computer networks chapter one: overview prepared by: dr. bahjat qazzaz book: computer networking a...

Computer Networks

Chapter One: Overview

Prepared By: Dr. Bahjat Qazzaz

Book: Computer NetworkingA Top-Down Approach Featuring the Internet

By: James Kurose & Keith RossThird Edition

Overview: Index

• Internet: Services and protocols

• Network Edge: End system, client, servers

Connection/less services

• Network Core: Circuit vs Packet switching

Datagram NW & Virtual Circuit

NW

• ISP and Backbones

Overview: Index

• Delay and loss in packet-switched NW

• Protocol Layers: Layered architecture

• Summary

Overview: Internet

• The Internet: Is a worldwide network that interconnects millions of computing devices. (or a collection of different types of networks).

• The devices can be: PCs, Workstations,

• Also, PDAs, TV, mobile computers, cell phones,

Overview: Internet

• Moreover, automobiles, environmental sensing devices, home electrical and security systems.

• These devices are referred to as:– Hosts or– End systems

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Overview: Internet

• End systems are connected together by Communication Links (not directly) of different types:– Copper wire, fiber optics, Coax, radio

spectrum– Different links can transmit data at different

rates (called Transmission rate and measured in bits/second).

Overview: Internet

• End systems (Hosts) are connected through intermediate devices known as: Packet Switches

• A packet switch takes a chunk of information arriving on one of its incoming communication link and forward it on one of its outgoing communication link.

• The chunk of information is called Packet• Example of packet switches: Routers

Overview: Internet

• A Router is an electronic device (e.g. computer) that forwards packets toward their ultimate destination.

• The communication links and packet switches between sending host and receiving host is called route or path

• Internet uses packet switching technique rather than dedicated path between hosts

Overview: Internet

• ISP (Internet Service Provider): End systems access the internet through ISPs.

• Examples of ISPs– Local Telephone company (e.g Paltel)– Corporate Company– University…etc

• Hosts can access the NW through: Dial-up modem, ADSL, High-speed LAN, Wireless

Overview: Internet

• To allow users to access worldwide Internet content:– Local ISP (Lower-tier ISP) is interconnected to

National ISP– And the National ISP (Upper-tier ISP) is

interconnected to International ISP

• An upper-tier ISPs consist of high-speed routers interconnected with high-speed fiber-optic links

• Each ISP network is managed independently, runs IP protocols, has name and IP address

Overview: Internet

• End systems and packet switches run protocols that control the sending and receiving of information within the Internet

• The TCP (transmission Control Protocol) and IP (Internet Protocol) protocols (Known as TCP/IP) are two of the most protocols in the Internet.

• IP protocol specifies the format of the packets that are sent and received among routers and end systems.

Internet services

• Distributed Applications: The Internet allows distributed applications running on its end systems to exchange data with each other. Applications include: E-mail, remote login, distributed games, video streaming, …etc

• The Internet provides two services to its distributed applications:– Connection oriented reliable service &– Connection unreliable service

TCP/IP Network, and in particular the Internet, provide two types of services to end-system applications

• Connection oriented– It guarantees that data transmitted from a

sender to a receiver will eventually be delivered to the receiver in order and in its entirety.

• Connectionless– It does not make any guarantees about

eventual delivery!!! The distributed app. Makes use of one of

these applications.

What is a Protocol

• 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

What is a Protocol

• all communication activity in Internet governed by protocols

• Examples:

Hi

Hi

Got thetime?

2:00

TCP connection request

TCP connectionresponse

<file>

time

Protocol examples:

• Hardware-implemented protocols in the NIC of two physically connected computers: control the flow of bits on the wire between the two NICs.

• Congestion-control protocols in end systems: control the rate at which packets are transmitted between sender and receiver

Protocol examples:

• Protocols in routers: determine a packet’s path from source to destination.

Web example!!

• What does happen when you request a web page?– You type a URL of a web page into a browser– Your computer will send a connection request

message to the web server and wait for reply– The server receives your connection request and

return a connection reply message– Your computer now sends the name of the web page

it wants to fetch from the web server– The web server sends the web page to your computer

The Network Edge

• End Systems, Clients, and Servers– Computers connected to the Internet are

called end systems, because they sit at the edge of the Internet.

