COMPUTER NETWORKS

COMPUTER NETWORKS

802.1

Is one of the IEEE standards for LANs and MANs which gives the introduction to the

set of standards and defines the interface primitives.

802.2

The standard describes the upper part of the datalink layer and uses the Logical Link

Control(LCC) protocol.

802.3

Is a IEEE standard called CSMA/CD for LANs. Uses the Ethernet cabling (10Base5

indicates 10Mbps, uses baseband signalling and can support segments upto 500

meters) also called thick ethernet which are coaxial cables with markings at every 2.5

meters to show where the taps go and are connected by the Vampire taps. Other

types of cablings include 10Base2, called thin ethernet which can handle 30 terminals

per cable segment and are connected by BNC connectors. Both these methods require

time domain reflectometry to detect the breaks in the cable. Another cable standard

is 10BaseT also called twisted pair running into a central hub, connected by

transceivers. This can run only upto 100 or 150 meters. The fourth cabling option is

10Base-F which uses the fibre optics.

802.4

LAN standard also called Token Bus.

802.5

LAN standard called Token Ring.

ALOHA

Aloha is the multiple access protocol devised by Norman Abramson to solve the

channel allocation problem using the ground-based radio broadcasting and has two

versions: pure which considers the time as continuous and slotted in which the time

is devided into descrete slots into which all frames fit; this requires the global time

synchronization.

ARPANET

ARPANET (Advanced Research Projects Agency) was sponsored by DoD (U.S.

Department of Defense) which connected hundreds of Universities and government

organizations using leased telephone lines. With the advent of satellites and radio

networks, the existing protocols had trouble interworking with them, a new

reference model TCP/IP with the ability to connect to multiple networks together in

a seamless way as the main goal was proposed and named after its two primary

protocols.

ATM

Asynchronous TransferMode is the technology that makes Broadband Integrated

Services Digital Network (ISDN) offer video on demand, live television from many

sources, full motion multimedia electronic mail, CD-quality music, LAN inter

connection, high speed data transport for science and industry and many other

services that have not been thought of, all over the telephone line.

Backbone

Backbones are the fast routers connected by high-bandwidth lines on which the

LANs are built.

Broadcasting

Sending a packet to all destinations simultaneously is called broadcasting.

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Collision-Free Protocols

The two protocols devised to send the data in an contention free manner inMAC

sublayer are: bitmap method, where a N bit contention period is begun each time the

transmission of all the N stations end in which they reserve their time slot and binary

countdown where the stations send their addresses in binary form which are ORed

together to get the station with the highest address which ultimately gets the

channel.

Congestion Control

The congestion control algorithms can be devided into two groups: open loop and

close loop. Open loop solutions attempt to solve the problem by good design by

making sure the congestion does not occur in the first place. Open loop algorithms

are also divided into that act at source versus ones that act at destination. Closed

loop solutions are based on the feedback loop. Closed loop algorithms are also

divided into two subcatagories: explicit feedback and implicit feedback. Some

congestions control algorithms are traffic shaping, Leaky bucket, Token bucket, Flow

specification (all openloop). The close loop algorithms are: Admission control, Choke

packets, Weighted fair queuing, Hop-by-Hop choke packets, Load shedding

Contention

Systems in which the multiple users share a common channel in a way that can lead

to conflicts are called contention systems.

CRC

Cyclic Redundancy Code (Polynomial code) is a way to remove the errors in a frame

by considering the bit string as representations of polynomials with co-efficients of 0

and 1 only.

CSMA Protocols

Career sense multiple access protocols suite consists of persistent and nonpersistent

CSMA

Datagram Service

The unreliable connectionless service is called datagram service.

DQDB

DQDB is Distributed Queue Dual Bus, a standard used in MAN and is denoted by

802.6 in IEEE standards.

Fragmentaion

When data packet is to be moved inbetween two or more different netrowks, each

network may impose some restriction on the maximum size of the data that can be

sent through that network due to a number of regions as hardware, OS, protocols,

compilance with internal standards, desire to reduce transmission errors, desire to

prevent one packet from occupying the channel for a long time.

Frame

Frame is the collection of data bits from physical layer well demarkated by some rule

to form a unit of data transmission in data link layer. Frames is the basic way to

check errors in the data transmission.

Functions of Layers

Application Layer: Is concerned with the terminals. An abstract Network Virtual

Terminal is defined that editors and other programs can be written to deal with,

where a peice of software is written to map the functions of the network virtual

terminal onto the real terminal. Other functions of application layers include file

transfer, electronic mail, remote job entry, directory lookup and various other general

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purpose & special purpose facilites.

Presentation Layer: Is concerned with the syntax and semantics of the information

transmitted, encoding of data, deals with the exchange of data representation such as

ASCII and Unicode.

Session Layer: Remote log in, file transfer, token management, synchronization.

Transport Layer: accept data from the session layer and split it into small pieces and

send those to network layer and ensure the correct arrival at the other end. Making

multiplexing transparent to session layer. Transport layer header tells which

message belongs to which layer?. Flow control.

Network layer: Is concerned with the operation of the network, how the packets are

routed from one machine to other, congestion control, billing information, handling

of problems occuring due to heterogeneous networks.

Datalink Layer: create and recognize the frame boundaries, handling the problems of

duplicate frames, flow control, buffer space management, control the access to a

shared channel.

Physical Layer: Actual transmission of data, howmany volts should be used to

represent 1 and 0, how many microseconds a bit should last, how the initial

connection is established & how it's torn, how many pins the network connector

should have.

Interface

The primitive operations and services the lower layer offers to upper layer.

