Computer NetworksAn Introduction

Prepared by S.Perry (February 2010)

Unit Objectives

To present a comprehensive view of the principles and fundamental concepts in Computer Networks

To learn about the basics in design and implementation of network protocols

To provide an understanding of the components of a network and how they are connected.

To acquire some hands-on experience

Presentation OutlineIntroduction

◦Fundamental concepts◦Basic definitions◦Network architecture

Communication Basics◦Media and signals◦Asynchronous and synchronous communication◦Relationship among bandwidth, throughput,

and noise◦Frequency-division and time-division

multiplexing

Networking and network technologies◦Packing switching◦Framing, parity, and error detection◦Local and wide area technologies◦Network addressing◦Connection, wiring and extension (repeaters,

bridges, hubs, switches)◦Forwarding and measuring of delay and

throughput

Presentation Outline (continued)

Internets and Internetworking◦Motivation and concept◦Internet Protocol (IP) datagram format and

addressing◦Internet routers and routing◦Transmission Control Protocol (TCP)

Presentation Outline (continued)

Network Applications◦Domain Name System (DNS)◦File Transfer Protocol (FTP)◦Remote Login Protocol (TELNET)◦Email Transfer (SMTP)◦Web technologies and protocol (HTTP)

Putting all pieces together

Presentation Outline (continued)

What is a Computer Network?A collection of computers (PCs, workstations) and

other devices (e.g. printers, credit card readers) are all interconnected (each is called a “node”)

Components:◦Hosts (computers)◦Links (coaxial cable, twisted pair, optical fiber, radio,

satellite)◦Switches/routers (intermediate systems)

Goal: provide ubiquitous access to resources (e.g., database servers, Web), allow remote users to communicate (e.g., email)

User runs applications

What is a Computer Network?Application Application

Networks connect applicationson different stations

Frame

What is a Computer Network?

ClientStation

Mobile ClientStation

ServerStation

ServerStationStations are computers

and other devicessuch as cellphones and PDAs

Frame

What is a Computer Network?

ClientStation

Mobile ClientStation

ServerStation

ServerStationStations communicate by

sending messages calledFrames

Frame

What is a Computer Network?

Frame Switch

Switch

Switch

Switch

Frames may passthrough multiple switches;

Each switch reads the frameAnd passes it on

What is a Computer Network?

TrunkLink

TrunkLink

Trunk Link

TrunkLink

TrunkLink

Trunk linksconnect switches

Higher capacitythan access links

Often opticalfiber

What is a Computer Network?In summary, a network is a system of

hardware, software and transmission components that collectively allow two application programs on two different stations connected to the network to communicate well

What is a Computer Network?

Point-to-Point Communication

Multiple Access Communication

What is a Computer Network?Switched Networks

◦Circuit - switched network: public telephone network (dedicated circuit per call)

◦Packet switched network: Internet (collection of networks where data is sent in chunks)

Circuit-SwitchingSet up a connection path (circuit) between the

source and the destination (permanent for the lifetime of the connection)

All bytes follow the same dedicated pathUsed in telephonyAdvantages: dedicated resourcesDisadvantages: not very efficient (lower

utilization, e.g., a person talks < 35% of the time during a call)

While A talks to C, B cannot talk to D on the same line.

PacketsPackets

◦Data are chopped up into small blocks called packets (e.g., ~ 4500 bytes)

◦Each packet carries extra information to allow it to reach its destination

Packet-SwitchingPackets from different sources are interleaved

Efficient use of resources (since they are used on a demand): statistical multiplexing. Nobody reserves a lane on a freeway

Can accommodate bursty traffic (as opposed to circuit-switching where transmission is at constant rate).

Features of a Packet-SwitchingStore and forward: intermediate nodes (e.g.,

routers) store (buffer) incoming packets, process them and forward them to the appropriate outgoing link.

Allows for flexibility and robustness. Packets can travel through alternate paths (adaptive routing).

Undesired situations such congestion, long delays may occur.

Packet Switched Networks: Example

Packets can travel on different networks/links that may have different line speeds

Packet-Switched Networks: Topologies

What is the Internet?In the 60’s and 70’s the Internet (ARPANET) was a

small network connecting universities, research labs and government agencies. Main application: email, FTP. Motivation: share & research

Today it is a global, non-regulated communications network with millions of hosts and users. Main applications: Web, multimedia (audio/video), email. Motivation: commercialization

A large number of different network technologies and standards exist: LANs, WANs, B-ISDN, Optical Nets, Wireless, Satellite.

