CCM 4300 Lecture 4Computer Networks, Wireless and Mobile Communication

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and Mobile Communication Systems

Dr Shahedur Rahman

School of Science and Technology

Recap of Last Session

�defined a computer network and identified some of

the basic components.

� explored the history of computers and computer

networks and how they have evolved.

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networks and how they have evolved.

� identified some of the advantages and disadvantages

of using computer networks.

� introduced the various network standards, how they

are created and by whom

Session Content� Lesson Objectives

� Physical layer

� Analogue or Digital?

� bandwidth and throughput

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� bandwidth and throughput

� Twisted pair, Coaxial cabling and optical fiber

� Data link layer and access control

�MAC – CSMA/CD, Token Passing

Lesson objectives� At the completion of this lesson you should be able to

- understand the difference between analogue and digital communication

- understand the ISO Reference Model Physical Layer specification

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- understand the ISO Reference Model Data Link Layer specification

- understand the concept of contention protocol

Physical Layer

� It defines everything that is required to support the

transmission and reception of signals (i.e. 1s and 0s)

� The Physical layer has four functional areas:

1. Electrical – signal type, amplitude, etc

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2. Mechanical – connectors, cabling, etc

3. Procedural – control and timing

4. Functional - requirements for activating, maintaining, and

deactivating a physical link between end systems.

� The physical layer is usually a combination of software

and hardware programming and may include

electromechanical devices.

Physical Layer - cont

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� All wiring, power, cabling and connections are part of the

physical layer. Without the physical layer functioning

properly none of the upper layers will respond correctly.

� It has no mechanism for determining the significance of

the bits it transmits or receives. The onus for this is

passed on to higher layer protocols

If you were given one word to describe the

physical layer what would it be?

Hint: Think like an electrical engineer!

Question?

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Physical Layer FunctionFor transmission, the physical layer generally:

� convert framed data from Data Link Layer to a

binary stream

� transmit framed data serially (that is, one bit at

a time) as a binary system

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a time) as a binary system

For reception, the physical layer generally:

� listens for inbound transmission that are

addressed to its host device

� accept appropriately addressed streams

� pass the binary stream up to the Data Link

Layer for reassembly into frames

Analogue or Digital?

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� The term analogue refers to any physical device or signal

that can continuously vary in strength or quantity, for

example, voltage in a circuit

Analogue or Digital? - cont

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Source :

Data Communications,

Computer Networks and

Open Systems

1992

� The term digital refers to any physical device

or signal that is coded in a binary form (i.e. 1s

and 0s)

What is “speed” or capacity?

� In analogue communication bandwidth is the total capacity (or theoretical capacity) of a communication

channel.

bandwidth = highest frequency – lowest frequency

The greater the bandwidth, the more signals

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The greater the bandwidth, the more signals that can be carried

� Example:

Typical ordinary telephone lines (often called a voice-

grade line) transmit frequencies from 300Hz to

3300Hz.

bandwidth = 3300Hz – 300Hz => 3000Hz, or 3kHz

What is “speed” or capacity? - cont

� In digital communication, bandwidth is referred to as

data rate

� Data rate – amount of data that can be transmitted over

a communications medium in a given period.

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a communications medium in a given period.

� Data rates measured in bits per second (bps) an can

vary considerably from one type of channel to another.

� For example, the bandwidth of dialup connections using

a modem ranges from 300bps to 33,600bps (33.6kbps) or

56kbps.

What is “speed” or capacity? - cont

� If we measure data rates in bits per second, then what is

baud rate?

� The speed in baud is equal to the number of times the line

condition (i.e. frequency, amplitude, voltage, or phase) changes

per second. i.e., the number of distinctive events per sec.

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� Named after French engineer Jean Maurice Emile Baudot

(1845 – 1903)

� For example a communication channel transmitting at 2400

baud. If each signal is used to represent one bit, then the baud

rate is equal to the data rate - 2400bps.

� If each signal represents four bits, then the baud rate – 2400,

but the data rate is 4x2400bps = 9600bps.

What is “speed” or capacity? - cont

� Is there any difference between bandwidth and throughput?

� Bandwidth represents a theoretical capacity of a

communications channel.

The “reality rate” is known as throughput.

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� The “reality rate” is known as throughput.

