layer 2 : concept

Layer 2 : Concept

Andres, Wen-Yuan Liao

Department of Computer Science and Engineering

De Lin Institute of Technology

[email protected]

http://www.cse.dlit.edu.tw/~andres

Overview

Reliable transit of data

Physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control

6.1 LAN Standards

Layer 2

Comparing OSI Layers 1 and 2 with various LAN standards

Comparing the IEEE model with the OSI model

Logical Link Control (LLC)

MAC sublayers

LLC as one of four concepts of Layer 2

Layer 2 Addresses these limitations of layer 1Logical Link Control (LLC): Communicate with the upper-level layers

Framing: describe streams of bits

Media Access Control (MAC):decide which computer will transmit binary

data

MAC address: name or identify computers

Comparing OSI Layers 1/2 with various LAN standards

IEEE divides the OSI data link layer into two separate sublayers

Media Access Control (MAC)transitions down to media

Logical Link Control (LLC)transitions up to the network layer

Comparing the IEEE model with the OSI model

LLC: Protocol Data Unit (PDU)

MAC : 802.3 and 802.5, cross over the Layer2/Layer1 interface

NIC as both a Layer 1 and a Layer 2 device MAC address & Transceiver


Allow part of the data link layer to function independently from existing technologies

Two addressing Destination Service Access Point (DSAP)Source Service Access Point (SSAP)

LLC

Manages communications Connectionless and connection-

oriented services

Enable multiple higher-layer protocols to share a single physical data linkIEEE 802.2

MAC Sublayers

Deals with the protocols that a host follows in order to access the physical media

LLC as one of Four Concepts of Layer 2

1. Communicates with the upper-level layers through LLC

2. Uses a flat addressing convention

3. Use framing to organize or group the data

4. Use MAC to choose which computer will transmit binary data

6.2 Hexadecimal Numbers

Hexadecimal numbers as MAC addresses

Basic hexadecimal (hex) numbering

Converting decimal numbers to hexadecimal numbers

Converting hexadecimal numbers to decimal numbers

Methods for working with hexadecimal and binary numbers

6.3 MAC Address

Organization Unique Identifier (OUI):Administered by the IEEE

Burned-in addresses (BIAs)Be burned into read-only memory (R

OM) and are copied into random-access memory (RAM) when the NIC initializes

MAC Addressing

Data link layer MAC identifiers

MAC address and NICs

How the NIC uses MAC addresses

Layer 2 address encapsulation and decapsulation

Limitations of MAC addressing

MAC address and NICs

Every computer has a unique way of identifying itself

No two physical addresses are alike

The physical address is located on the Network Interface Card (NIC)

Two formats for MAC addresses: 0000.0c12.3456 or 00-00-0c-12-34-56

How the NIC uses MAC addresses

Ethernet and 802.3 LANs are broadcast networks

All stations see all frames

Each station must examine every frame to determine whether that station is a destination

Limitations of MAC addressing

The number of possible addresses is not going to run out anytime soon

They have no structure, and are considered flat address spaces

6.4 Framing Why framing is necessary

Frame format diagram

Three analogies for frames

A generic frame format

Frame start fields, Address fields

Length/type fields, Data fields

Frame error problems and solutions

Stop frame field

Why framing is necessary

Which computers are communicating with one another

When communication between individual computers begins and when it terminates

A record of errors

Whose turn it is to "talk" in a computer "conversation"

Frame start fieldsGrab the attention of other computers

Address fieldSource and destination MAC address

Length/type fields Length field: exact length of a frame Type field: specifies the Layer 3 protocol

Data fieldsHigher-layer data/padding bytes

Frame Check Sequence (FCS) field Error detectionContains a number that is based on the

data in the frame

Three primary ways Cyclic redundancy check (CRC) Two-dimensional parity Internet checksum

Stop frame field Indicate the end of frame

6.5 Media Access Control (MAC)

Definition of MAC

Three analogies for MAC

Deterministic MAC protocols

Non-deterministic MAC protocols

Three specific technical implementations and their MACs

Definition of MAC

Protocols that determine which computer on a shared-medium environment (collision domain) is allowed to transmit the data

Two broad categories of MACDeterministic (taking turns)Non-deterministic (first come, first

served)

MAC

Classification of MAC protocols

Topologies of broadcast links

Contention MAC

Round Robin MAC

Reservation MAC

Dedicated MAC

Classification of MAC Protocols

MAC protocols

Contetion (Non-Deterministic)

ALOHA, CSMA/CD, CSMA/CA

Round Robin (Deterministic)

token ring, token bus, FDDI

Reservation DQDB

Dedicated TDMA, FDMA, WDMA, CDMA

Note : Some systems use a combination of above schemes.

Topologies of Broadcast

station

(a) Busrepeater

station

(b) Ring

base station

mobile station

(d) Wireless--celluar(c) Star

Star coupler (active

or passive)

station

Topologies and Standard Protocols

Bus CSMA/CD (802.3), Token Bus (802.4), DQDB(802.6)

Ring Token Ring (802.5), FDDI

Star WDMA

Wireless ALOHA, CSMA/CA (802.11), FDMA+TDMA (GSM),

CDMA

Dedicated MAC

WDMA Wavelength Division Multiple Access

FDMA on passive fiber optic LANsFrequency Division Multiple Access

TDMA on passive fiber optic LANsTime Division Multiple Access

CDMACode Division Multiple Access

Deterministic MAC protocols

A form of "taking your turn"

Token Ring network

A special data token circulates around the ring

When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then places the token back in the ring

Token Ring

receiver

sender(expecting token)

token

sender(releasing token)

token

receiver

sender(txing data)

receiver

sender(draining data)

receiverframe copied

Token Bus

A logical ring on a physical bus, a ring maintenan

ce protocol is required.

Run a token protocol similar to token ring, except

that the token rotation time is controlled. FDDI’s t

imed token rotation protocol follows the same me

chanism as token bus.

FDDI

backup ringoperational ring

Fiber Distributed Dual Interface

Non-deterministic MAC protocols

First-come, first-served (FCFS) approach Allow anyone to transmit at willThis led to collisions Carrier Sense Multiple Access with Collision Detection (CSMA/CD)Everyone else on the system also hears the collision, waits for silence, and then tries to transmit

ALOHA

Transmit whenever it wants

Listen to know if it’s a success or collisio

n

Wait a random time to retx, if collided

CSMA/CACarrier Sense Multiple Access with Collision Avoidance

Before sending the data frame, send a RTS (Request To Send) mini-frame to receiver, (RTS declares the length of data frame) If Collided, backoff exponentially and retry.

Receiver sends back a CTS (Clear To Send) mini-frame to ask the other stations keep silent for the period of data frame. If collided, backoff exponentially and retry.

Transmit the data frame when the CTS is received.

C A B. . .

.E

A B D. . .

.E

RTS CTSD .

C .

Three specific technical implementations

EthernetLogical bus topology Physical star or extended star

Token Ring Logical ring topology Physical star topology

FDDILogical ring topology Physical dual-ring topology

Summary

Institute of Electrical and Electronic Engineers (IEEE)

Media Access Control (MAC)


layer 2 : concept

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