NAME: - IBRAHIM SA’ID AHMAD
REG NO: - SCI/06/COM/03361
COURSE: - CSC 4306 COMPUTER NETWORK DESIGN ASSIGNMENT
DATE: - 24/12/2010
QUESTION 1
Bandwidth: it is a capacity of the measurement of the width or the capacity of the communication
channel. Effectively, it means how much information can move across the channel ina given amount of
time. Example to transmit a text document, a slower bandwidth is acceptable, while to transmit audio or
video signals a wider bandwidth is needed. There are 3 categories of bandwidth:
Voice band also known as voice grade or low bandwidth, is used for standard telephone
communication. Micro computers with standard modems and dial – up service use this
bandwidth. It is effective for transmission of texts =. Typical speed is 56 – 96 kbps
Medium band is used in special leased lines to connect minicomputers and mainframes as well
as transmit data over long distances. Not typically used by individuals.
Broad band is used for high capacity transmission. Microcomputers with DLL, cable and satellite
connections and other more specialized high – speed devices use this bandwidth. It is capable of
meeting most of today’s communication needs including transmitting high quality video and
audio over long distances. Typical speed is 2.5 mbps and there is full capacity of much higher.
Types of connections commonly available
Dial – up services use standard telephone lines and conventional modems. They are quite slow
and inadequate.
T1, T2, T3, T4 support very high speed all – digital transmission. They do not require
conventional modem and they provide very high capacity, but very expensive.
More affordable technologies are
Digital subscriber line (DSL): uses existing telephone line. Widely used in all means. ADSL is the
most commonly used.
Cable modems: use existing cable lines to provide high speed connections, as fast as T1 and DSL,
and at much lower cost.
Satellite and air connection: uses satellite to download data.
QUSTION 1B
Elements of communication system: communication system is an electronic system that transmits data
over communication lines from one location to another. It usually comprises of 4 basic elements:
1. Sending and receiving devices
2. Communication channel
3. Connecting device
4. Data transmission specification
Numerous applications depend on communication system example E – Mail, instant message, internet
telephony, E – Commerce.
COMMUNICATION CHANNEL: it is an essential element of every communication system. It carries data
from one location to another. There are two categories of communication channel:
a) Physical connection: it uses solid medium to connect sending and receiving devices. These
include:
Telephone lines: it is made up of twisted pair cable which is made of hundreds of copper wires.
A single twisted pair culminates in the wall jack into which you can plug your phone or
computer. This have the standard transmission media for years for both voice and data but are
now being replaced by more technically advance and more reliable media.
Coaxial cable: it is high frequency transmission (communication) cable. It replaces the multiple
wires of telephone lines with a single solid copper wire. It has over 80 times the transmission
capacity of twisted pair. It is used to deliver TV signals as well as connect computers in a
network.
Fibre optic cables: Fibre optic cables transmit data and information as pulses of light through
tiny tubes of glass. In terms of telephone connections, it is 26, 000 times the transmission
capacity of twisted pair (about halt diameter of human hair). They are immune to electrical
interference which make them even more secured.
Compared to coaxial cables, they are lighter and more reliable. They transmit information using
beam of light at light speed instead of electricity. This makes them much faster than copper
cables. They are rapidly replacing twisted pair.
b) Wireless Connection: they do not use solid medium to connect the sending and receiving device.
Instead they us air. Primary technologies used for wireless connection are:
Infrared: uses infrared light waves to communicate over short distances. Sometimes
referred to as line-of-sight communication, because the light waves canonly travel in a
straight line. Most computers are now equipped with infrared port (IRDA port).
Microwave: uses high frequency microwave. Like infrared, it provides line-of-sight
communication because microwaves travel in straight lines. It can only be used over
short distances in which microwaves are relayed by means of microwave stations with
microwave dishes which can be installed on towers. Bluetooth is a short range wireless
communication that uses microwave to transmit over short distances of up to 35 feets.
Broadcast radio: uses special sending and receiving towers called transceivers.
Transceivers send and receive radio signals from wireless devices. E.g. cellular and many
web-enabled device use broadcast radio to place call
Satellite: uses satellite orbiting about 22,000 miles above the earth as microwave relay
station, many of these are offered by Intelsat(International satellite consortium) which is
owned by 114 governments and forms a worldwide communication system. GPS is one
of the most interesting application of satellite communication.
CONNECTION DEVICES: a great deal of computer communication takes over telephone lines. Telephone
sends and receives analog signals which are continuous electric waves. But computers send and receive
digital signals which take only finite sets of discrete values. To converts digital to analog and vice versa,
we need a modem. The word modem is a short form of modulator – demodulator. The modem allows
digital micro computers to communicate across analog telephone lines. This communication include
both voice and data communication. The speed with which modems transmit data varies. This speed,
also known as transfer speed or transfer rate is measured in bits per second (bps).
There are four basic types of modems, which are external modems, internal modems, PC card modem
and wireless modem.
Audio over long distances. Typical speed is 2.5 mbps and there is full capacity of much higher.
Types of connections commonly available
Dial – up services use standard telephone lines and conventional modems. They are quite slow
and inadequate.
T1, T2, T3, T4 support very high speed all – digital transmission. They do not require
conventional modem and they provide very high capacity, but very expensive.
More affordable technologies are
Digital subscriber line (DSL): uses existing telephone line. Widely used in all means. ADSL is the
most commonly used.
Cable modems: use existing cable lines to provide high speed connections, as fast as T1 and DSL,
and at much lower cost.
Satellite and air connection: uses satellite to download data.
