CCM 4300 Lecture 2Computer Networks: Wireless and Mobile Communication

Systems

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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.

Session Content

� Classification of computer networks

� Switched

� circuit switching

� packet switching

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� packet switching

� broadcast networks

� scale

� Network standards

Lesson objectives

� To introduce a taxonomy of computer network :

�identify basic transmission technologies

�Classify different networks based on their scale.

�Understand the importance of standards

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�Understand the importance of standards

� There is no generally accepted taxonomy into which all

computer networks fit, but two dimensions stand out as

important:

� Transmission technology

� Switched network

Classification of computer networks

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� Switched network

� Broadcast network

� Scale

�LANs, MANs, WANs, HANs, PANs, CANs, WLANs

� Switched networks

– data are transferred from source to destinationthough a series of intermediate nodes. The nodesprovide the switching facility that will move data totheir destination

circuit-switched network – a dedicated, physical

Transmission Technology

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circuit-switched network – a dedicated, physicalcircuit is first established between the source anddestination nodes before the data transmissiontakes place

packet-switched network – messages are firstpartitioned into smaller units called packets, whichare that sent to the destination node one at a time viaintermediate switches

Circuit Switching

End-end resources reserved for “call”

� call setup required

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� call setup required

� dedicated resources: no sharing

� Release of resources

Circuit Switching

network resources (e.g., bandwidth) divided into “pieces” and pieces allocated to calls

� For instance we can divide link bandwidth using

� frequency division

time division

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� time division

� One of the main advantages of circuit switching is that some QoS can be guaranteed

� Example of Circuit switching is the PSTN (thetelephone network). Here each user has 64Kbsguaranteed during the call

� What are the disadvantages of Circuit switching?

Why packet switching?

� Computer data transmission is normally bursty innature

� Bursty traffic refers to an uneven pattern of datatransmission: sometime very high data transmissionrate while other time it might be very low.

In this case circuit network is wasteful of network

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� In this case circuit network is wasteful of networkbandwidth since must have sufficient channelbandwidth to handle burst level of traffic which isthen used most of the time

� The solution is packet switching which allowssharing

Packet Switching

A

B

C10 MbsEthernet

1.5 Mbs

45 MbsPackets are bufferedwaiting for a free output

link

When the link is free the packet is sent

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� Packet-switching is like sending a standard letter through the post

� other analogies?

D E

� This schema of data transfer is often called store-and-forward

Packet Switchingeach end-end data stream divided into packets

� user A, B packets sharenetwork resources

� each packet uses full link

resource contention:

� aggregate resource demand can exceed amount available

� congestion: packets

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Bandwidth division into “pieces”

Dedicated allocation

Resource reservation

� each packet uses full link bandwidth

� resources used as needed

� congestion: packets queue, wait for link use

Packet Switching

• What if messages were sent as single units?

• Very long packets can increase the delay

incurred at each switch until the link is free

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• Short packets can reduce the delay and fixed

packets can allow hardware optimisation

Packet switching versus circuit switching

� E.g., 1 Mbit link

� each user:

� 100Kbps when “active”

� active 10% of time

Packet switching allows more users to use network!

1−35

i

p

i1 − p( )

35− i

i=0

10

∑

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� active 10% of time

� circuit-switching:

� 10 users

� packet switching:

� with 35 users, probability > 10 active less than .0004� (probability that out of 35, ten or less more transmitting at the same time is

0.0004)

N users

1 Mbps link

Packet switching vs circuit switching

A circuit-switched network can guarantee a certain amountof end-to-end bandwidth for the duration of a call.

Most packet-switched networks today (including theInternet) cannot make any end-to-end guarantees forbandwidth.

Why should I choose circuit switching?

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bandwidth.

Extra mechanisms are required for flow control, congestioncontrol, and QoS.

A circuit-switched network is suitable for application withlong sessions with predictable smooth band-width requirements.

Packet switching vs circuit switching

When connections are short-lived the setup delaymay represent a large part of the total connectiontime, thus reducing the network's capacity.

