Data-link Layer

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Data-link Layer
Computer Networks
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Where are we?
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The Data Link Interface
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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"

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Shared Medium Access

• A Shared Medium Used by All
• Only One Station Transmits at a Time
• Stations "Take Turns?
• MAC Protocol defines fairness policy

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Topology Review
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Data Link Bit Encoding
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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

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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

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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

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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 station?s signal
  interferes
• Back off from interference and try again

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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-off
• 5. Go to step 1 until max-try counter is reached.

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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 lt r lt 2k-1
• Where r is the random number generated, where k
  MIN(n,10) and where n is the n-th retransmission
  attempt

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The Collision Domain

• Minimum Length Frame Must Be gt 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

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An Aside - Collisions

• They are NOT bad, unless they?re late
• Collision statistics are mostly meaningless
• Monitor utilization
• Distance Matters
• Becoming irrelevant with switching
• The name "Collision? is misleading

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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

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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

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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

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Ethernet Destination Addresses
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Promiscuous Mode

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

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IEEE 802.3 Frame Format

• Sender fills in
• Sender?s source address
• Recipient?s destination address
• Type of data in the frame type field
• Cyclic Redundancy in FCS field

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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

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Ethernet Wiring - 10BASE5

• Thick Ethernet (Thicknet)
• Heavy coaxial cable

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Ethernet Wiring - 10BASE2

• Thin Ethernet (Thinnet)
• Smaller coaxial cable

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Ethernet Wiring - 10BASE-T

• Uses a hub
• Twisted-pair wiring

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Ethernet Office Wiring
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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!

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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?

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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!

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Token Ring Transmission

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

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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

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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

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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

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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

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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

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

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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

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Token Ring Destination Addresses
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Token Ring Frame Format

• Sender fills in
• Sender?s source address
• Recipient?s destination address
• Cyclic Redundancy in FCS field
• Other stations may change
• Frame Status

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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

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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!

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Token Ring - Final Notes

• Jitter
• Early Token Release
• Backup Path
• Token Transmission Timer
• Needs LLC - we haven?t talked about it yet
• news//comp.dcom.lans.token-ring

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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

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FDDI Dual Ring Operation
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Logical Link Control

• Standardized by IEEE 802.2
• Often used for MACs that don?t use type field

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LLC with SNAP
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What else?

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