� Just because a medium or LAN architecture is specified to

operate at a certain data rate, it is not a valid assumption to

assume that this rate will be the actual throughput achieved.

Transmission medium

� Transmission medium can be:

� Simplex

- transmission in one direction only

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- transmission in one direction only

� Half-duplex

- transmission in both direction; but not at the same

time

� Full-duplex (duplex)

- simultaneous transmission in both directions.

Twisted Pair

� Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular

� Two insulated wires are twisted around each other, and

combined with others into cable

� Each pair is twisted with a different number of twists per inch -

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� Each pair is twisted with a different number of twists per inch -

eliminates interference from adjacent pairs and other electrical

devices.

Unshielded twisted pair RJ-45 Connector

Twisted Pair......cont.....

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Twisted Pair - cont

� Several techniques can be used to improve throughput:

� Increase the thickness of the conductor

� Increase the twist of rate

� Use several different twist rates in bundle of multiple pairs

� Shield the pairs with a metallic barrier

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� Shield the pairs with a metallic barrier

Type Use

Category 1 Voice Only (Telephone Wire)

Category 2 Data to 4 Mbps (LocalTalk)

Category 3 Data to 10 Mbps (Ethernet)

Category 4 Data to 20 Mbps (16 Mbps Token Ring)

Category 5 Data to 100 Mbps (Fast Ethernet)

Coaxial Cable

� Coaxial cabling has a single copper conductor at its center

� A plastic layer provides insulation between the center

conductor and a braided metal shield.

� The metal shield helps to block any outside interference

from fluorescent lights, motors, and other computers

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from fluorescent lights, motors, and other computers

Bayone-Neill-Concelman (BNC) Coaxial cable

Coaxial Cable

� The two types of coaxial cabling are thick coaxial and thin

coaxial (refers to diameter - 0.25inch and 0.5inch).

- Thin coaxial cable is also referred to as thinnet. 10Base2

refers to the specifications for thin coaxial cable carrying

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Ethernet signals. The 2 refers to the approximate maximum

segment length being 200 meters.

- Thick coaxial cable is also referred to as thicknet. 10Base5

refers to the specifications for thick coaxial cable carrying

Ethernet signals.

Optical Fibre

� Optical fiber is a thin, flexible medium capable of conducting

an optical ray

� It transmits light rather than electronic signals eliminating the

problem of electrical interference

� Very high bandwidth (currently up to 10Gbps)

Used for long-distance trunks, local area networks, high-

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� Used for long-distance trunks, local area networks, high-

speed transmissions

Fibre Optic CableSC Connector

ST Connector

Optical Fibre Connector

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SC (SubscriberConnector) Connector

SC Duplex Connector

ST (Straight Tip) Connector

Question?

� Which of the following is not defined at the physical layer of

the OSI reference model?

A. hardware addresses

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B. bitstream transmission

C. voltage levels

D. physical interface

Data Link Layer � Regulates and format transmission of data from software on

a node to the network cabling facilities.

� It acts like a “Glue” between the wire and the software on a

node.

� Some of the services the data link layer provides to the

network layer include:

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network layer include:

� framing – involves partitioning data into frames with

recognized frame boundaries and exchange these

frames over the link

� frame sequencing – involves maintaining the correct

ordering of frames as they are being exchanged

� establishing and maintaining an acceptable level of flow

control as frames are being exchanged across a link

Data Link Layer - cont

� detecting (and possibly correcting) errors in the

physical layer, which includes error notification when errors

are detected but not corrected

� selecting quality of services (QoS) parameters

associated – ensuring sufficient bandwidth is available and

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that transmission delays (i.e latency) are predictable and

guaranteed.

the data link layer enables data frames to be transmitted error-free between two end nodes over the physical layer

Data Link Layer - cont

� How is the data link layer implemented within a network?

� Typically implemented on a node as device drive (i.e.

firmware layer of the network interface card), which is a

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firmware layer of the network interface card), which is a

software component that is specified to both a piece of

hardware (e.g. network interface card), and the

operating system of the computer in which it is installed.

