Chap1 networking

Data Communicationsand

Networking

Copy rights: Waleej Haider

Chapter 1. Introduction

1. Data communications2. Networks3. The Internet4. Protocols and standards


1. Data Communications• Data

– Information presented in whatever form, agreed upon by the parties who are creating and using the data

• Communication– Sharing information– Sharing can be local or remote– Local Communication: usually occurs face to face– Remote communication: takes place over distance (prime concern)

• Data Communication– Exchange (sharing) of data between two devices– Via some form of transmission medium (wired or wireless)

• Communication System includes:– Hardware : Physical equipments/devices– Software : Programs


Cont..

• Fundamental characteristics of effective data communicationThe effectiveness of a data Comm. System depends on 4 fundamental

chars.:– Delivery: Delivery of data to the correct and only that destination– Accuracy: No loss of information/data– Timeliness: Without significant delay & in the same order

(audio/video)– Jitter : Variation in the packet arrival time (uneven delay)

• Telecommunication:– Communication at a distance (‘tele’ in Greek means “far”)– using telephony, telegraphy, television, computer.

Data Communications

• The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point- The Mathematical Theory of Communication

by Claude Shannon


Five Components of Data Communication• Message: Information (data) to be communicated (text, numbers, pics, audio, video)• Sender: that sends the data message• Receiver: that receives the message• Transmission medium: Physical path by which a message travels• Protocol: A set of rules that govern data communication. An agreement between the

communicating devices. Without a protocol, two devices may be connected but not communicated


A Communication Model

(Digital bits to analog signals)

(Analog signals to Digital bits)

Cont..

• Source - generates data to be transmitted• Transmitter - converts data into transmittable

signals• Transmission System - carries data from source

to destination• Receiver - converts received signals into data• Destination - takes incoming data

Direction of Data Flow

Data Flow• Simplex

– Unidirectional– As on a one-way street

• Half-duplex– Both transmit and receive possible, but not at the same time– When there is no need for communication in both direction at the same time– Like a one-lane road with two-directional traffic but at different times– Entire capacity of the channel can be utilized for each direction– Walkie-talkie, CB (citizens band) radio

• Full-duplex– Transmit and receive simultaneously– Like a two-way street; telephone network– Channel capacity must be divided between two directions

2. Networks• Network: A set of devices (nodes) connected by communication links• Node: Computer, printer, or any device capable of sending and/or

receiving data• Network Criteria: To be considered effective and efficient, a network

must meet a number of criteria

Network Criteria

• Performance can be measured in:– Transit time: amount of time required for a message to

travel from one device to another– Response time: elapsed time between an inquiry and a

response– More throughput (no. of bits/sec)– Less delay– Other factors: no. of users, type of medium, connected

hardware, and efficiency of the software

Cont..

• Reliability is measured by:– Frequency of link failure– Recovery time from a failure– Network robustness in a crash

• Security issues include:– Protecting data from unauthorized access– Protecting data from damage– Implementing policies for recovery from data losses

Type of Connection

Types of Connection• Point-to-point

– Dedicated link between two devices– The entire capacity of the channel is reserved– Wired or wireless– Eg: Microwave or satellite link, link between TV and remote control

• Multipoint – More than two devices share a single link– Capacity of the channel is shared either

• Spatially shared: Several devices can use the link simultaneously (cable TV channels using diff freq. bandwidths)

• Temporally shared: Users must take their turns, also called timeshared (telephone networks)

Physical Topology• Two or more devices connect to a link • Two or more links form a topology• A Method of physical (LAN) arrangement of computers/devices• The topology of a network is the geometric representation of the

relationship of all the links and linking devices (nodes)

Mesh Topology• Dedicated point-to-point link to every other nodes • Dedicated link carries traffic only b/w the two devices it connects • A mesh network with n nodes needs n(n-1)/2 links. As each node

requires (n-1) I/O ports (links) • Advantages: No traffic problems, robust, security, easy fault

identification & isolation because of dedicated links• Disadvantages: Difficult installation/reconfiguration, more space,

expensive • As it is expensive therefore used in limited fashion • Mostly as a backbone connecting the main computers of a hybrid

network that can include several other topologies

Star Topology• Dedicated point-to-point link only from each device to a central

controller, called a hub (or switch)• Hub acts as an exchange-because no direct traffic between devices• Advantages: Less expensive; robust; each device requires only one

link that is only one I/O port therefore easy to install/reconfigure; easy fault identification and isolation