Why should I choose packet switching?

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time, thus reducing the network's capacity.

Reserved resources cannot be used by any otherusers even if the circuit is inactive, which mayfurther reduce link utilisation.

� Broadcast networks

– there are no intermediate switching nodes. The

network share a single communication medium or

channel

packet radio and satellite networks – stations

Transmission Technology

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packet radio and satellite networks – stations

transmit and receive via antenna and share the same

channel or radio frequency

Local Area Networks (LANs) – transmission by any

one station propagates the length of the medium

� Local Area Networks (LANs)

� LANs interconnects computing

resources within a moderately (IEEE

usually relates this to 10 km or about

6 miles or less in radius) sized

geographical area

Scale

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� This can include a room, several

rooms within a building, or several

buildings of a campus (e.g. Hendon

campus)

� Various topologies are possible for

LANs such as bus, star, ring, mesh

� MANs interconnects computing resources that span a

metropolitan area

� The main reason for ever distinguishing MANs as special

category is that a standard had been adopted for them

(called Distributed Queue Dual Bus (DQDB, IEEE 802.6)

Metropolitan Area Network (MAN)

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� MANs generally refer to networks that span a larger

geographical area than LANs but a smaller geographical

area than WANs

. A MAN might be owned and operated by a single

organization, but it usually will be used by many individuals and organizations.

� MANs might also be owned and operated as public

utilities. They will often provide means for internetworkingof local networks.

Metropolitan Area Network (MAN)

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of local networks.

� For instances this include

�Companies with buildings

located throughout a local

county or city

�A large campus

Internet StructureSprint London backbone network

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� WANs Interconnects computing resources that arewidely separated geographically (usually over 100 Km). This includes towns, cities, states and countries (e.g.The Internet)

� A WAN can be thought as consisting of a collection ofLANs

Wide Area Networks (WANs)

�Most WANs consists of twodistinct components:

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distinct components:

�transmission lines, whichmoves data between

nodes

�switching elements(router), which usesspecialised computers toconnect two or moretransmission lines.

WAN vs LAN

1. Two computers communicating through a WAN

2. Two computers communicating through a LAN

• Which computers are exchanging data faster?

• LAN usually operates at much higher ratescompared to WAN. This is mainly due to the

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compared to WAN. This is mainly due to theproximity of the computers and the lack ofcongestion. It is common to experience up to 90mbps in a LAN while achieving 10 to 20mbps isalready a great achievement for WAN.

WAN vs LAN

• Is a LAN more secure than a WAN?

• Conceptually LAN could be more securedue to the fact that all the computers are

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due to the fact that all the computers arewithin a specific area and are physicallyeasier to secure. The data on a wide areanetwork could pass insecure networks inorder to reach its intended destination.

PAN – Personal Area Network�A personal area network (PAN) is a computer network used forcommunication among computer devices, including telephones andpersonal digital assistants, in proximity to an individual's body.

�The devices may or may not belong to the person in question. PANscan be used for communication among the personal devices themselves(intrapersonal communication), or for connecting to a higher levelnetwork and the Internet (an uplink).

�A PAN covers the few meters that surround the user’s workspace

� It provides the ability to synchronise computers, access localperipherals such a printers and pocket devices

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peripherals such a printers and pocket devices

�It can be made possible using IrDA, Bluetooth, ZigBee, USB

� A home area network (HAN) is a residential local area network

�It is used for communication between digital devices typically deployed

in the home, usually a small number of personal computers and

accessories, such as printers and mobile computing devices.

� An important function is the sharing of Internet access, often a

broadband service through a cable tv or Digital Subscriber Line (DSL)

provider.

�Additionally, a home server may be added for increased functionality.

�Intended to provide the home with an infrastructure to interconnect a

variety of homes appliances

HAN – Home Area Network

variety of homes appliances

� Enable them to be accessed through the Internet through a central

home gateway.

� Therefore, a home computer network, will in the future allow multiple

computers as well as multiple devices to connect with a network

protocol.

