computer networks midterm revision revision topics · pdf filecomputer networks midterm...

3201 Computer Network – 2014/2015 Handout: Midterm Revision

Computer Networks Midterm Revision

Revision topics:

- Computer networks definition

- Computer network uses

- Classification of networks

- Difference between computer networks and distributed systems

- Network Topology

- Ethernet Network

- Cables Configuration

- Network Operating Systems

- Protocols

- Design issues for layers

- OSI reference model

- TCP/IP reference model

- Reliable vs. Unreliable connection

- Different switching techniques

- Modulation techniques

- Communication satellite vs. terrestrial links

- Abbreviations

Computer networks definition:

- An interconnection of various computer systems located at different places.

- Group of computers connected to each other in order to exchange information such as e-mail messages or documents, or share resources such as disk storage or printers.

Computer network uses:

• Business Applications

• Home Applications

• Mobile Users

• Social Issues

Computer network classification:

- Host rule:

o Peer-to-Peer

o Client-Server

- Geography distance:

o Local Area Network (LAN)

o Metropolitan Area Network (MAN)

o Wide Area Network (WAN)

o Wireless networks


o Home networks

o Internetworks

- Participation:

o Private

o Public

Computer networks vs distributed systems:

Distributed System Computer Networks

- A collection of independent computers

appears to its users as a single

coherent system

- The coherent model and software are

absent

- Layer of software on top of OS called

“middleware” is responsible for

implementing this model.

- Users are exposed to the actual

machines

- The distinction between a network and a distributed system lies with the software,

rather than the hardware.

Network Topology:

o The term Topology is used to describe how devices are connected & how

messages flow from device to device.

Physical Topologies (Layouts):

o Bus Topology:

Consists of a trunk cable with nodes wither inserted directly into the

trunk, or nodes trapping into the trunk using offshoot cables called drop

cables.

Signals travel from one node to all other nodes on the bus.

A device called a terminator is placed at both ends of the trunk cable.

Terminators absorb signals & prevent them from reflecting repeatedly

back & forth on the cable.

Advantages:

Use few amount of cables

Not need to purchase any additional devices such as “Hub”

Ease of Installation

If a computer fails, the network stays functional

Disadvantages:

The difficulty to troubleshoot it


Not scalable

If a line breaks, the network goes down.

o Ring Topology:

Connects neighboring nodes using cables until they form a ring

A circle has no start & no end, because there are no ends, terminators are

not necessary

Signals travel in one direction around the ring

Each device on the network acts as a repeater to send the signal to the

next device.

Advantages:

Signal degeneration is low because each workstation is

responsible for regenerating or booting the signal

Disadvantages:

If one computer fails or the cable link is broken, the entire

network could go down.

Installation requires careful planning to create a continuous ring.

Isolating problems can require going to several physical locations

along the ring.

A malfunctioning node or cable break can prevent signals from

reaching nodes further along on the ring.

o Star Topology:

All computers are connected through one central device known as a “Hub”

or “Switch”

Each workstation has a cable that goes from the network card to the hub

device.

Advantages:

Scalable as nodes can be added to or removed from the

network easily


All network connections are located in a single place, which

makes it easy to troubleshoot & reconfigure

Cabling problems usually affect only one node.

Disadvantages:

If the hub fails, the entire network comes down “Central Point of

Failure”

To connect each workstation to the network, need to ensure that

there is a hub with an available port

Need to ensure that there is a cable from the workstation to the

hub.

o Mesh Topology:

Exists when there are multiple paths between any two nodes on a network

Created using point-to-point connections

Variations of Mesh topologies:

Partial Mesh:

Some redundant paths exist.

Full Mesh:

Every node has a point-to-point connection with other node

Advantages:

Increases the network fault tolerance

Network impossible to go down due to a cable fault

Disadvantages:

The cost of the additional cabling & network interfaces to create

the multiple pathways “connections”

Every hard to administer & manage because of the numerous

connections.

o Mesh Topology:

Advantages of a Wireless Topology:

• The nice thing about wireless networks is the lack of cabling. The wireless network requires only

base backbone segments to connect the wireless cells to the wired network if there is one.

• Troubleshooting failed devices and cells is very easy and makes failed components easy to find

and replace.

• Easy Management


Disadvantages of a Wireless Topology:

• Signal interference. Other devices and machinery that emit radio frequencies or “noise” can cause

interference and static, which can disrupt the bubble of communication around the cell. Another

source of noise is lightning during storms. This noise is the same static you hear when lightning

strikes while you are speaking on a phone.

