COSC 3213: Computer Networks I

Course Instructor: Amir Asif

Teaching Assistant: TBA

Contact Information: Instructor: Section M Teaching Assistant:Office: CSE 3028 TBAasif@cs.yorku.ca(416) 736-2100 X70128

URL: http://www.cs.yorku.ca/course/3213Text: A. Leon-Garcia and I. Widjaja, Communication Networks:

Fundamentals Concepts and Key Architectures, McGraw Hill, 2nd edition

Class Schedule: TH 10:00 � 11:30 a.m., Room PSE 321

Assessment: 15% Assignment / Quiz; 35% Mid-term Exam; 50% Final Exam

Office Hours: Instructor: CSE 3028, TH 15:00 � 16:00TA: TBA

2

Course Objectives

! Introduce communication networks and understand how different components work including the underlying technology including hardware and software used.

! Review basic concepts of digital communications including channel encoding, modulation, error detection and error correction schemes.

! Understand different topologies used in local area networks (LAN)

! Comprehend how an internet (wide area network or WAN) is formed

! Present various networking standards

! Transmission Control Protocol / Internet Protocol (TCP/IP)

! Open Systems Interconnection Reference model (OSI)

! Demystify terminology !!!!!!

3

Course Outline

Circuit Switching, Message Switching, and Packet Switching. OSI and TCP/IP Reference Model. Theoretical limits (Nyquist Signaling rate and Shannon Channel Capacity theorems). Time and Frequency Representations of Signals. Intro to Fourier Analysis. Line Coding (Unipolar NRZ, Polar NRZ, Bipolar, Manchester, Differential Manchester) Digital Communication (ASK, PSK, FSK). Analogue Communication (AM, FM, PM). Modems (V.90) and Transmission Media (Microwave, Twisted pair, Coaxial, Optical Fiber). Error Detection (Parity Bit Checking, 2D Parity Codes, Checksum, Polynomial Codes, CRC). Flow Control (ARQ, Sliding-window, Go-back-N, Selective Repeat). Multiplexing: FDM and TDM; Group, Supergroup, and Mastergroup configurations in FDM;

DS1, DS2, and DS3 configurations in TDM. Telephone Network and Circuit Switches: Space Division Switches, Time-Division Switches. LAN: MAC, Data Link Layer and Logical Link Control (LLC). LAN: Medium Access Control I � ALOHA, Slotted ALOHA, CSMA, CSMA/CD LAN Technologies: Ethernet (802.3), Token Ring (802.5), Wireless LANs (802.11) TCP/IP: Network and transport layers (TCP, UDP and IP Protocols)

4

What is a Communication Network (1)?

! Communication network is a set of equipment, software, and facilities that allows transfer of information between geographically distant users.

! Why Communication networks:

Resource Sharing: Data sharing between distant sites.

High Reliability: Provide alternative sources of data.

Parallel Processing: Use multiple computers for a single application.

Scalability: Ability to increase system size based on demand.

! Familiar Applications:

Radio and TV broadcasting: Single source transmits to multiple users; Real-time (low latency), Unidirectional (simplex).

Telephone connection: One to one connection (connection-oriented), Real-time (low latency), Bidirectional (Duplex), Fixed location

Cellular Telephone: One to one connection through microwave (roaming), similar in service to telephone connection

5

What is a Communication Network (2)?

! Examples (contd):

Electronic mail (Email): Not real-time, Not connection-oriented, Reliable.

World wide web (WWW): accessed through a uniform resource locator (URL) identifying a home page developed using a hypertext language.

Video-on-demand: Not real-time, Unreliable.

Streamed audiovisual services:

Audio and video conferencing: Real-time, Unreliable.

Peer-to-peer applications such as Napster, Gnutella, Kazaa file exchange

Searching for ExtraTerrestrial Intelligence (SETI)

6

Services versus Applications

! Service: is defined as the basic information transfer capability of a communication network. Internet transfer of individual block of information. Internet reliable transfer of a stream of bytes. Real-time transfer of a voice signal.

! Applications use communication services to built a complete feature, hiding most of the technical details from the users. E-mail and web built on reliable stream service. Fax and modems built on basic telephone service.