• Disadvantages: dependency of the whole on one single point, the hub; If hub goes down the whole system is dead

• High-speed LANs often use a star topology

Bus Topology• Bus topology is multipoint• One long cable acts as a backbone to link all nodes • Nodes are connected by drop lines and taps to the bus cable• Cable ends act as terminators that remove any signal from the bus• Advantages: Easy installation; less cabling; cheap • Disadvantages: Limited no. of devices and the distance b/w the devices;

difficult reconfiguration (for new nodes) ; no fault isolation; a fault or break in the bus stops all transmissions; taps cause degradation in quality

• Less popular due to above disadvantages

Flow of data

Ring Topology• Each device has a dedicated point-to-point link with only the two devices on either

side of it • Signal is passed in one direction (clock or counter clockwise) from device to device

until it reaches its destination e.g; using token passing access method• Each device incorporates a repeater • A repeater regenerates the bits and passes signals along if signals are intended for

another device• Advantages: Easy installation, reconfiguration, fault isolation• Disadvantage: Unidirectional traffic, a break in the ring can disable the entire

network• Today, the need for higher speed LANs has made this topology less popular

Hybrid Topology• Collection of more than one topology• Used to share advantages from various topologies• Example: Main star topology with each branch connecting

several stations in a bus topology

Categories of Networks

LAN• Usually privately owned• A network for a single office, building, or campus a few Kms (2kms)• LANs are designed to share resources b/w PCs• Resources can be a printer, a software, data, etc.• Beside size, LANs are also distinguished from other networks by their

transmission media and topology• Common LAN topologies: Bus, Ring, Star, Mesh • Data rate: 100 or 1000 Mbps• Wired or Wireless LANs• E.g; an isolated LAN connecting 12 computers to a hub in a closet

MAN• A network with a size between LAN and WAN• Designed to extend to an entire city or a town• Topology: Hybrid• Mainly used for Cable TV network; High speed internet (broadband);

a company’s connected LANs, high speed DSL line provided by telephone company network

• Owned by a private or a public company

WAN• Long distance transmission, e.g., a country, a continent, the world• A WAN that is owned and used by one company is called Enterprise network • Switched WAN: connects the end systems, which comprise a router that

connects to another LAN or WAN (highlighted devices are switches)• Router: used for connecting various networks ( LANs/MANs/WANs). It is an

internetworking connecting device


WAN (cont..)

• Examples of Packets Switched WANs: X.25, Frame Relay, and ATM (are networks designed to provide connectivity b/w end users)

• Examples of Circuit Switched WANs: country wide PTCL network

• Point-to-point WAN: it is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an internet service provider (ISP)– this type of WAN is mainly used for internet purpose

Comparisons

Networking Tasks – phone net. sol’n

Addressing - identify the end user phone number 1-201-222-2673 = country code + region code + exchange + number

Routing – Find route from source to destination. determined from phone number by static routing tables

Forwarding – how information is moved circuit switching::a fixed circuit along path to destination

Information Units - How information is sent voice samples; no addressing attached samples sent continuously , 8000/sec network must prepare source-dest. circuit in advance

Networking Tasks – Internet Solution

Addressing - identify the end user IP addresses 132.66.48.37, = network number || host # Routing- How to get from source to destination routers learn automatically network topology build routing tables / updated frequentlyForwarding – how information is moved packet switching: move packets 1 by 1 through routers.Information Units - How information is sent. self-descriptive packet = data + header header contains destination address

Telephone networks support a high end-to-end quality of service, but is expensive

Internet supports no quality of service but is flexible and cheap

Future networks will have to support a wide range of service qualities at a reasonable cost

Internetwork• Internetwork (internet) : Interconnection of networks ( LANs/MANs/WANs) • The Internet:

– a specific worldwide network– Network of networks

• two or more networks are connected to one another by internetworking devices • Internetworking devices: router, gateway, etc.