– End systems are referred to as Hosts because they host application programs (e.g. web server prog, e-mail server prog, web browser prog

– Hosts are also can be divided to clients and servers

Client/Server paradigm

• A client program is a program running on one end system that requests and receives a service from a server program

• A server program is a program listening on a connection waiting for a request and replying for the request.

• !!!!! Notice the difference between sw & hw

Application examples:

• Web, E-mail, File transfer, remote login (Telnet), and many other applications follow the

Client / Server Paradigm

Since the client program runs on a computer and the server program runs on another, these applications are called

Distributed Applications

Application examples:

• Notice that:

An application may behave as a

Client and server at the same time

• For example PeerToPeer (P2P) file sharing applications:

• It works as a client when it requests a file from another peer, and works as a server when it sends the file to another peer.

Connection and connectionless

1. Connection oriented & handshaking (by sending control messages)

It comes with services:– Reliable data transfer (ack.)– Flow control (No overwhelming)– Congestion control (prevents entering in

gridlock)– It has the name: Transmission Control

Protocol (TCP)

Connection and connectionless

2. Connectionless– No handshakig (just sending when ready)

– E.g. MM App.

Network Structure• Host or End-System

– a computer that a user logs into to do work– attached to network, not part of network (usually)

• Subnet – everything between hosts– transport data from one host to another

Subnet• Point-to-Point

– Two machines, one at each end of a “wire”– Often many point-to-points in a subnet

Subnet• Broadcast

– Many (3+) machines connected by a common link– When one “speaks”, all hear

• Multicast targets only some• Unicast send to only one

Types of Network Structures

• LAN - Local Area Network

• MAN - Metropolitan Area Network

• WAN - Wide Area Network

• Wireless / Mobile Networks

Local Area Networks (LANs)

• Small geographic regions (e.g., building(s))

• High data rates (10-100 Mbps and up)– Much higher than connection to ISP

• Low cost (thousands of dollars)

• Typically broadcast

Metropolitan Area Networks (MANs, not MEN)

• Medium-size geographic regions (e.g., entire cities)

• Still no switches, single “wires”

• Example: local cable system

• IEEE 802.6--Distributed Queue Dual Bus (DQDB)– Uses two broadcast buses, one for each

direction

Wide Area Networks (WANs)

• Larger geographic distance (e.g. entire countries)

• Low data rates (56 kbps - 1.5 Mbps (T1), bundle T1 links to get higher rates),

• High cost (tens or hundreds of thousands of dollars per year)

• The Internet is a specific WAN

Internetworking

• The connection of different types of networks

• The Internet

Network Core: Circuit switching and Packet switching

• Circuit-switched networks:– The resources needed along a path to provide

for communication between the end systems are reserved for the duration of the communication session.

– So, when two host want to communicate, the network establishes a dedicated end-to-end connection between them.

Network Core: Circuit switching and Packet switching

• Packet-switched networks:– The resources are not reserved ahead of the

time. Instead, the resources are allocated on demand. So, a session’s message may have to wait (at the sending buffer) for access to a communication link (suffering a delay)

Network Core: Multiplexing in circuit-switched networks

• A circuit in a link can be implemented in one of the two ways:

Frequency-Division Multiplexing (FDM)

Or

Time Division Multiplexing (TDM)

Network Core: Frequency-Division Multiplexing

• FDM: The frequency spectrum of a link is shared among the connections. That is, each circuit continuously gets a fraction of the bandwidth.

Network Core: Time-Division Multiplexing

• TDM: Time is divided into frames of fixed duration, and each frame is divided into fixed number of time slots, then the networks dedicate the whole bandwidth during the time of the slot to the connection.

Network Core: FDM vs TDM

• FDM: – is wasteful during silent period– Is static

• TDM: leads to increment in the number of users

Example in: end of Page 17

Network Core: Packet switching

• In modern networks:– Messages are broken into packet– Each packet travels through communication

link and packet switch (router) at full rate (best efforts).