! Newer applications use multiple (sometimes heterogeneous) networks. SMS builds on Internet reliable stream service and cellular telephone text

messaging

7

Representation of a Communication Network

! Equipment (hardware & software) and facilities that provide the basic communication service is represented by a white cloud.

! The equipment inside a communication network is virtually invisible to the user.! Equipment includes repeaters, routers, bridges, servers, switches, multiplexers, hubs,

modems, interconnecting wires, �! Network architecture specifies the plan used to built and operate a communication network.

TCP/IP and OSI are example of network architectures.! Since the overall communication process is complex, network architecture partitions the

overall communication process into separate functional areas called layers. TCP/IP consists of five layers, while OSI has seven layers.

CommunicationNetwork

Host Server

8

Evolution of Network Architectures (1)

1.0E+00

1.0E+02

1.0E+04

1.0E+06

1.0E+08

1.0E+10

1.0E+12

1.0E+14

1850 1875 1900 1925 1950 1975 2000

Telegraphnetworks

Telephonenetworks

Internet, Optical& Wireless

networks

Info

rmat

ion

trans

fer r

ate

inbi

ts p

er se

cond

(bps

)

Next Generation

Internet

?

9

Evolution of Network Architectures (2)

Communication networks can basically be classified in the following categories:1. Telegraph Networks

Used for digital transmission. Uses message switching to transfer data from one node to the next one.

2. Telephone Networks Used for transmission of both analog and digital information. Uses circuit switching to transfer data.

3. Internet Primarily used for computer applications. Uses packet switching to transfer data. Analogous to the postal mail service. Provides �best-effort� service with equal priority to every user.4. Next-Generation Internet Planned to provide �prioritized communication� instead of �best effort� service. Will be based on multiservice packet switching network technology.

10

Telegraph Networks and Message Switching (1)

! Pre-telegraph services used physical transport of the message using messenger pigeons, pony express, or ship service.

! In telegraphs, the message is typically converted into a digital signal and then transmitted across a network.

! In the eighteen century, optical telegraphs were initially used.1. Claude Chappe invented the first optical telegraph in the 1790�s.2. Semaphore mimicked a person with outstretched arms with flags in each

hand.3. Different angle combinations of arms and hands generated hundreds of

possible signals.4. Code for enciphering messages kept secret.! Later, electrical telegraphs using electrical signals were invented.1. William Sturgeon Electro-magnet (1825): used electrical current in a wire

wrapped around a piece of iron to generate a magnetic force.2. Joseph Henry (1830): transmitted current over 1 mile of wire to ring a bell.3. Samuel Morse (1835) used pulses of current to deflect electromagnet to

generate dots & dashes.

Optical Telegraph

Electric Telegraph

11

Telegraph Networks and Message Switching (2)

! In electric telegraphs, the message is typically converted into a digital signal and then transmitted across a network.

! Initially, Morse code was used to encode the alphanumeric message.! Electrical signals were used to transmit the alphanumeric message.! Later on 7-bit ASCII code was used to encode the messages.

— — — — —0· — — — —1· — ·R· ·I

— — — — ·9— — · ·Z— — · —Q· · · ·H

— — — · ·8— · — —Y· — — ·P— — ·G

— — · · ·7— · · —X— — —O· · — ·F

— · · · ·6· — —W— ·N·E

· · · · ·5· · · —V— —M— · ·D

· · · · —4· · —U· — · ·L— · — ·C

· · · — —3—T— · —K— · · ·B

· · — — —2· · ·S· — — —J· —A

Morse Code

Morse Code

Morse Code

MorseCode

12

Telegraph Networks and Message Switching (3)

! Since point-to-point communications between every pair of subscriber is physically unrealizable, network nodes were created where several telegraph lines met.

! Each node operated on the principle of the store-and-forward operation.1. Message arriving on each line was decoded to extract the destination address.

(addressing)2. Next-hop in route determined by the destination address of the message (routing).3. Each message was stored until the next-hop is available for transmission (store).4. On availability of the next-hop, entire message is transmitted to the next node

(forwarding).