• Blockage can occur in structures that are made of thick stone or metal, which do not allow radio

frequencies to pass through easily. This drawback usually can be overcome somewhat by

changing the frequency used by the devices to a higher frequency.

• Signal interception. means unwanted third parties could intercept wireless communications

without physically being on the premises; they would simply have to be within the signal range.

Ethernet Network:

Follows a naming convention as “##BaseXX”

## stands for the speed of the network

Base stands for the Baseband transmission

XX stands for the cabling type or medium

Ex.:

If X 5

Represent Thicknet as max = 500 meter

If X 2

Represent Thinnet as max = 200 meter or 185 meter

If X T

Represent Twisted Pair cable

If X T4

Represent Twisted Pair cable with 4 Pairs

If X F

Represent Fiber-Optic cable

If X TX

Represent a higher grade of connection 100 Mbps

100BaseFX

100BaseTX

Configuring / Creating Cables:

Using a Straight through Cable to Connect Two Hosts:


Using a Crossover Cable to Connect Two Hosts:

OR

Network Operating Systems:

Windows 2000 Server and Windows Server 2003/2008

Novell NetWare

UNIX


Protocols:

- Rules and conventions used in the conversation between two machines.

- Is an agreement between the communicating parties on how communication is to proceed.

Design issues for layers:

- Addressing:

Every layer needs a mechanism for identifying senders & receivers.

- Rules for data transfer:

o Unidirectional:

Data only travel in one direction.

o Bidirectional:

Data travel in both directions but not simultaneously.

- Error-control:

There are many error-detecting & error-correcting codes are known, but both ends of the

connection must agree on which one is being used.

- Order of message:

o Not all communication channels preserve the order of messages sent on them.

o In order to deal with a possible loss of sequence: the protocol must make explicit

provision for the receiver to allow the pieces to be reassembled properly.

o An obvious solution is to number the pieces.

- Flow control:

How to keep a fast sender from swamping a slow receiver with data.

- Inability of accepting long messages:

Leads to mechanisms for disassembling, transmitting & then reassembling messages.

- Multiplexing & De-multiplexing:

When it is inconvenient or expensive to setup a separate connection for each pair of

communicating processes.

- Routing:

When there are multiple paths between source & destination, a route must be chosen.

OSI reference model:

- International standards organization – open system interconnection.

- It’s called open system because it deals with connecting open systems “Systems that

are open for communication with other systems”.


- Physical layer:

Concerned with transmitting raw bits over a communication channels.

The design issues must take in consideration:

logic.

or.

- Data link layer:

o Data link layer takes “raw transmission bits” & break it up into “Data frames”.

o Creates & recognizes frames boundaries, this can be done by attaching special bit

patterns to the beginning & the end of the frame “01111110”.

o Problem of flooding slow receivers from fast senders must be taken into

consideration.

o Problem of transmission on both directions must be solved.


- Network layer:

Controls the operation of the subnet.

o The design issues must take in consideration:

billing information.

- Transport layer:

o Called “Host-to-Host”, or “Source-to-Destination”, or “End-to-End” layer.

o We mean by “end-to-end” or from “source-to-destination” a program or a source

machine carries only a conversation with the similar program on a destination

machine using the message headers & control messages.

o In the lower layers (1, 2 & 3) the protocols are between each machine & its

immediate neighbors & not by the ultimate source & destination machine, which

may be separated by many routers.

- Session layer:

o Users interfaces to the network.

o The user negotiate with the session layer to establish a connection with a process

on another machine to establish a session, the user must provide the remote

address he wants to connect to the operation of setting up a session between 2

processes is often called “binding”.

o In some networks the transport layer & session layer are merged together in one

layer.

o offer various services:

dialog control: keeping track of whose turn is to transmit.

critical operation at the same time.

continue from where they were after a crash.

- Presentation layer:

o Concerned with the syntax & semantics of the information transmitted.

o Manages theses abstract data structures & allows higher-level data structures to be

defined & exchanged, as computers with different data representations to

communicate.

o Example :

Text compression

- Application layer:


Contains a variety of protocols that are commonly needed by users.

TCP/IO model:

Goals of designing this model: The network will be able to survive loss of subnet hardware, with existing conversions not being broken off. Connections remain intact as long as the source & destination machines were functioning, even if some of the machines or transmission lines in between were suddenly put of operation.