Internet

Internet is network of networks connected by black boxes called repeaters which

copy individual bits between cable segments, Bridges store and froward data link

frames between LANs,Mulitprotocol routers forward packets between dissimilar

networks, Transport gateways connect byte streams in the transport layer,

Application gateways allow interworking on networks. Conventionally gateway is

used to mean a device that connects two or more dissimilar networks.

IP Address

Each IP packet will contain IP addresses of both the source and destination. Each

address is of 32 bytes and are devided into five classes A, B, C, D, E. Each host on the

network will have its unique network number provided by the NIC (Network

Information Center). The IP address 0 means this network or this host and -1 is used

for broadcast addresses to mean all hosts on the indicated network. The network

address 0.0.0.0 is used by hosts while booting but not afterwards.

Hamming Distance

The number of bit positions by which the two code words differ is called hamming

distance. In another way the number of bits that need to be inverted in one code

word so that it will represent its nearest or next code word is called hamming

distance.

MAC

Medium Access Control is a sublayer of datalink layer, deals with the broadcast

channels, sometimes refered to as multiaccess channels or random access channels.

Manchester Encoding

Manchester encoding is the way to represent the bits in a data frame such that it

willn't produce any redundency in the bit transmissions in the physical layer.

Multicasting

Multicasting is sending messages to well-defined groups that are numerically large

in size but small compared to the network as a whole.

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Network architecture

A set of layers and protocols

Optimality Principle

It states that if router J is on the optimal path from router I to router K, then the

optimal path from J to K also falls along the same route. The set of optimal routes

from all sources to a given destination form a tree rooted at the destination, is called

Sink Tree.

OSI

Open System Interconnection reference model is developed by Inernational

Standards Organization to standardize the protocols used in various layers.

Pipelining

In Slinding window protocol suite, the goback n protocol causes the transmission of

all the frames to the destination even if the acknowledgement for a previous frame

hasn't been received yet, till the sender's window gets filled is called pipelining.

Port

Ports indicate which service is desired. port-23 is for Telnet, port-79 is for Finger,

port-119 is for USENET news, etc.

Protocol stack

A list of protocols, one per layer in certain system.

Routing Algorithm

The main function of network layer is routing packets from the source machine to the

destination machine.The routing algorithm is that part of the network layer S/W

resposible for deciding which output line an incoming packet should be transmitted

on. They can be grouped into two classes: Nonadaptive algorithm which don't base

their routing decisions on measurements or estimates of the current traffic and

topology. This is also called static routing. Adaptive algorithms change their routing

divisions to reflect the changes in the topology, and usually the traffic as well. Static

or Nonadaptive algorithms are Shortest Path Routing, Flooding, Selective Flooding,

Flow-Based Routing. The adaptive algorithms are Distance Vector Routing

algorithm, Link State Routing.

Simplex Protocol

The simplest protocol in data link layer where data is transmitted in only one

direction with the assumption that the sending and receiving networks are always

ready, ignoring the processing time, having infinite buffer space and the

communication channel between the data link layers never damages or looses the

frames.

Sliding Window Protocol

The goal of the protocol suite is to maximize the use of bandwidth through a

technique called piggibacking where the outgoing acknowledgement is temporarily

delayed so that they can be hooked on to next outgoing data frame.

SNA

System Network Architecture, as seven layer protocol designed by IBM.

Stop andWait Protocol

Is equivalent to Simplex Protocol with the restriction that buffer space is limited.

Traffic Shaping

The open loop method to manage congestion is forcing the packets to transmit in

more predictable way. This approach widely used in ATM networks is called traffic

shaping.Monitoring the traffic flow is called traffic policing.

Tunneling

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Tunneling is defined as a way of sending data packet of a network to the destination

network following the same protocol, through a network inbetween, which follows a

different protocol, by wrapping the data packet in the payload field of the network

inbetween.

Protocol Acronyms

ARP

In addition to IP, which is used for data transfer, the Ineternet has several other

control protocols used in the network layer, one of which is Address Resolution

Protocol. ARP maps the IP address into the host's LAN address.

BGP

Boarder Gateway Protocol: is the Exterior Gateway Protocol used between different

Autonomous Systems(AS) which mainly concerns with the transit packets.

DNS

Domain Name Service: maping of host names onto their network addresses.

FTP

File Transfer Protocol: Transfer of files

HTTP

Hyper Text Transfer Protocol: used for fetching the pages on world wide web.

ICMP

Internet Control Message Prortocol: is used to test the internet and monitor the

routers closely.

NCP

Network Core Protocol: is heart of NetWare (Network System from Novell) provides

various services along with user data transport.

NNTP

Network News Transfer Protocol: moving news articles around

OSPF

Open Shortest Path First: is the interior gateway protocol which gives a standard to

LANs that are Autonomous Systems(AS) operating in different organization.

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RARP

Reverse Address Resolution Protocol: allows the newly booted workstations to

broadcast its ethernet address, where the RARP server sees this 48 bit address and

finds out the 32 bit IP address of the host machine.

RSVP

Resource reSerVation Protocol: is a network layer protocol which allows multiple

senders to transmit to multiple groups of receivers, permits individual receivers to

switch channels freely, and optimizes bandwidth use while at the same time

eliminating congestion.

SMTP

Simple Mail Transfer Protocol: used for electronic mail transfer.

TCP/IP

Transmission Control Protocol and Internet Protocol: The two main protocols in

network layer of TCP/IP model which defines an official packet format.

TELNET

TELecommunication NETwork: remote login or virtual machine.

UDP

User Datagram Protocol: A unreliable connectionless protocol for application that do

not require the sequencing or flow control of data.