Nodes/Switches

Message

Destination

Source

Message

Message

Message

Message Switching

13

Telephone Networks and Circuit Switching (1)

! Alexander Graham Bell (1875) discovered that voice signals can be transmitted over electrical wires.

! Microphones used to voice pressure variations (sound) into an analogous electrical signal.

! Electrical circuits communicated the signals between the communications parties.! Loudspeakers converted the electrical signals back into audible sound.

! Telephone patent was first granted in 1876 with the Bell Telephone Company founded in 1877.! To provide interconnectivity between multiple users, two types of connections are possible.1

2

34

N

. . .

Pairwise interconnectionsNo. of connections = N(N − 1)/2

Access networkNo. of connections = N

Switch

1

2

34

N

. . .

14

Telephone Networks and Circuit Switching (2)

Pick up phone

Listen for Dial tone.

Dial number of the destination.

Exchange voice signals

1.

2.

3.

4.

5.

Telephonenetwork

Telephonenetwork

Telephonenetwork

Telephonenetwork

Telephonenetwork

Hang up to terminate the connection.6.

Connection set up

Information transfer

Connection release

15

Computer Networks and Packet Switching (1)

! The initial computer networks (SAGE, SABRE, etc.) were terminal-oriented networks (main frames) with a single central computer shared by multiple users.

! Each user had access to a dummy terminal (keyboard with monitor) connected with a dedicated line to the central computer.

! A user would type instructions using the dummy terminal.! The instructions would be passed on to the central computer, where they were executed, and

the results, returned to the dummy terminal, were displayed on the terminal. ! To allow fair access to the central computer, time-sharing techniques were employed.

Central computer

Terminal

Terminal. .

.

TerminalModem ModemTelephoneNetwork

16

Computer Networks and Packet Switching (2)

Modification # 1: Use a multidrop line to connect terminals with the central computer! Dedicated communication lines were expensive so two simplex lines were used to interconnect

the dummy terminals with the central computer.! Time sharing (medium access control) was provided by polling frames that polled each

terminal in a sequential order.! Address in the header of the polling frame was used to identify the terminal.! If a terminal wanted to communicate to the central computer, it would do so when polled.! The central computer would execute the instruction and pass the result back to the terminal.

Host computer

TerminalTerminal . . . Terminal

Terminals at different locations in a cityMust avoid collisions on inbound line

Polling frames & output frames

input frames

17

Computer Networks and Packet Switching (3)

Modification # 2: Use multiplexing to (a) transmit multiple messages simultaneously and (b) to detect communication errors.

! Multiplexers provided a second approach for sharing the communication line.! Message from each terminals was encapsulated into a frame with destination address and an

error detection code (for example the CRC code).! The central computer would sort out the messages from each terminal, perform possible

detection of errors, carry out the necessary processing, and return the results inside a frame.! The multiplexer would separate the results and direct each result to the appropriate terminal.

. . .

CRC Information Header

Header Information CRC

Central computer

Terminal

Terminal

Terminal

Multiplexer

Frame

18

Computer Networks and Packet Switching (3)

The second generation of networks were Computer-to-computer networks.! As cost of computers dropped, dumb terminals were replaced by self-computing terminals.! Interconnecting such computers was required to support: (1) file transfers; (2) remote telnet to

allow execution of a program on another computer; and (3) parallel processing to execute a single program over multiple computers.

! Circuit or message switching resulted in long communication delays. ! Packet switching was introduced in which long messages are broken into multiple packets.! Each packet has the destination address and is transmitted independently on the network.

19

Computer Networks and Packet Switching (4)

! ARPANET was the first research wide area network (WAN) commecting several universities.! ARPANET operated on the principle of packet switching.! Host generates message.! Each message is converted into several smaller packets.! Each packet contains source and destination addresses and is transferred independently. ! At the destination computer, the packets are combined into a single message.

UCLA RAND TINKER

USC

NBS

UCSB

HARV

SCD

BBN

STAN

AMES

AMES McCLELLAN UTAH BOULDER GWC CASE

CARN

MITRE

ETAC

MIT

ILLLINC

RADC

20

History of Networks

1. SAGE (Semi-Automatic Ground Environment System):

First computer network developed in 1950 for air-defense purposes

2. SABRE: an airline reservation system introduced in 1964

3. ARPANET:

Developed in mid 1960s at height of cold war to survive a nuclear war.

