SIMPLEX: only allows the transmission of data in one direction between two devices
DUPLEX (sometimes called Full Duplex)
Allows transmission of data in both directions simultaneously.
HALF DUPLEX (HDX)
Allows bothway transmission of data between two devices but only in one direction at a time. (2-wire).
ASYNCHRONOUS:
Irregular or not synchronised. In computer communications, the term is usually applied to data transmitted irregularly rather than as a steady stream. Asynchronous communication uses start bits and stop bits to indicate the beginning and end of each data block.
SYNCHRONOUS: Regular. Most communication within a computer system is synchronous, controlled by the computer's own internal clock, while communication between computers is usually asynchronous. Synchronous telecommunications are, however, becoming more widely used.
PARALLEL TRANSMISSION: Transmission mode in which a number of bits of information are sent simultaneously over separate lines (for example, eight bits over eight lines); usually unidirectional.
Protocols used in data transfer over LANs (token ring, CSMA/CD) and WANs (TCP/IP)
LOCAL AREA NETWORK (LAN) the cable interconnection of items of computing equipment over a small local area such as a single building or site. Such systems enable the sharing of data, software and equipment resources.
TOKEN RING
A network access mechanism and topology in which a supervisory frame or token is passed from station to station in sequential order. Stations wishing to gain access to the network must wait for the token to arrive before transmitting data. In a token ring, the next logical station receiving the token is also the next physical station on the ring. The standard was developed by IBM and ratified as an IEEE standard, 802.5. Token Ring interconnects PCs via special twisted-wire cable in a star topology, connecting all computers to a central wiring hub.
CSMA/CD (Carrier Sensing Multiple Access with Collision Detection)
A LAN contention protocol by which workstations connected to the same channel are able to sense transmission activity on that channel and so defer their own transmission while the channel is active. If two nodes try to transmit simultaneously a collision is detected and both stations stop transmission for a period. After a random period of time each station will make a repeat attempt. It is unlikely that they will attempt to transmit at the same time and further collision usually does not occur.
Wide Area Network (WAN)
A WAN connects computer networks or devices that are located in different geographical areas. A WAN uses world-wide communications usually telephone lines, to connect computer resources. A WAN may connect similar computer systems (LANs to LANs) or dissimilar systems (LANs to mini computers or mainframes)
A WAN provides communication speeds at usually less than 2Mbps (a common communication line is called T1 that gives 1.544Mbps) Higher speed lines are available for WANs but those are provided by the telephone companies at a very high price.
TCP/IP (Transmission Control Protocol/Internet Protocol): A layered set of protocols that allows sharing of applications among PCs, hosts, or workstations in a high-speed communications environment. Because TCP/IP's protocols are standardized across all its layers, including those that provide terminal emulation and file transfer, different vendors' computing devices (all running TCP/IP) can exist on the same cable and communicate with one another across that cable. Corresponds to Layers 4 (Transport) and 3 (Network) of the OSI reference model.
The main features of packet switching
Packet switching is similar to message switching using short messages. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission; the packets, each with an associated header, are then transmitted individually through the network. The fundamental difference in packet communication is that the data is formed into packets with a pre-defined header format (i.e. PCI), and well-known "idle" patterns which are used to occupy the link when there is no data to be communicated.
A packet network equipment discards the "idle" patterns between packets and processes the entire packet as one piece of data. The equipment examines the packet header information (PCI) and then either removes the header (in an end system) or forwards the packet to another system. If the out-going link is not available, then the packet is placed in a queue until the link becomes free. A packet network is formed by links which connect packet network equipment.
Communication between A and D using circuits which are shared using packet switching.
Packet-switched communication between systems A and D
(The message in this case has been broken into three parts labelled 1-3)
There are two important benefits from packet switching.
- The first and most important benefit is that since packets are short, the communication links between the nodes are only allocated to transferring a single message for a short period of time while transmitting each packet. Longer messages require a series of packets to be sent, but do not require the link to be dedicated between the transmission of each packet. The implication is that packets belonging to other messages may be sent between the packets of the message being sent from A to D. This provides a much fairer sharing of the resources of each of the links.
- Another benefit of packet switching is known as "pipelining". Pipelining is visible in the figure above. At the time packet 1 is sent from B to C, packet 2 is sent from A to B; packet 1 is sent from C to D while packet 2 is sent from B to C, and packet 3 is sent from A to B, and so forth. This simultaneous use of communications links represents a gain in efficiency, the total delay for transmission across a packet network may be considerably less than for message switching, despite the inclusion of a header in each packet rather than in each message.
Meaning of bandwidth and the impact of bandwidth on data transmission
Bandwidth has a general meaning of how much information can be carried in a given time period (usually a second) over a wired or wireless communications link. For example, a link with a broad bandwidth - that is, a broadband link - is one that may be able to carry enough information to sustain the succession of images in a video presentation.
