COMMUNICATIONS PROTOCOL

A communications protocol is a set of rules and procedures for exchanging information among computers on a network. Protocols define how communications channel is established, how information is transmitted and how errors are detected and corrected.

Different kinds of computers can communicate with each other using the same protocol.  Two widely used protocols for LANs are Ethernet and Token ring. The TCP/IP is a set of protocols widely used on the Internet.

Ethernet is a LAN protocol that allows personal computers to contend for access to the network.  Ethernet was the first industry standard LAN protocol developed by Xerox in 1976.

Advantages of Ethernet

- It is relatively in expensive and easy to maintain and install.  Some times it is more efficient and economical to use a bridge to connect two separate LANs instead of creating one large LAN that combines the two separate LANs.

COMMUNICATIONS CHANNELS

A Communications channel is the communications path between two devices. It is composed of one or more transmission media

The width of the communications channel is called The Bandwidth.  The higher the bandwidth, the more data and information the channel can transmit.

For analog signals, bandwidth is expressed in hertz (Hz) or cycles per second.  Foe digital signals, bandwidth is expressed in bits per second (bps)

FACTORS AFFECTING TRANSMISSION RATE OF A NETWORK

(a)    Transfer rate.  Is the speed of transmitting data and information.  Transfer rates are usually expressed in bits per second (bps), kilobytes per second (kbps) or megabytes per second (mbps). The transfer rate of a transmission medium depends on the mediums bandwidth and its speed. The following contributes to the transfer rate of a network.

(i) Network topology.  Since data travels in both direction in a bus network, if data collides, then it as to be sent again, and slow the network down.  On the other hand, start networks have fewer collisions and usually run faster

(ii) Capacity of hardware.  Hubs, switches and network interface cards have their own maximum speeds.

(iii) The server. Amount of Ram and the speed of the hard disk.

(iv) Location of software and files. Storing software on the workstation hard disks reduce network traffic and speed up performance.

Transfer rates of various connection lines to the Internet

Types of lines	Transfer rates
Dial up	Up to 56kbps
ISDN	Up to 128kbps
ADSL	128kbps to 9mbps
Cable TV
T1	1.544mbps
T3	44mbps
ATM	155mbps to 622mbps

(a)    A dial up Line.  This is a temporary connection that uses one or more analog telephone lines for communication.  Using a dial-up line to transmit data is similar to using the telephone to make a call.

Advantages of dial-up lines

-       It cost no more than making a regular telephone call.

-       Computers at any two locations can establish a connection using modems and the telephone network.

Disadvantages

-       The user cannot control the quality of the connection because switching the telephone company’s switching office randomly selects the line for connection.

(b)   ISDN (Integrated services Digital Network) is a set of standards for digital transmission of data over standard copper telephone lines.  It requires both ends of the connection to have an ISDN Modem.

(c)    DSL (Digital subscriber lines) provides high speed connections to the internet over a regular copper telephone line

(d)   ADSL (Asymmetric Digital Subscriber line) is a type of DSL that supports faster transfer rates when receiving data (i.e down stream rate) than when sending data (i.e upstream rate)

(e)    Cable TV network also provides high speed internet connections fro users equipped with cable modem.

(f)    A T-carrier line is a digital line that carries multiple signals over a single communication line using the multiplexing technique.  T1 line can carry 24 separate signals at a transfer rate of 64kbps each for a transmission rate of 24mbps.  A t3 can carry 672 individual signals at a transmission rate of 43mbps.  The internet backbone also use T-3 lines

                                    Advantages

▪      T-carrier lines provide very fast transfer rates.

Disadvantages

▪      T-carrier lines are so expensive that only medium to large companies can afford the investment.

(g)    ATM (Asynchronous Transfer mode).  This is a dedicated connection switching technology that organizes digital data into 53 bytes cell units and transmits them over a physical medium using digital signal technology.

TRANSFER RATE

This is the speed of transmitting data and information usually expressed in bits per second (bps) and kilobytes per second (kbps) or megabits per second (mbps)

Transfer rates of various communications media.

Type of transmission media		Transfer rate
Twisted Pair cable	10 Base-T (Ethernet)	10mbps
	100 Base-T (Fast Ethernet)	100mbps
	1000 Base-T  (Gigabit Ethernet)	1000mbps
	Token ring	4-16mbps
Coaxial Cable	10 Base 2 (Thin wire Ethernet)	10mbps
	10 Base 5 (Thick wire Ethernet)	10mbps
Fibre optic cable	10 Base F (Ethernet)	10mbps
	10 Base Fx (Fast Ethernet)	100mbps
	FDDI(Fibre Distributed Data Interface)	100mbps
Broadcast radio		Up to 2mbps
Micro wave		45 mbps
Communication satellites		50mbps
Cellular radio		9,600bps to 14.4kbps
Infrared		1-4mbps

1. TWISTED PAIR CABLE.  This consists of one or more twisted wires bundled together.  Each twisted pair wire consists of two separate insulated copper wires of diameter o.4 – 0.8mm that are twisted together (to reduce noise).  They are of two types.

