For unguided media, transmission and reception are achieved by means of an antenna. For transmission, the antenna radiates electromagnetic energy into the medium (usually air), and for reception, the antenna picks up electromagnetic radiation waves from the surrounding medium. There are basically two types of configurations for wireless transmission: directional and omnidirectional. For the directional configuration, the transmitting antenna puts out a focused electromagnetic beam; the transmitting and receiving antennas must therefore be carefully aligned.
# Fiber optic communication
An optical fiber is a thin (2 to 125 pm), flexible medium capable of conducting an optical ray. Various glasses and plastics can be used to make optical fibers. The lowest losses have been obtained using fibers of ultra pure fused silica. Ultra pure fiber is difficult to manufacture; higher-loss multi component glass fibers are more economical and still provide good performance. Plastic fiber is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.
An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket (Figure 13.4)T. he core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core. The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing, and other environmental dangers.
# Guided transmission media:
For guided transmission media, the transmission capacity, in terms of either data rate or bandwidth, depends critically on the distance and on whether the medium is point-to-point or multipoint, such as in a local area network (LAN). Table 3.1 indicates the type of performance typical for the common guided medium for long distance point-to-point applications; The three guided media commonly used for data transmission are twisted pair,
coaxial cable, and optical fiber (Figure 13.3). We examine each of these in turn.
Transmission medium is the physical path between transmitter and receiver in a data transmission system. Transmission media can be classified or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable, and optical fiber. The atmosphere and outer space are examples of unguided media that provide a means of transmitting electromagnetic signals but do not guide them; this form of transmission is usually referred to as wireless transmission systems.
4) Star Topology:
In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion.
A transmission of a frame from one station to the node is retransmitted on all of the outgoing
links. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.
The basic types of network topology as shown in fig 12.1. For the physical layer, we confine for to an introduction of the basic LAN topologies. The common topologies for LANs are bus, tree, ring, and star (Figure 12.1). The bus is a special case of the tree, with only one trunk and no branches; we shall use the term bus/tree when the distinction is unimportant.
Finally, a protocol may be either standard or nonstandard. A nonstandard protocol is one built for a specific communications situation or, at most, a particular model of a computer. Thus, if K different kinds of information sources have to communicate with L types of information receivers, K × L different protocols are needed without standards and a total of 2 × K × L implementations are required (Figure 11.2a). If all systems shared a common protocol, only K + L implementations would be needed (Figure 11.2b). The increasing use of distributed processing and the decreasing inclination of customers to remain locked into a single vendor
dictate that all vendors implement protocols that conform to an agreed-upon standard.
Before turning to a discussion of communications architecture and the various levels of protocols, let us consider a rather small set of functions that form the basis of all protocols. Not all protocols have all functions; this would involve a significant duplication of effort. There are, nevertheless, many instances of the same type of function being present in protocols at different levels.
This discussion will, of necessity, be rather abstract; it does, however, provide an integrated overview of the characteristics and functions of communications protocols. The concept of protocol is fundamental to all of the remainder of Part Four, and, as we proceed, specific examples of all these functions will be seen.
# We can group protocol functions into the following categories:
- Segmentation and reassembly
- Connection control
- Ordered delivery
- Flow control
- Error control
The concepts of distributed processing and computer network system imply that entities in different systems need to communicate. We use the terms entity and system in a very general sense. Examples of entities are user application programs, file transfer packages, data base management systems, electronic mail facilities, and terminals. Examples of systems are computers, terminals, and remote sensors.
Note that in some cases the entity and the system in which it resides are coextensive (e.g., terminals). In general, an entity is anything capable of sending or receiving information and a system is a physically distinct object that contains one or more entities. For two entities to successfully communicate, they must “speak the same language.” What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable set of conventions between the entities involved. The set of conventions is referred to as a protocol, which may be defined as a set of rules governing the exchange of data between two entities.
HDLC protocol is an example of a protocol. The data to be exchanged must be sent in frames of a specific format (syntax). The control field provides a variety of regulatory functions, such as setting a mode and establishing a connection (semantics). Provisions are also included for flow control (timing). Most of Part Four will be devoted to discussing other examples of protocols.
# Some important characteristics of a protocol are:
- Direct /indirect
- Standard /nonstandard