Autumn 2000John Kristoff1 Data-link Layer Computer Networks.

Autumn 2000 John Kristoff 1

Data-link Layer

Computer Networks


Where are we?


The Data Link Interface


The Local Area Network

Popular (most data links are LANs) High Throughput Low Cost Short Distances Often shared medium access

Most new installations usually "switched"


Shared Medium Access

A Shared Medium Used by All Only One Station Transmits at a

Time Stations "Take Turns MAC Protocol defines fairness policy


Topology Review


Data Link Bit Encoding


Example Bus: Ethernet

Most Popular LAN IEEE Standardized as 802.3 Several Generations

Same frame format (mostly) Changing data rates Different physical layer requirements

The book: Gigabit Ethernet, Rich Seifert


Ethernet Transmission

Only one station transmits at a time Signal propagates entire cable length All stations receive all transmissions CSMA/CD medium access control scheme


CSMA/CD

Carrier Sense (CS) Wait until medium is idle Begin to transmit frame

Multiple Access (MA) Multiple stations attached to shared media Each station uses the same access algorithm

Simultaneous Transmission is Possible


CSMA/CD [continued]

Simultaneous Transmission: Interfere with each other Known as a collision

CSMA with Collision Detect (CD) Listen to media during transmission Detect whether another stations signal

interferes Back off from interference and try again


Transmission Logic

1. If media is idle, transmit.2. Else, continue to listen to the media and

when it is available, transmit.3. Listen to media while transmitting.4. If collision is detected while transmitting,

send jam and back-off5. Go to step 1 until max-try counter is

reached.


Exponential Back-off Algorithm

Let 1 Slot Time = 512 bit times Upon 1st collision, randomly choose among {0,1} slot delay Upon 2nd collision, randomly choose among {0,1,2,3} slot

delay Up to a maximum of 16 transmission attempts with a range

of delay from {0 to 1024} bit times

0 <= r < 2k-1

Where r is the random number generated, where k = MIN(n,10) and where n is the n-th retransmission attempt


The Collision Domain

Minimum Length Frame Must Be >= Maximum RTT of the Ethernet segment

Minimum Frame is 512 bits Requires 46 bytes of data whether the

upper layer has them or not Distances decrease as speed increases Full-duplex mode eliminates the

collision domain


An Aside - Collisions

They are NOT bad, unless theyre late Collision statistics are mostly

meaningless Monitor utilization Distance Matters Becoming irrelevant with switching The name "Collision is misleading


Ethernet Addressing

Standardized by IEEE Each station assigned a unique 48-bit address

First 24-bits are the OUI Second 24-bits are vendor assigned

Usually set when NIC is manufactured Canonical address format


Ethernet Address Recognition

Each Frame Contains a Destination Address

All Stations Receive All Transmissions Station Discards Any Frame Not Destined

for It Important: interface hardware, not

software, checks address


Possible Destinations

1. Single destination (unicast) 2. All stations on the Ethernet

(broadcast) 3. Subset of stations on the Ethernet

(multicast)

MAC address is used to distinguish between the destinations


Ethernet Destination Addresses


Promiscuous Mode

Designed for testing/debugging Allows interface to accept all frames Available on most Ethernet hardware


IEEE 802.3 Frame Format

Sender fills in: Senders source

address Recipients

destination address Type of data in the

frame type field Cyclic Redundancy

in FCS field


Demultiplexing on Frame Type Field

Network Interface Hardware Receives a copy of each transmitted frame Examines address and either accepts or

discards Passes accepted frame to system software

Network device software Examines frame type Passes frame to correct software module


Ethernet Wiring - 10BASE5

Thick Ethernet (Thicknet) Heavy coaxial cable


Ethernet Wiring - 10BASE2

Thin Ethernet (Thinnet) Smaller coaxial cable


Ethernet Wiring - 10BASE-T

Uses a hub Twisted-pair wiring


Ethernet Office Wiring


High-speed Ethernet

Fast Ethernet Operates at 100 Mb/s Standardized in IEEE 802.3 as 100BASE-T and

100BASE-F standards 10/100 Devices available

Gigabit Ethernet Operates at 1 Gb/s Mostly fiber systems using switches

Even higher speeds coming!


Ethernet - Final Notes

Data Link Layer Usually Implemented with Physical Layer

Link Beat Interframe Gap Time Capture Effect Modern Ethernet is a star-shaped bus news://comp.dcom.lans.ethernet IETF increasing maximum frame size?


Example Ring: Token Ring

Popular in IBM environments IEEE Standardized as 802.5 Operates at 4Mb/s, 16Mb/s Quickly Being Abandoned

802.5 working group moved to "hibernation" status in July 2000

Still worth learning about!


Token Ring Transmission

Station waits for token before sending Signal travels the entire ring Sender receives its own transmission


Token Passing Paradigm

Frames travel in a unidirectional fashion around the ring

Stations must wait for token to transmit

Stations can reserve the token Token will circle indefinitely until a

station wants to transmit


MAC Frames

Ring management and control frames Beacon, Ring purge, claim token, report

error Ring Poll every 7 seconds

Active monitor present Standby monitor present NAUN notification process


Active and Standby Monitor

Only 1 Active Monitor per ring AM is the master clock for the ring AM inserts 24-bit delay to

transmissions AM ensures tokens/frames are present AM removes circulating frames SMs are ready to take over if AM fails


Monitor Contention

Ring elects a new Active Monitor Initiated when:

Loss of signal is detected Active monitor not detected Time-outs of token timer, NAUN, etc.

Highest MAC address wins Everyone else is Standby Monitor


Token Ring Insertion Process

Phase 0 - Media Lobe Check Phase 1 - Physical Insertion Phase 2 - Address Verification Phase 3 - Participation in Ring Poll Phase 4 - Request Initialization


The Token Frame

When no station is transmitting, the token frame travels continuously around the ring.


Token Ring Addressing

Standardized by IEEE Each station assigned a unique 48-bit address

First 24-bits are the OUI Second 24-bits are vendor assigned

Usually set when NIC is manufactured Non-canonical address format


Token Ring Address Recognition

Each Frame Contains a Destination Address

All Stations Receive and Repeat All Transmissions

Stations Copy Any Frame Destined for It Important: interface hardware, not

software, checks address


Token Ring Destination Addresses


Token Ring Frame Format

Sender fills in: Senders source

address Recipients destination

address Cyclic Redundancy in

FCS field

Other stations may change: Frame Status


High-speed Token Ring

HSTR Operates at 100 Mb/s 1 Gb/s was being worked on Standardized in IEEE 802.5 Some 4/16/100 devices


Why Token Ring Lost

IBM was the only systems manufacturer that promoted it

Cost Complexity Support throughout the industry Only one vendor left to develop

product!


Token Ring - Final Notes

Jitter Early Token Release Backup Path Token Transmission Timer Needs LLC - we havent talked about

it yet news://comp.dcom.lans.token-ring


Example Ring: FDDI

Uses Optical Fiber cabling High reliability (dual rings) Immune to interference Standardized by ANSI Transmission rate of 100 Mb/s Similar to token ring


FDDI Dual Ring Operation


Logical Link Control

Standardized by IEEE 802.2 Often used for MACs that dont use type field


LLC with SNAP


What else?

ATM Wireless (802.11) Fiber Channel HIPPI Token Bus (802.4) IEEE 802 standards may become

free!

Autumn 2000John Kristoff1 Data-link Layer Computer Networks.

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