                                                                             Fig:  A twisted pair cable

   (i) Shielded twisted pair (STP) has a metal wrapper around each twisted pair wire to further reduce noise. STP cables are used in environment susceptible to noise, such as local area network.

  (ii) Unshielded twisted pair (UTP) cable doesn’t have this metal wrapper for shielding noise.  UTP cables are commonly used in telephone networks and data communication between computers because it is inexpensive and easy to install.

Advantages of twisted pair cable

-       Data cannot be easily distorted due to reduced noise interface.

-       Twisted pair wire is inexpensive.

-       Easy to install.

-       Used in transmission of data and voice

Disadvantages

-       Susceptible to noise.

-       Slows data transmission between devices.

-       Limited to short distances.

2. COAXIAL CABLE.  A coaxial is a high quality communication line that consists of a single copper wire conductor surrounded by atleast three layers;

-       A non conducting insulating material

-       A woven or braided metal outer conductor

-       Plastic outer coating

                                                                        Fig:  Coaxial cables

It is insulated more heavily than twisted-pair.  It is not susceptible to electrical interferences and transmits data faster over longer distances.  Cable TV wiring often uses coaxial cable because it can be cabled over long distances than twisted-pair cable.

Advantages of coaxial cables

-       Can be cabled over long distances.

-       Less susceptible to electric interference.

-       Can transmit much more data at a time

Disadvantages

-       More expensive than a twisted pair cable.

-       Not easy to install

-       Need boosters to transmit data.

-       They are bulky

3. FIBRE OPTIC CABLE.  This consists of dozens of hundreds of smooth thin strands of glass or plastic that use light to transmit signals.  Each strand called an optical fibre is as thin as human hair.  An insulating glass cladding and a protective coating surround each optic fibre. Each optic fibre can carry several hundred thousand-voice communications simultaneously.  Fibre optic cables are used by many local and long distance telephone companies, cable Tv and in high traffic networks or as the main cable in the network.

Figure:  Fibre optic cable

Advantages of fibre optic cable

-       Carry significantly more signals than wireless cables.

-       Less susceptible to noise

-       Better security for signals during transmission

-       Smaller size and much thinner and lighter than wire cables.

Disadvantages

-       Cost more than wire cables

-       Difficult to install and modify.

-       More fragile than other wire based communication channels.

4. COMMUNICATION SATELITES. These receive microwave signals from earth based communications facilities, amplify the signals and retransmit the signals back to the communications facilities.  The earth based stations use large dish shaped antennas to transmit and receive data from satellites.  The transmission to the satellites is called uplink and the transmission from the satellite is called a down link.

Communication satellites are usually placed about 22,300 miles above the earth’s equator and moves at the same rate as the earth.

Applications of communication satellite

▪      Television.

▪      Radio broadcast.

▪      Video conferencing.

▪      Paging and global positioning systems.

Advantages of Communications satellites

-       Lots of data can be sent simultaneously

-       Allow high quality broadband communication across continents.

-       Covers a large geographical area for data transmission.

-        

Disadvantages of communication satellite

-       The fee to launch a satellite is extremely expensive.

-       The infrastructure needed to access satellite communications is also expensive.

5. MICROWAVES.  These are radio waves that provide a high-speed transmission of both voice communications and data signals.  Microwaves transmission involves sending signals from one earth based microwave station called a terrestrial microwave to another.  It is fast (up to 4,500 times faster than a dial-up modem) but it limited to line of sight transmission, which means that the micro wave must transmit a straight line with no obstructions such as buildings between microwave antennas.  To avoid obstructions, microwaves stations are often located on tops of buildings, towers or mountains to avoid possible obstructions.

Advantages of microwaves

-       Provides high-speed communication transmission.

-       No need to install cable.

-       Lower installation and maintenance costs.

-        

Disadvantages of microwaves

-       Limited to line-of-sight transmission

-       May be affected by temporary atmospheric disturbances

N.B

(a)   BASEBAND TRANSMISSION.  This transmits only one signal at a time.

(b)   BROADBAND TRANSMISSION.  This can transmit multiple signals simultaneously.  They transmit signals at a much faster speed.  Two widespread applications of broadband transmission are;

-       Digital subscriber lines

-       Cable television networks.

(c)    SIMPLEX TRANSMISSION.  In simplex transmission, data flow in one direction from the sending device to the receiving device.  It is used only when the sending device does not require a response from the receiving device.  Examples include:

▪      Security systems

▪      Fire alarms

▪      Temperature sensors that contain a sensor

▪      Printing systems, pagers.

(d)   HALF DUPLEX TRANSMISSION.  In half duplex transmission, data can flow in either direction, from the sending device to the receiving device and back but only in one direction at a time.  Examples include:

▪      Fax machines

▪      Radio calls

▪      Credit card verification systems.

▪      Automatic teller machines

(e)    FULL DUPLEX TRANSMISSION. In full duplex transmission, data can flow in both directions at the same time.  A regular telephone line supports full duplex transmission so that both parties can talk at the same time.  It is used for most interactive computer applications and for computer-to-computer data transmission i.e a regular telephone line.