Based on packet switching technology such that part of the network is working under any circumstances.

4. NSFNET:

Developed in mid 1970s to connect research institutions and universities in U.S.

5. ARPANET and NSFNET connected in 1983.

6. Internet was the gluing technology for (5). Growth continued exponentially with the size of networks doubling almost every year.

7. Other networks connected to Internet include Aurora (MIT, IBM, UPenn, Bell core); Blanca (resulting from XUNET project in AT&T), CASA (a network for supercomputers based in CA), Nectar (CMU to UPitt), Vistanet (Univ. in North Caroliona), and many others

21

Comparison of Switching Techniques (1)

1. Circuit switching (desined by telephone companies)

End-to-end path is established between the transmitter and receiver .

Complete block of data is transmitted and circuit terminated.

Tx and Rx are inaccessible for the duration of the connection .

2. Message switching (designed for telegraphic networks):

No physical path is established between Tx and Rx.

Connection is established between the Tx and first switching office (router).

Entire block of data is transmitted to the switching office.

Block is forwarded one hop at a time.

No limit on block size, switching stations inaccessible for duration of transfer.

3. Packet switching (used in Internet):

A tight limit is placed on maximum block size.

Data is broken in different sub-blocks and each sub-block is transmitted one hop at a time, one after the other.

22

Comparison of Switching Techniques (2)

Timing of events for: (a) circuit switched; (b) message switched; and (c) packet switched networks

23

Comparison of Switching Techniques (3)

Packet SwitchingPacket SwitchingMessage SwitchingMessage SwitchingCircuit SwitchingCircuit Switching

Intelligent adaptive routing algorithms are required at each router for each packet.

Intelligent adaptive routing algorithms are required at each switching station.

Complete path is established once at initialization of connection.

Routing

Each packet may use a different path and hence arrive out of order

No problemNo problemSynchronization

A big issue since numerous packets from different sources are roaming in the network.

Can be an issue.No problemCongestion

Highest probability of overflow and data loss.

A router will be accepting data from various sources and may overflow.

No problem until a connection is dropped in the middle of transfer

Data Loss

Best under normal working conditions.

Improvement over circuit switching by placing a limit on maximum packet size.

Highly inefficient due to long setup time.

Efficiency

Connectionless service.Connectionless service.Connection-oriented service.

24

Functions required in Communication Networks

Basic user services: including smtp, ftp, telnet, http, video conferencing, �

Switching: transfer information between communication lines.

Transmission: ability to transmit (store and forward) information across a medium

Addressing: identify communication lines and stations

Multiplexing: means for coupling information from different sources together

Routing: identify the shortest path between the source and destination

Congestion control: identify congestion of data and /or ways to prevent it

Flow control: prevent overwhelming of a slower computer

Quality of service (QoS): allocate different class of service to different users

Compatibility: connect heterogeneous networks

Error detection: identify errors and / or correct them

Security: prevent eavesdropping

Management: monitor and recover from faults, manage bills etc.

25

Types of Networks (1)

! Networks are typically classified in three types: LAN, MAN, and WAN

1. Local area networks (LAN)

Small networks confined to a few kilometer (<= 1 km).

Speeds confined to 100 Mbps. Newer LANs run at up to 10 Gbps.

Uses the principal of broadcasting (one transmits, others listen)

Various Topologies including Token bus and Token ring are possible

Terminator

Token Bus Token Ring

26

Types of Networks (2)

2. Metropolitan area networks (MAN)

Covers up to a city (<= 10 km), Example: Cable TV network, IEEE 802.16

Cable network were initially designed for TV and later extended to Internet

Internet is fed into a head for subsequent re-distribution

27

Types of Networks (3)

3. Wide area networks (MAN)

Spans a continent (<= 10000 km), Example: Internet

Interconnects various LAN using switches (routers) and transmission lines

Uses packet switching in conjunction with the store and forward technology