Introduction to OSI 7 layer model and Data Link Layer in Detail

VNIT ACM Student Chapter- Mustaqim Moulavi

Introduction to Number system

• Binary number system

Introduction to Number system• Hexadecimal number system0000 = 0 = (0)10

0001 = 1 = (1)10

0010 = 2 .1010 = A = (10)10

.1110 = E = (14)10

1111 = F = (15)10

7 Layer OSI Model of Networking Architecture

• ISO/ Open System Interconnection Reference Model

• It divides network architecture into seven layers which, from top to bottom• A layer is a collection of

conceptually similar functions that provide services to the layer above it and receives service from the layer below it.

7 Layer OSI Model of Networking Architecture

Outlines WHAT needs to be done to send data from one computer to another.

Each layer Provides services to the layer above it

And gets servicesFrom layer below it– You can see it likeEach layer is customerOf the layer below itAnd provides services toLayer above it

7 Layer OSI Model of Networking Architecture

What each layer does?

Remembering the 7 Layers

7 - Application All

6 - Presentation People

5 - Session Seem

4 - Transport To

3 - Network Need

2 - Data Link Data

1 - Physical Processing

Topics for today’s seminar• Data Link Layer in Detail in 3 parts

1. The Basic Design principles for Layer 2 i.e. Link Layer communication between only 2 nodes and dealing with problems

2. Working of Real networks - Ethernet (LAN) system at the Link Layer level

3. Wireshark Demo showing the actual live data flowing in the network in the form of packets, decoding them and understanding what all this data packets means.*

Some terminology:• hosts and routers are nodes• communication channels that

connect adjacent nodes along communication path are links– wired links– wireless links– LANs

• layer-2 packet is a frame, encapsulates datagram

“link”

What is link layer?• data-link layer has

responsibility of transferring datagram from one node to adjacent node over a link

• Deals with communication between adjacent nodes joined by a physical(LAN wire, WIFI, Bluetooth) link.

“link”

Link layer: context Datagram transferred by

different link protocols over different links:◦ e.g., Ethernet on first link,

frame relay on intermediate links, 802.11 on last link

Each link protocol provides different services◦ e.g., may or may not provide rdt

over link

transportation analogy trip from kanyakumari to delhi◦ ship: kanyakumari to chennai◦ plane: chennai to mumbai◦ train: mumbai to delhi

tourist = datagram transport segment =

communication link transportation mode = link

layer protocol travel agent = routing

algorithm

Why data link layer is required?• The data sent in the physical layer is in the form of 0

and 1. physical layer doesn't understand what it means.• It is like 2 blind persons communicating with only 2

words YES and NO.• No one knows where a sentence starts and where it

ends and what it means.• Moreover Communication circuits make errors

occasionally.• The function of the link layer is to provide error free

communication and make some meaning out of the data sent/receieved.

Specific functions of the link layer

• The task of data link layer is to convert the raw bit stream offered by the physical layer into a stream of frames for use by the network layer

• Providing reliable service to the layer above it i.e. network layer

• Dealing with transmission errors.• Regulating the flow of data so that slow

receivers are not swamped by fast senders

Link Layer Services

• Framing, link access: – encapsulate datagram into frame, adding header, trailer– channel access if shared medium– “MAC” addresses used in frame headers to identify source, dest

• different from IP address!• Reliable delivery between adjacent nodes

– seldom used on low bit error link (fiber, some twisted pair)– wireless links: high error rates

• Q: why both link-level and end-end reliability?

Link Layer Services (more)

• Flow Control: – pacing between adjacent sending and receiving nodes

• Error Detection: – errors caused by signal attenuation, noise. – receiver detects presence of errors:

• signals sender for retransmission or drops frame (used in fiber optics and wired networks)

• Error Correction: – receiver identifies and corrects bit error(s) without resorting to

retransmission (used in wifi)

Adaptors Communicating

• sending side:– encapsulates datagram in a frame– adds error checking bits, flow control, etc.

• receiving side– looks for errors, flow control, etc– extracts datagram, passes to receiving node

sendingnode

frame

datagram

frame

adapter adapter

link layer protocol

One of the Methods of Framing• Starting and ending flags with bit stuffingEach frame begins and ends with a special bit pattern (flag

byte) 01111110• Whenever the senders data link layer encounters five

consecutive ones in the data it automatically stuffs a zero bit into the outgoing bit stream

Thus if the receiver looses track of where it is all it has to do is to scan input for flag sequences since they can only occur at frame boundaries and never within the data.

