Information is carried out in data communication systems as signals between two or more points. To understand transmission one needs to study electrical form that the messages take while they are in transit and of media and transmission technologies.
Signals: Signals are electric or electromagnetic encoding of data, and signalling is propagation of signal along suitable communication medium.
Transmission: Transmission is communication of data achieved by the propagation and processing of signals.
Parallel and Serial Transmission:
In parallel data transmission, there are multiple parallel lines connecting the transmitting and receiving units. Each wire carries a bit of information.
In series data transmission, each bit is sent sequentially one after another and it requires one pair of wires for connecting the receiving and transmitting units.
Series transmission is slower than parallel transmission and is used primarily for transferring data between devices at the same site. Communication between computers is almost always serial.
Synchronous and Asynchronous Transmission: The mode of transmission is the way in which coded characters are assembled for the purpose of transmission and permits receiving devices to identify where the coding for each character begins and ends within the torrent of bits. When two computers communicate, they must have a way to synchronize the flow of data so that the receiving computer can read at the same speed at which the sending computer transmits.
In synchronous transmission characters are transmitted as groups, with control characters in the beginning and at the end of the bit train. The transmission and receiving intervals between each bit are precisely timed permitting the grouping of bits into identifiable characters. In synchronous mode, intervals between characters are uniform within space between consecutive bytes.
In asynchronous transmission, each character is transmitted separately, that is, one character at a time. Each character begins with a start bit and ends with a stop bit.
The start and stop bits and the interval of time between consecutive characters allow the receiving and sending computers to synchronize the transmission.
Simplex, Half-duplex, Full-duplex Communication
The direction in which information can flow over a transmission path is determined by the properties of both the transmitting and the receiving devices. There are three basic options:
n Simplex mode, the communication channel is used in one direction. The receiver receives the signals from the transmitting device. The simplex mode is rarely used for data communication.
In Half-duplex mode, the communication channel is used in both directions, but in one direction at a time. This requires the receiving and transmitting devices to switch between send and receive modes after each transmission.
In Full-duplex mode, the communication channel is used in both directions at the same time. Typical example of this mode of transmission is the telephone in which both parties talk to each other at the same time.
Local Area Network (LAN): A LAN is a data communication network, which connects many computers of workstations (computer terminal, printer etc.) and permits exchange of data and information among themselves, within a localised area, typically confined to a building, or a cluster of buildings.
LAN topology: A network topology refers to the physical layout of the network in which all the devices are connected. This includes all the hardware that makes up the network. The points of connection to the network by the stations are called Nodes or link stations. There are several types of typographical design and strategies used to implement LAN. The majority of these are based on three types of topologies:
Star Topology: In this topology, a number of stations are connected directly to a central station or controller. Communications on the connecting links between the stations and the central station of a star topography can be bi-directional and are point-to-point.
A station on this type of network passes an information to the central controller, which then forwards the information to the destination station. The central controller manages and controls all communications between stations on the network.
Bus Topology: All stations are connected to a single communication line. This single communication line is referred to as a bus. Information frames originating at a station are propagated away from the station in both directions on the bus. Each station on the bus interrogates the information frame destination address field for its own address. If the destination address field matches the station address, only then it accepts the information frame and processes the frame.
Ring Topology: Local area networks that have each station attached to an adjacent station using point-to-point links form a physical ring. Each station attached and active to the ring
regenerates the information frame, and then re-transmits the information frame on the ring. The ring itself is logically circular and the information travels in one direction.
Failure of a station in a ring topology disrupts the ring because the information frame is not regenerated. Additions or deletions of stations to the ring can be disruptive, if the changes are not managed properly.
Lan Access Method: Access methods are the means or way by which stations actually gain the use of the common channel to transmit messages. The right to transmit is an issue only in broadcast where work stations share a single channel.
Many techniques have been proposed, but two of these are commonly used.
* Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)
* Token passing.
CSMA/CD: CSMA/CD access method is used with bus networks. The bus operates in a Multiple Access (MA) mode. A node is allowed to transmit on the bus, if it senses that the medium is free (carrier sense). Occasionally, two or more nodes may simultaneously sense that the medium is free and begin to transmit. This creates a collision as the contents of transmitted information frames will collide resulting in corruption of the information frame. This collision is detected (collision detection) by the transmitting node. The two (or more) nodes involved then wait for a further short random time interval before trying to retransmit a frame.
Token Passing: Another way of controlling access to a share medium is by the use of a control (permission) token. The control token is passed from one node to another according to a defined set of rules understood and adhered to by all nodes.
In token passing, a logical ring of all nodes connected to the physical medium is first established and a single token is generated; The control token passes from one node to another traversing the logical ring. The token keeps on circulating the logical ring until it is received by a node waiting to send an information frame. After receiving the token, waiting station transmits its waiting frames on the physical medium after which it passes the control token to the next node in the logical ring.
Wide Area Network: A Wide Area Network, or WAN, is a network that links separate geographical locations. A WAN can be a public system such as the Public Switched Telephone Network (the PSTN) or one of the various packet switched services provided by the public telecommunication authorities. WANs can also use most other types of circuit including satellite networks, ISDN, Value Added Networks (VANs/ VADs).
The main distinguishing feature between a WAN and LAN is that the LAN is under the complete control of the organisation which owns it, whereas the WAN needs the involvement of another authority like the telephone company.
Communication Switching Techniques: In a WAN, two computing devices are not connected directly. A network of switching nodes provide a transfer path between the two devices.
The process of transferring data blocks from one node to another is called data switching.
There are three switching techniques commonly employed and these are:
* Circuit Switching: In Circuit switching there is a dedicated communication path between the sending and receiving devices. The dedicated path is a connected sequence of links between switching nodes. A conventional telephone network, where a dedicated path is set between the caller and the called party for the duration of a telephone call, is an example of circuit switching.
Circuit switching is mainly used for voice telephone network, but is not all that effective for data communication networks, as channel capacities are not fully utilised.
* Message Switching: Message Switching is an alternative switching technique, where it is not necessary to establish a dedicated path between the sending and receiving devices. In Message Switching, the sending device appends the destination address to the message and passes it to the network; the message is then passed through the network from one node to another till it reaches the intended destination. Each switching node receives a message, stores it briefly and then transmits it to the next node. Examples of a message are electronic mails, computer files, telegrams and transaction queries and responses.
* Packet Switching: Packet Switching combines the advantages of message and circuit switching. Packet switching is functionally similar to message switching, in which data is transmitted in block,stored by the first switching node it meets in the network and is forwarded to the next and subsequent downstream nodes until it reaches the destination.
Network/ File server system: For sharing data in a LAN, users stores files on a file server. A file server is a central node (Computer in the network) that stores data files where all users can access them. Typically, the file server in LAN acts as a central hub for sharing peripherals like printers, print queues and modems. In a LAN's file server, in many cases entire files are pumped across the network on behalf of the operations taking place on LAN computers. A file server does not involve in processing of an application. It simply stores files for applications that run on LAN computer.
Client/ Server system: Any local area network could be considered as client/ server system, since the workstation (clients) requests services such as data, programme files, or printing from server. A client/ server (C/S) has three distinct components, each focussing on a specific job: a database server, a client application and a network.
A server gives stress on efficiently managing a resource such as a database of information. Its main job is to manage its resources optimally among various clients that concurrently request the server for the same resource.