of this network is the radio link to which both the centre and the taxis are attached.

Having described the concept of a communications network we will now define the concept of an entity and the concept of a medium in more detail.