The Internet Today-- ComplicatedA huge and arbitrary collection of

heterogeneous nets. A network of networks!◦More than 240 million hosts◦Growing exponentially– doubling every 18

monthsHierarchically structured

◦LANs (e.g., Ethernet)◦CANs (e.g., FDDI)◦National/global (e.g., ATM or optical backbone)

Fully distributed operation (i.e., no centralized system or computer)

An Internet

Routers

RouteSingle Network

Single Network

Multiple NetworksConnected by RoutersPath of a Packet is its Route

Packet

A Network Example

IssuesResource sharing (i.e., accommodate

many users over the same link or through the same router)

Addressing and routing (i.e., how does an email message finds its way to the receiver)

Reliability and recovery: guarantee end-to-end delivery

Traffic management: monitoring and policing the network! Regulate traffic

Network PerformanceThere is a number of measures that

characterize and capture the performance of a network

It is not enough that networks work◦They must work well

Quality of service (QoS) defines quantitative measures of service quality◦Speed◦Delay (Latency)◦ Reliability

Security (not a QoS measure but crucial)

Network PerformanceSpeed

◦Bits per second (bps)◦Multiples of 1,000 (not 1,024)◦Kilobits per second (kbps) Note the lower case

“k”◦Megabits per second (Mbps)◦Gigabits per second (Gbps)◦Terabits per second (Tbps)

Related to link bandwidth

Network PerformanceCongestion and Latency

◦Congestion because traffic chronically or momentarily exceeds capacity

◦Latency delay measured in milliseconds (ms), microseconds ( ).

Especially bad for some services such as voice communication or highly interactive applications

s

Network PerformanceDelay:

◦Transmission time: time it takes to transmit a packet (depends on the link speed) = packet size/ speed

◦Propagation delay: time for a bit to travel across a link (depends on the distance, physical medium)

◦Queuing delay: waiting time inside a buffer◦Processing delay: time to process a packet

RTT (round-trip time): time for a bit to travel to the destination and come back

Reliability and RecoveryReliability

◦Availability – percentage of time the network is available to users for transmission and reception

◦Error rate – percentage of lost or damaged messages or bits.

Examples:◦Bit errors (bits are flipped, e.g., due to electrical

signal interference.)◦Packet loss (packets may be dropped due to

insufficient buffer space.)◦Packet delays (e.g., due to large queue size)◦Nodes or links can fail (go down)◦Malicious users

Reliability and RecoveryAs a consequence:

◦Packets delivered to the wrong destination◦Long delays on packets◦Packets delivered out-of-order◦Duplicate packets

Recovery:◦Implement error-control mechanism

Hop by hop (I.e., between nodes) End-to-end (source-to-destination).

◦Retransmissions◦End-to-end security (e.g., encryption, authentication)

Overload and CongestionOverload: Too many packets occur in

a subnetwork in the same time, which prevent each other and in such a way the throughput decreases

Congestion: the queues in the routers are too long, the buffers are full.◦As a consequence some packages are

dropped if the buffers of the routers are overloaded

In extreme case: grid-lock, lock-up (often used in a DNS (Denial of Service attack)

User ApplicationsUsers run application programs (web, email, ftp) at

the hosts interconnected through a networkHosts need to communicate in a meaningful way. User

should not be concerned with the underlying networkNetwork supports process-to-process (uni- or bi-

directional) communication among the hostsApplications need to take into consideration

limitations imposed by the networks physical characteristics

The Need for a Protocol ArchitectureProcedures to exchange data between

devices can be complex

High degree of cooperation required between communicating systems◦destination addressing, path◦readiness to receive◦file formats, structure of data◦how commands are sent/received and

acknowledged

What is a Protocol?Set of rules that specify the format and

meaning of messages exchanged between computers across a network ◦Format is sometimes called syntax ◦Meaning is sometimes called semantics

Example from everyday life: traffic laws!

Internet Protocol ArchitectureCurrently, Internet is mostly based on the

TCP/IP protocol suite (designed in late 70’s)TCP/IP became popular as it was bundled

with the UNIX/C environmentISO is still influential in designing networksOther architectures: ATM. Frame Relay

Key Features of a Protocol

Set of rules or conventions to exchange blocks of formatted data

Syntax: data formatSemantics: control information

(coordination, error handling)Timing: speed matching, sequencingActions: what happens when an event

occurs

Network ToolsRepeater: connects network segments

logically to one networkHub: multiport repeaterBridge: datalink level connection of two

networksSwitch: multiport bridgeRouter: connects networks that are

compatible in transport level◦subnetworks are connected to the interfaces of

the repeaterGateway (proxy server): router

between two individual network. The “Way Out”

Characteristics of High-Speed LANs

Fast Ethernet Gigabit Ethernet Fibre Channel Wireless LAN

Data Rate 100 Mbps 1 Gbps, 10 Gbps 100 Mbps – 3.2 Gbps 1 Mbps – 2 Gbps

Transmission Mode

UTP,STP, Optical Fiber

UTP, shielded cable, optical

fiberOptical fiber,

coaxial cable, STP2.4 GHz, 5 GHz

Microwave

Access Method CSMA/CD CSMA/CD Switched CSMA/CA Polling

Supporting Standard IEEE 802.3 IEEE 802.3 Fibre Channel

Association IEEE 802.11

Performance Considerations The following affect performance metrics

◦ Overhead: CPU time to put packet on wire◦ Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing

delay) or by congestion at routers◦ Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop

Contributions to Latency◦ Wire latency: depends on speed of light on wire about 1–1.5 ns/foot

◦ Router latency: depends on internals of router Could be < 1 ms (for a good router)

Router Router

LR1 LR2LW1 LW2 Lw3