What’s the Internet: “nuts and bolts” view

A world-wide computer network, i.e., a network that interconnects millions of computing devices throughout the world.

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

router

PC

server

wirelesslaptopcellular handheld

wiredlinks

access points

e.g. desktop PCs, Unix-based workstations, PDAs, TVs, mobile computers, cell phones, automobiles etc.

What’s the Internet: “nuts and bolts” view

hosts = end systems are computers connected to the internet running network apps

End systems are connected by communication links Fiber optics, coaxial cable, copper, radio,

satellite Different links can transmit data at different

rates known as transmission rate(bits/sec) = bandwidth

What’s the Internet: “nuts and bolts” view protocols control sending,

receiving of msgs e.g., TCP, IP, HTTP, Skype,

Ethernet Internet: “network of

networks” loosely hierarchical public Internet versus

private intranet Internet standards

RFC: Request for comments IETF: Internet Engineering

Task Force

Home network

Institutional network

Mobile network

Global ISP

Regional ISP

What’s the Internet: a service view communication

infrastructure enables distributed applications: Web, VoIP, email, games,

e-commerce, file sharing communication services

provided to apps: reliable data delivery

from source to destination

“best effort” (unreliable) data delivery

Internetwork Example• There are 3 entities (2 LANs and 1 president office)• For this heterogeneous network following are required:

– One switched WAN has been leased (country wide telephone network)– 3 point-to-point WANs required to connect LANs to switched WAN

Internet Today• ISPs (Internet service

providers) are local ISPs to provide direct service to the end users

• RISPs (Regional ISPs) are mostly city wide ISPs

• NISP (National ISP) is mostly country wide ISP

• NAP (network access point) is a third party to connect all NISPs

What’s a protocol?human protocols: “what’s the time?” “I have a question” Introductions… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather than

humans all communication

activity in Internet governed by protocols

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

4. Protocols

• Key elements of a protocol– Syntax: Structure or format of data– Semantics: meaning of each section of bits in the

structure– Timing: when data should be sent and how fast they can

be sent

What’s a protocol?a human protocol and a computer network protocol:

Hi

HiGot thetime?2:00

TCP connection requestTCP connectionresponse

Get http://www.awl.com/kurose-ross

<file>time

Chapter 1: roadmap1.1 What is the Internet?1.2 Network edge

end systems, access networks, links1.3 Network core

circuit switching, packet switching, network structure1.4 Delay, loss and throughput in packet-

switched networks1.5 Protocol layers, service models1.6 Networks under attack: security1.7 History

1.2 Network edgeComputers and other devices connected to the Internet are often referred to as end systems (because they sit at the edge of the Internet). desktop computers (e.g., desktop PCs, Macs, and

Linux boxes), servers (e.g., Web and e-mail servers), and mobile computers (e.g., portable computers, PDAs,

and phones with wireless Internet connections).

Continued… End systems are also referred to as hosts because

they host (that is, run) application programs :

• Web browser program, a Web server program, an e-mail reader program, or an e-mail server program.

Two categories: clients (desktop and mobile PCs, PDAs, and so on ) and servers (machines that store and distribute Web pages, stream video, relay e-mail, and so on)

Client and Server Programs A client program is a program running on

one end system that requests and receives a service from a server program running on another end system.

The Web, e-mail, file transfer, remote login, newsgroups, and many other popular applications adopt the client- server model.

peer-to-peer (P2P) applications Increasingly, many applications are peer-to-

peer (P2P) applications, in which end systems interact and run programs that perform both client and server functions.

For example, in P2P file-sharing applications (such as BitTorrent and eMule), the program in the user's end system acts as a client when it requests a file from another peer; and the program acts as a server when it sends a file to another peer.

The Network Core mesh of packet switches

and links that interconnects the Internet's end systems.