Application Layer:

Contains all the higher level protocols.

Transport Layer:

Allow peer entities on the source & destination hosts to carry on a

conversation.

Two protocols are defined:

- TCP: is a reliable connection-oriented protocol that:

Allows a byte stream on one machine to be

delivered without error on any other machine in

the internet.

Fragments the incoming byte stream into discrete

messages & passes each one on the internet layer.

At the destination the receiving TCP process

reassembles the received messages into the output

stream.


Handles flow control to make sure a fast sender

can’t swamp a slow receiver with more messages

than it can handle.

- UDP: is an unreliable connectionless protocol for

applications that do not want TCP’s sequencing or flow

control & wish to provide their own.

Internet Layer:

Uses packet switching techniques, based on a connectionless

internetworking

Defines packet format & protocol called “IP”.

The job is to deliver IP packets where they are supposed to go.

Host-To-Network Layer:

The host has to connect to the network using some protocol, so it

can send IP packets to it.

Reliable vs Unreliable connection:

Connection vs Connectionless oriented services

Reliable connection:

Connection that never loses data & data are known even if failure occurrs as acknowledgemnt

sent from receiver to sender.

Unreliable connection opposite of reliable

Switching techniques:


- Circuit switching:

once a call has been setup, a dedicated path between both ends exists and will

continue to exists until the call is finished.

- Packet switching:

o Store and forward networks:

No physical path is established inadvance between the sender and reciever, but

packets are stored in an available IMPs and then forwareded later.

o Packet switching networks:

Packets buffered in IMPs main memory.

- Message switching

Packet switching Circuit switching

Doesn’t setup a physical connection between

sender and receiver inadvance

Setup physical connection inadvance

Acquire the bandwidth only on transmission

and then releases it

Holds the bandwidth until the end of

transmission

No bandwidth waste Bandwidth is wasted

Circuits are never dedicated Circuits are dedicated

IMPs provide speed, code conversion as well

as some correction

No

IMPs provide buffering in the main memory

install and forward routing

Buffering is to be in the secondary memory

for sender and receiver only

Cost doesn’t depend on the distance between

the two ends

Charge = f ( traffic, time )

Cost doesn’t depend on the traffic (no. of bits)

sent

Charge = f ( distance, time )

Used for coputer networks Rarely used

Packets may arrive to the rciever in order

different than transmission order

Message arrive in order

Modulation:


Satellite vs terrestial links:

Terristal links Satellite

Propagation delay 6 Msec/Km = 𝟏𝟎𝟑

𝟑∗𝟏𝟎𝟖

Typical propagation delay 270 Msec

To send x kbits over a 9600 bps is (𝒙∗𝟏𝟔𝟎𝟎

𝟗𝟔𝟎𝟎) To send x kbits over a 5-Mbps is 270 MS +

𝑥

𝟓∗𝟏𝟎6ms

The satellite is faster and the propagation delay is independent of distance between sender and

receiver

Given a channel of bandwidth f = 8 khz and bitrate b= 300, 600, 1200, 2400, & 38000 bps and

we want to transmit n = 1 byte through that channel.

Calculate for each above bitrates the following

- Number of harmonics passed

- Time of transmission T [msec]

- Frequency of the first harmonic

B (bps) T [msec] 1st harmonic #harmonics passed

300 26.67 37.5 213

600 13.33 75 106

1200 6.67 150 53

2400 3.33 300 26

38000 0.21 4800 1

Time =(𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒)

1st harmonic =( 𝑏𝑖𝑡𝑟𝑎𝑡𝑒

𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠)

# harmonics =f * (𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒)

Draw a schematic wave diagram to transmit a binary 01100010 to show the shape when passing

only the first harmonic, first and second harmonics, 4 harmonics and 8 harmonics.

(a) A binary signal

(b) Amplitude modulation

(c) Frequency modulation

(d) Phase modulation


Some Abbreviations:

SAP service access point

IDU interface data unti

SDU service data unit

PDU protocol data unit

ICI interface control information

UDP user datagram protocol

STP shielded twisted pair

UTP unshielded twisted pair

DNS domain name space

HTTP hypertext transfer protocol

HTTPS hypertext transfer protocol secured

IP internet protocol

TCP transmission control protocol

FTP file transfer protocol

NOS network operating system

NIC network interface cards

computer networks midterm revision revision topics · pdf filecomputer networks midterm...

Documents