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� provides access up to a range of few hundredmeters, e.g. an office block. 802.11b

�A single access point, wired at a fixed location,supports a number of wireless users within aspecified range without the need of cumbersome

CAN – Company Area Network

specified range without the need of cumbersomeEthernet cabling to form a star topology.

�CANs have also emerged as hotspots in variouslocations such as coffee shops and airportsproviding users with Internet access

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� Is a vehicle bus standard designed to allowmicrocontrollers and devices to communicate witheach other within a vehicle without a hostcomputer

� CAN uses message based protocols, designedspecifically for automotive applications but nowalso used in other areas such as industrialautomation and medical equipment.

CAN – Controller Area Network

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� A modern automobile mayhave as many as 70electronic control unit(engine control unit, airbags,cruise control). A CAN can beused to connect them together.

Network standards� “Many network vendors exist each with its own ideas

about how things should be done. Withoutcoordination there would be a complete chaos. Theonly way out is to agree on some networkstandards.”

Andrew S. Tanenbaum – Computer network 2003

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� A standard is a stated norm or requirements abouttechnical systems. It is usually a document thatestablishes uniform criteria, methods, processes andpractises.

� Not only standards allow different computers tocommunicate, but they also increase the market forthe products that comply with the standard

Network standards� Network standards fall into two categories:

� De facto (Latin for “from the fact”): they havenot been approved by any organised body butthey have been adopted as a standard because oftheir widespread use. For instance Unix, MP3,PDF, DOC, IBM PC format

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PDF, DOC, IBM PC format

� De jure standards (Latin for ‘by Law’): they areformal, legal standards adopted by someauthorised standardisation body.

Network Standards - cont� Recognised body can be classified into four major categories: (1) National, (2) Regional, (3) International, (4) Industry, Trade, and Professional.

� National Standards Organisations – generally responsible for standards within a nation

- British Standards Institute (BSI)

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- British Standards Institute (BSI)

- American National Standards Institute (ANSI)

Network Standards - cont�Regional Standards Organisation – restrict their activity to a specific geographical region but generally influences standards outside their regions

- European Telecommunications Standards Institute (ETSI) Previously CEPT

- European Committee for Standardization (CEN)

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- European Committee for Standardization (CEN)

- European Computer Manufactures’ Association (ECMA)

� International Standards Organization – promotes standards for worldwide use

- International Standards Organisation (ISO)

- International Telecommunications Union (ITU) formally known as the CCITT - Comité Consultatif Internationale Télégraphique et Téléphonique.

Network Standards

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Télégraphique et Téléphonique.

• ITU major divisions:

•Consists of ITU -T, which is responsible for

communications, interfaces, and other standards

relating to telecommunications

•I series - Integrated Services Digital Network

Network Standards

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•I series - Integrated Services Digital Network

(ISDN)

• X series - Data communication networks (e.g.

X.25)

• ITU – R, which is responsible for allocation of

frequency band in the electromagnetic spectrum for

telecommunications, and for making recommendation

relating to radio communications.

� Industry, Trade and Professional Standards Organizations –restrict their activity to member interest areas but generally influence other areas

- Electronic Industry Association (EIA)

- The Institute of Electrical and Electronics Engineers

- Institute of Electrical Engineers (IEE)

Network Standards - cont

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- Institute of Electrical Engineers (IEE)

- Internet Engineering Task Force (IETF)

� Standards organizations are composed of delegates, from the government, academia and vendors who will be developing products based on the proposed protocols.

� The formal standards process, which is designed to ensure that a consensus is reached, is often lengthy and sometimes can take years

for approval.

Network standards� Why do we need to bother with networking standards?

- enables interoperability

- independence from vendor proprietary approaches

- enables open procurement

-Standards should be international in scope

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-Standards should be international in scope

� Important to track emerging standards

- know when it is “safe” to use them

- need to know where they come from

� Classification of computer networks

� Switched

� scale

Summary!

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� Network standards

Are there any questions?