The fundamental question: how is data transferred through net? circuit switching

packet-switching

In circuit-switched networks, the resources needed along a path (buffers, link transmission rate) to provide communication between the end systems are reserved for the duration of the communication session between the end-systems. E.g. telephone net

In packet-switched networks, these resources are not reserved; a session's messages use the resources on demand, and as a consequence, may have to wait (that is, queue) for access to a communication link. E.g. Internet

Circuit Switching In this network, the four circuit

switches are interconnected by four links.

Each of these links has n circuits, so that each link can support n simultaneous connections.

The hosts are each directly connected to one of the switches.

When two hosts want to communicate, the network establishes a dedicated end-to-end connection between the two hosts. E.g. Conference calls between A and B.

(Continued…) for Host A to send messages to Host B, the

network must first reserve one circuit on each of two links.

Because each link has n circuits, for each link used by the end-to-end connection, the connection gets a fraction 1/n of the link's bandwidth for the duration of the connection.

Multiplexing in Circuit-Switched Networks

A circuit in a link is implemented : frequency-division multiplexing (FDM) time-division multiplexing (TDM).

With FDM, the frequency spectrum of a link is divided up among the connections established across the link.

The link dedicates a frequency band to each connection for the duration of the connection. In telephone networks, this frequency band typically

has a width of 4 kHz

FM radio stations also use FDM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being allocated a specific frequency band.

For a TDM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots.

When the network establishes a connection across a

link, the network dedicates one time slot in every frame to this connection.

Introduction 1-53

Circuit Switching: FDM and TDMFDM

frequency

timeTDM

frequency

time

4 usersExample:

Numerical example How long does it take to send a file of

640,000 bits from host A to host B over a circuit-switched network? All links are 1.536 Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit

Let’s work it out!

Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps

so it takes (640,000 bits)/(64 kbps) = 10

seconds to transmit the file.

To this 10 seconds we add the circuit establishment time, giving 10.5 seconds to send the file.

Packet Switching In computer networks, the source breaks

long messages into smaller chunks of data known as packets.

Between source and destination, each of these packets travels through communication links and packet switches.

Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link.

Continued… Most packet switches use store-and-forward

transmission at the inputs to the links.

Store-and-forward transmission means that the switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.

Thus store-and-forward packet switches introduce a store-and-forward delay at the input to each link along the packet's route.

Example:

Consider how long it takes to send a packet of L bits from one host to another host across a packet-switched network.

Let's suppose that there are Q links between the two hosts, each of rate R bps. Assume that this is the only packet in the network.

Solution: The packet must first be transmitted onto

the first link emanating from Host A; this takes L/R seconds.

It must then be transmitted on each of the Q - 1 remaining links; that is, it must be stored and forwarded Q - 1 times, each time with a store-and-forward delay of L/R.

Thus the total delay is QL/R.

Packet Switches Each packet switch has multiple links

attached to it.

For each attached link, the packet switch has an output buffer (also called an output queue), which stores packets that the router is about to send into that link.

If an arriving packet needs to be transmitted across a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer.

..continued In addition to the store-and-forward delays, packets

suffer output buffer queuing delays.

• These delays are variable and depend on the level of congestion in the network.

Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely filled with other packets waiting for transmission.

In this case, packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped.

…continued Suppose Hosts A and B are sending packets to Host

E.

Hosts A and B first send their packets along 10 Mbps Ethernet links to the first packet switch.

Introduction 1-62

The packet switch then directs these packets to the 1.5 Mbps output link, congestion will occur as packets queue in the link's output buffer before being transmitted onto the link.

Packet Switching Versus Circuit Switching: Statistical Multiplexing

Why Packet switching is not suitable for real-time services (for example, telephone calls and video conference calls)? It is because of its variable and unpredictable end-to-

end delays (due primarily to variable and unpredictable queuing delays).

Packet switching it offers better sharing of bandwidth than

circuit switching and it is simpler, more efficient, and less costly

to implement than circuit switching.

Why is packet switching more efficient? Suppose users share a 1 Mbps link. Also suppose that each user alternates

between periods of activity, when a user generates data at a constant rate of 100 kbps, and periods of inactivity, when a user generates no data.

Suppose further that a user is active only 10 percent of the time (and is idly drinking coffee during the remaining 90 percent of the time).