– Most packet switches use

Store-And-Forwards technique


• Store-and-forward:– The entire packet is received by the switch– It is checked for errors then,– It is forwarded to its destination.– Of course, this technique introduces some

delay. (S&F delay)– Output buffer or output queue (queuing delay)– Packet loss

Examples of page: 19+20+21


• Two classes of packet-switched networks

Datagram networks

And

Virtual-circuit networks

They differ in whether their switches use destination address or virtual-circuit numbers to forward packets toward their destination


• Datagram networks:– The network that forwards packets according

to host destination address is called datagram network.

– The internet is a datagram network.


• Virtual circuit networks:– The network that forwards packets according

to virtual circuit numbers called virtual-circuit network.

– This is done by giving a VC ID to end systems and to each and every switch along the VC’s source-to-destination path.

– The x.25, frame relay, and ATM are all examples of packet-switching technologies that use virtual-circuit.

Network Taxonomy

Telecommunicationsnetworks

Circuit-switchednetworks

Packet-switchednetworks

FDM TDMNetwork with

VCsDatagramNetworks

Access Network and Physical Media

• Now we want to consider Access Networks: the physical link(s) that connect an end system to its edge router, which the first router on a path from end system to any other distant end system.

• Access links include:– Fiber– coaxial– Twisted pair– Radio spectrum


Access networks can be classified into 3 categories:

• Residential Access: connecting home end systems to the NW

• Company Access: connecting end systems in a business or educational institution into the network

• Wireless Access: connecting end systems (that are often mobile) into the NW


• Residential Access:

Forms of residential access:– Dial-up modem

• Over analog telephone line• Twisted-pair, two modems, point-to-point• Converts from analog to digital and vice versa• Slow (56Kbps)

• New broadband tech. provides higher bit rate– Digital subscriber line (DSL) (more in Europe)– Hybrid fiber-coaxial cable (more in USA)



New broadband tech. provides higher bit rate– Digital subscriber line (DSL) (more in Europe)

• Access is provided by a telephone company• It runs over existing twisted-pair telephone lines• It receives and sends data at much higher rates, theoritically,

over 10Mbps from ISP to home (download) and 1Mbps from home to ISP (Up-load). In practice 1to 2 Mbps download, and hundreds of Kbps upload.

• It uses FDM

• DSL divides the communication link into 3 frequency bands:– Downstream channel(50khzto 1Mhz), upstream(4khzto50khz),

two-way telephone channel (0 to 4khz) band.



New broadband tech. provides higher bit rate– Hybrid fiber-coaxial cable (more in USA)

• Extension of cable network used for broadcasting cable tv• Cable head end broadcasts through a NW of Coaxial cable

and amplifiers to residence. Fig 1-10. (fiber node is used), then coaxial cable can support 500 to 5000 homes

• Cable modems are used, it is normaly an external device connected to the PC through Ethernet-port (chapter 5)

• Provides downstream and upstream.• The cable is shared, multiple access protocols (e.g. CSMA)

are used


• Company Access– Companies and universities use LAN to connect an

end system to the edge router– Different techs are used (ethernet is prevalent)– Ethernet operates at 10M, 100M, 1G and 10 Gbps.– Twisted-pair copper wire or coaxial cable– Shared medium, but recently switched ethernet tech.– The edge router routs packets that have destination

address outside of the LAN.


• Wireless Access• More eople have PCs, mobiles, and PDAs• Two types: wireless LAN, and wide-area wireless access

network.

– In wireless LAN• Users transmit packets to a base station (or receive from it)• The base station is within a radius of a few tens of meters• The station is typically connected to wired network.• Wireless LANs is based on IEEE 802.11 technology (Also

known as Wireless ethernet and Wi-Fi).• It provides a shared transmission rate of 11Mbps.


• Wireless Access– WAP: wireless access protocol

• Users can make use of portable phone infrastructure, accessing base station that are up to 10s of kilometers away.

• WAP is widely used in Europe, i-mode in Japan• Instead of HTML, wap phones use a special mark

up language WML(Wap Markup Language).

Physical Media

• Go to the layering architecture then back

computer networks chapter one: overview prepared by: dr. bahjat qazzaz book: computer networking a...

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