Reliable delivery

• The outgoing frames are numbered and the receiver sends an acknowledgement for that particular frame

• So the sender knows which frames the receiver has received.

• Sender knows which frame to send again and which shouldn’t be sent again.

• This ensures the reliable delivery of the data

Flow control• Sender wants to send fast than the capacity of

the receiver.• The frames might be lost in this case• Feedback based flow control• Receiver sends back information to sender giving

it permission to send more data• For example when a connection is setup the

receiver might say “you may send me n frames now but after they have been sent do not send any more until I have told you to continue”

Error Detection• EDC= Error Detection and Correction bits (redundancy)• D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction

Parity CheckingSingle Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

Chances are 50:50

Sender: treat segment contents as

sequence of 16-bit integers checksum: addition (1’s

complement sum) of segment contents

sender puts checksum value into UDP checksum field

Receiver: compute checksum of received

segment check if computed checksum equals

checksum field value:◦ NO - error detected◦ YES - no error detected. But

maybe errors nonetheless? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

Checksumming: Cyclic Redundancy Checkused at link layer

view data bits, D, as a binary number choose r+1 bit pattern (generator function), G goal: choose r CRC bits, R, such that

◦ <D+R> exactly divisible by G (modulo 2) ◦ receiver knows G, divides <D,R> by G. If non-zero remainder: error

detected! widely used in practice

CRC Example• D = 101110• Add r = 000 after D (for

purpose of division only)• G= 1001• Remainder R = 011• Add remainder R to D• D+R = 101110(011)• This bitpattern is divisible

by generator function and so will be error free

• IEEE 802 G function = 1000001001100000010001110110110111

Working of Ethernet MAC Addresses and ARP

• 32-bit IP address: – network-layer address– used to get datagram to destination IP subnet

• MAC (or LAN or physical or Ethernet) address: – used to get datagram from one interface to

another physically-connected interface (same network)

– 48 bit MAC address (for most LANs) burned in the adapter ROM

LAN Addresses and ARPEach adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

LAN Address (more)• MAC address allocation administered by IEEE• manufacturer buys portion of MAC address space (to assure

uniqueness)• Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address• MAC flat address (no hierarchy) ➜ portability

– can move LAN card from one LAN to another

• IP hierarchical address NOT portable– depends on IP subnet to which node is attached

ARP: Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP table

• ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>– TTL (Time To Live): time after

which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88

ARP protocol: Same LAN (network)

• A wants to send datagram to B, and B’s MAC address not in A’s ARP table.

• A broadcasts ARP query packet, containing B's IP address – Dest MAC address = FF-FF-

FF-FF-FF-FF– all machines on LAN

receive ARP query • B receives ARP packet, replies

to A with its (B's) MAC address– frame sent to A’s MAC address

(unicast)

• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that

times out (goes away) unless refreshed

• ARP is “plug-and-play”:– nodes create their ARP tables

without intervention from net administrator

Routing to another LANsend datagram from A to B via R assume A know’s B IP address

• In routing table at source Host, find router 111.111.111.110• Two ARP tables in router R, one for each IP network (LAN)• In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB

• A creates datagram with source A, destination B • A uses ARP to get R’s MAC address for 111.111.111.110• A creates link-layer frame with R's MAC address as dest, frame

contains A-to-B IP datagram• A’s adapter sends frame • R’s adapter receives frame • R extracts IP datagram from Ethernet frame, sees its destined to B• R uses ARP to get B’s MAC address • R creates frame containing A-to-B IP datagram sends to B

A

RB

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble: • 7 bytes with pattern 10101010 followed by one byte with

pattern 10101011• used to synchronize receiver, sender clock rates

5: DataLink Layer

Ethernet Frame Structure (more)• Addresses: 6 bytes

– if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol

– otherwise, adapter discards frame

• Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)

• CRC: checked at receiver, if error is detected, the frame is simply dropped

Wireshark Demo to see Ethernet packets

• References :1. Computer Networks – Andrew S. Tanenbaum2. Computer Networking : A Top-Down Approach

Featuring the Internet – James F. Kurose , Keith W. Ross