With circuit switching, 100 kbps must be reserved for each user at all times.

For example, with circuit-switched TDM, the circuit-switched link can support only 10 (= 1 Mbps/100 kbps) simultaneous users.

With packet switching, the probability that a specific user is active is 0.1 (that is, 10 percent).

If there are 35 users, the probability that there are 11 or more simultaneously active users is approximately 0.0004.

When there are 10 or fewer simultaneously active users (which happens with probability 0.9996), the aggregate arrival rate of data is less than or equal to 1 Mbps, the output rate of the link.

Overview of Delay in Packet-Swiched Networks

As a packet travels from one node (host or router) to the subsequent node (host or router) along a path, the packet suffers from several types of delays at each node along the path.

The most important of these delays are the: nodal processing delay, queuing delay, transmission delay, and propagation delay;

Together, these delays accumulate to give a total nodal delay.

Processing Delay The time required to examine the packet's

header and determine where to direct the packet is part of the processing delay.

The processing delay also include the time needed to check for bit-level errors in the packet that occurred in transmitting the packet's bits from the upstream node to router A.

Processing delays in high-speed routers are typically on the order of microseconds or less.

Queuing Delay At the queue, the packet experiences a queuing delay as it

waits to be transmitted onto the link.

The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission across the link.

If the queue is empty and no other packet is currently being transmitted, then our packet's queuing delay will be zero.

If the traffic is heavy and many other packets are also waiting to be transmitted, the queuing delay will be long.

Queuing delays can be on the order of microseconds to milliseconds in practice.

Transmission Delay The amount of time required to push (that is, transmit)

all of the packet's bits into the link.

A packet can be transmitted only after all the packets that have arrived before it have been transmitted.

Denote the length of the packet by L bits, and denote the transmission rate of the link from router A to router B by R bits/sec.

For example, for a 10 Mbps Ethernet link, the rate is R = 10 Mbps; for a 100 Mbps Ethernet link, the rate is R = 100 Mbps.

The transmission delay (also called the store-and-forward delay is L/R.

Transmission delays are typically on the order of microseconds to milliseconds in practice.

Propagation Delay- Once a bit is pushed into the link, it needs to propagate

to router B.

The time required to propagate from the beginning of the link to router B is the propagation delay.

The bit propagates at the propagation speed of the link.

The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair copper wire, and so on) and is in the range of

2 x 108 meters/sec to 3 x 108 meters/sec

The propagation delay is d/s, where d is the distance between router A and router B and s is the propagation speed of the link.

Caravan analogy

cars “propagate” at 100 km/hr

toll booth takes 12 sec to service car (transmission time)

car~bit; caravan ~ packet Q: How long until caravan

is lined up before 2nd toll booth?

Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec

Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr

A: 62 minutes

toll booth

toll booth

ten-car caravan

100 km

100 km

Caravan analogy (more)

Cars now “propagate” at 1000 km/hr

Toll booth now takes 1 min to service a car

Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?

Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth.

1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router! See Ethernet applet at AWL

Web site

toll booth

toll booth

ten-car caravan

100 km

100 km

Cont..

• Standards: agreed-upon rules– Rules must be standardized– Standards provide guidelines to manufactures, vendors,

government agencies, and other service providers to ensure the kind of interconnectivity necessary in communications

• Standards are of two categories– de jure (“by law” or “by regulation’) standards:

• Legislated by an officially recognized body– de facto (“by fact” or ‘by convention’) standards:

• That have not been approved by an organized body but have been adopted through its widespread use.

Standards Organizations

• Standards are developed through– Standards creation committees– Forums– Government regulatory agencies

Standards Creation Committees

• ISO (International Organization for Standardization)– Voluntary international organization

• ITU-T (International Telecommunication Union-Telecommunication Standards Sector)– Formerly, CCITT formed by UN

• ANSI (American National Standards Institute)– Private non-profit corporation in the US

• IEEE (Institute of Electrical and Electronics Engineers)– The largest engineering society in the world

• EIA (Electronic Industries Association)– Non-profit organization in the US

Chap1 networking

Science

Transcript of Chap1 networking