DATA TRANSMISSION SPECIFICATIONS: several factors affect how data is transmitted. These factors
include bandwidth, data flow, data direction and protocols.
a) Bandwidth: it is a capacity of the measurement of the width or the capacity of the
communication channel. Effectively, it means how much information can move across the
channel in a given amount of time. Example to transmit a text document, a a slower bandwidth
is acceptable, while to transmit audio or video signals a wider bandwidth is needed. There are 3
categories of bandwidth:
Voice band also known as voice grade or low bandwidth, is used for standard telephone
communication. Micro computers with standard modems and dial – up service use this
bandwidth. It is effective for transmission of texts =. Typical speed is 56 – 96 kbps
Medium band is used in special leased lines to connect minicomputers and mainframes as well
as transmit data over long distances. Not typically used by individuals.
Broad band is used for high capacity transmission. Microcomputers with DLL, cable and satellite
connections and other more specialized high – speed devices use this bandwidth. It is capable of
meeting most of today’s communication needs including transmitting high quality video and
audio over long distances. Typical speed is 2.5 mbps and there is full capacity of much higher.
b) Data Flow: data can flow in one of two ways, either serially or parallel;
Serial data transmission: in this transmission, bits flow in series or continuous stream like vehicles
crossing a single lane bridge. Thos is the way most data is sent over telephone lines. For this reason,
external modems typically connect microcomputers via a serial port. More technical names for the serial
port are RS – 232C connector or Asynchronous communication port (ACP).
Parallel transmission: in this transmission, bits flow via separate lines simultaneously. In other words,
bits resemble vehicles moving together at the same speed on a multilane highway. Typically limited to
transmission over short distance and not used for telephone lines.
c) Data direction: There are 3 directions or nodes of data transmission.
Simplex communication( one direction only)
Half – duplex communication (both directions but not simultaneously)
Full duplex communication ( both directions simultaneously)
d) Protocols: what, when and how should A send or receives message from B. For data
transmission to be successful, sending and receiving devices must follow a set of communication
rules for the exchanging of information. These rules for exchanging data between computers are
called protocols. The standard protocol for the internet for instance is the TCP/IP protocol. The
essential features of this protocol involve:
Identification: The sending devices and the receiving devices using unique IP addressing
systems.
e) Reformatting: information sent across the internet usually travels through numerous
interconnected networks. Before it is sent, it is reformatted, or broken down into small ports
called packets. Each packet is then sent separately over the internet possibly travelling different
routes to one common destination. At the receiving end, the packets are reassembled into the
correct order of the original message. There are numerous other protocols used, example HTTP,
HTTPS, FTP, WAP.
The behaviors that every protocol governs include, access initiation, flow control, acknowledge,
failure handling and error handling.
7A) If the data-link layer provides error checking and the transport layer provides error
checking, isn’t this redundant? Explain. Similarly, the data- link layer provides flow
control, and the transport layer provides flow control. Are these different forms of flow
control? Explain.
No, it is not redundant. Error detection is the process of detecting whether errors occurred during
the transmission of the bits across the wire. The Data Link layer uses a calculated value called
the CRC (Cyclic Redundancy Check) that's placed into the Data Link trailer that's added to the
message frame before it's sent to the Physical layer. The receiving computer recalculates the
CRC and compares it to the one sent with the data. If the two values are equal, it's assumed that
the data arrived without errors. Otherwise, the message frame may need to be retransmitted
under control of an upper layer. Although the Data Link layer implements error detection, it does
not include a function to perform error recovery which is primarily left on the Transport layer. In
addition to detection of error using the above method, transport layer provides recovery function
to recover the lost data. With this we can conclude by saying that even though they both perform
error checking, the error checking performed at data link layer does not include error recovery
unlike transport layer that does recovery making them not to be redundant. Also the data link
layer performs error checking when data arrives at the next computer on the transmission path.
The transport layer performs the error checking only when the data arrives at the final destination
machine.
Yes, there are two different flow controls. The transport layer of the OSI model manages the
mechanisms used to control the flow of data between two hosts. The flow control mechanisms
used in the transport layer vary for the different classes of service and the flow control becomes a
much more complex issue at this layer than at lower levels like the data link level because; Flow
control must interact with transport users, transport entities, and the network service, and also
there is long and variable transmission delays between transport entities
While Data Link layer defines the data values used in the flow control signaling between two
transmitting hosts. Base on the above explanation, it can be concluded that the two layers in
question handled flow control it different ways.
7B Describe the DQDB protocol. How is it different from token passing?
A distributed-queue dual-bus network (DQDB) is intended for use with a dual bus configuration,
where each bus transport data in only one direction and has been standardized by one of the
institute of electrical and electronic engineering as part of it’s MAN standard. Transmission on
each bus consist of a steady stream of fixed-sized slots which are generated at one end of each
bus and marked free and sent downstream, where they are marked busy and written to by nodes
that are ready to transmit data. Nodes read and copy data from the slots, which then continue to
travel toward the end of the bus, where they dissipate.
The mechanism is handled by a pair of first-in, first-out queues and a pair of counters, one for
each bus, at each node in the network. This is a very effective protocol providing negligible
delays under light loads and predictable queuing under heavy loads.
On the other hand, token passing network involves the generation of an electronic message
called “token” each time the network is turned on and is then passed along from node to node.
Only the node with the token is allowed to transmit, and after it has done so, it must pass the
token on to another node. These networks typically have either a bus or ring topology
References:
Computer networks by Andrew S Tanenbaum
Understanding operating systems by Ida M. Flynn/Ann McIver McHoes
http://en.wikipedia.org/wiki/CSMA/CD