OSI Reference Model vs IEEE 802 Model

� IEEE initiated its development of the LAN standards

with an architectural model, defined in IEEE 802.1

� The architectural model corresponds to the two

lowest layers of the OSI Model with the following

differences:

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differences:

� The IEEE divides OSI’s data link layer into two parts –

the logical link control (LLC) and the medium access

control (MAC) sublayers

Note:

The MAC sublayer has nothing to do with Apple

Computer’s Machintosh

OSI Reference Model vs IEEE 802 Modle - cont

Network Layer

Data Link Layer

Network Layer

Logical Link Control

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Data Link Layer

Physical Layer

Physical Layer

Media Access Control

OSI Model

IEEE 802 Model

LLC and MAC

� The LLC sublayer (i.e. upper half of data link layer)

encompass several functions – framing, flow control and error

control

� The MAC sublayer (i.e. lower half of data link layer) provides

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� The MAC sublayer (i.e. lower half of data link layer) provides

media access management protocols for accessing a shared

medium.

Logical Link Layer - Framing � Framing enables synchronize the transmission and reception

of data since frames have detectable boundaries.

� Integrity of frames - detection and correction

Data set to be transmitted: 1101110011011011

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Thus, the frame to be transmitted is:

01111110 1101110011011011 01111110

Start of frame User Data End of Frame

Logical Link Layer – Error Control� The term error control refers to the process of guaranteeing

reliable data delivery

� Two basic strategies:

� error control through retransmission (also known as error-

detecting codes) – provides enough information in the data

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stream to detect errors during transmission (e.g. parity,

cyclic redundancy check (CRC))

� autonomous error correction (also known as error-

correcting codes) – provides redundant information in the

data stream to detect and correct any errors autonomously

(e.g. hamming distance)

Media Access Control� The MAC sublayer provides the protocol that define the

manner in which nodes share the single physical transmission

medium.

� The IEEE 802 specifications recognizes three different forms

of media access:

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� Contention

� Demand priority (not so common anymore)

� Token passing

Media Access Control - Contention

� Contention based media access is embodied in the Carrier

Sense, Multiple Access with Collision Detection (CSMA/CD)

scheme 802.3

� As its name implies:

- requires station to check the wire to determine whether

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- requires station to check the wire to determine whether

any other station is already sending data

- If the station can sense an available carrier signal on a

wire, it is free to transmit

� The sensing of carrier signal does not necessarily guarantee

a free transmission media – collide with previously transmitted

signal on what appeared to be an idle medium.

Media Access Control – Token Passing

� Project 802’s specifications include three different token-

based protocols – 802.4 Token Bus, 802.5 Token Ring, and

Fiber Distributed Data Interface (FDDI); generally pronounced

as a word; fiddy.

A token is a special frame that is passed from device to

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� A token is a special frame that is passed from device to

device, in sequence along the ring.

� It can circulate only when the ring is idle

� A device must have this token to place data frames on the

network.

MAC Protocols: a taxonomy

Access Method

Random Access Taking-turnProtocols /

Channel

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Random Access Protocols / Contention Protocols /

Round Robin

CDMAFDMATDMA

TokenBus

TokenRing

SlottedRing

Channel Partitioning / Reservation

CSMA/CDAlohaSlotted

Aloha

Goal: efficient, fair, simple, decentralised

Desirable MAC characteristic

||| When only one node tx – throughput R bps

||| When M nodes tx – each node throughput

R/M bps (average tx rate over suitable

defined interval of time)

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||| Decentralised protocol – no master to bring

system down

||| Simple protocol – inexpensive to implement

Random Access protocols||| When node has packet to send

- transmit at full channel data rate R.

- no a priori coordination among nodes

||| two or more transmitting nodes -> “collision”,

||| random access MAC protocol specifies:

- how to detect collisions

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- how to detect collisions

- how to recover from collisions (e.g., via delayed retransmissions)

||| Examples of random access MAC protocols:

- pure ALOHA

- slotted ALOHA

- CSMA and CSMA/CD

We will be looking at these issues in the next lecture.

�Physical layer and its function

� Analogue or Digital?

�Speed, data and baud rate, bandwidth and throughput

�Transmission media - Twisted pair, Coaxial cabling and

Summary!

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�Transmission media - Twisted pair, Coaxial cabling and

optical fiber

� Data link layer, OSI reference model, LLC and MAC,

Token Passing, and access control methods.

Are there any questions?