Download - 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 ThroughputLow CostShort DistancesOften shared medium access
Most new installations usually "switched"
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Shared Medium Access
A Shared Medium Used by AllOnly One Station Transmits at a TimeStations "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 LANIEEE Standardized as 802.3Several Generations
Same frame format (mostly)Changing data ratesDifferent physical layer requirements
The book: Gigabit Ethernet, Rich Seifert
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Ethernet Transmission
Only one station transmits at a timeSignal propagates entire cable lengthAll stations receive all transmissionsCSMA/CD medium access control scheme
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CSMA/CD
Carrier Sense (CS)Wait until medium is idleBegin to transmit frame
Multiple Access (MA)Multiple stations attached to shared mediaEach station uses the same access
algorithm
Simultaneous Transmission is Possible
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CSMA/CD [continued]
Simultaneous Transmission:Interfere with each otherKnown as a collision
CSMA with Collision Detect (CD)Listen to media during transmissionDetect whether another station’s signal
interferesBack 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-off5. Go to step 1 until max-try counter is
reached.
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Exponential Back-off Algorithm
Let 1 Slot Time = 512 bit timesUpon 1st collision, randomly choose among {0,1} slot delayUpon 2nd collision, randomly choose among {0,1,2,3} slot
delayUp 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
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The Collision Domain
Minimum Length Frame Must Be >= Maximum RTT of the Ethernet segment
Minimum Frame is 512 bitsRequires 46 bytes of data whether the upper
layer has them or not
Distances decrease as speed increasesFull-duplex mode eliminates the collision
domain
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An Aside - Collisions
They are NOT bad, unless they’re lateCollision statistics are mostly
meaninglessMonitor utilizationDistance MattersBecoming irrelevant with switchingThe name "Collision” is misleading
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Ethernet Addressing
Standardized by IEEEEach station assigned a unique 48-bit address
First 24-bits are the OUISecond 24-bits are vendor assigned
Usually set when NIC is manufacturedCanonical address format
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Ethernet Address Recognition
Each Frame Contains a Destination AddressAll Stations Receive All TransmissionsStation Discards Any Frame Not Destined for
ItImportant: 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/debuggingAllows interface to accept all framesAvailable on most Ethernet hardware
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IEEE 802.3 Frame Format
Sender fills in:Sender’s source
addressRecipient’s
destination addressType of data in the
frame type fieldCyclic Redundancy in
FCS field
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Demultiplexing on Frame Type Field
Network Interface HardwareReceives a copy of each transmitted frameExamines address and either accepts or discardsPasses accepted frame to system software
Network device softwareExamines frame typePasses 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 hubTwisted-pair wiring
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Ethernet Office Wiring
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High-speed Ethernet
Fast EthernetOperates at 100 Mb/s
Standardized in IEEE 802.3 as 100BASE-T and 100BASE-F standards
10/100 Devices available
Gigabit EthernetOperates 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 BeatInterframe Gap TimeCapture EffectModern Ethernet is a star-shaped busnews://comp.dcom.lans.ethernetIETF increasing maximum frame size?
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Example Ring: Token Ring
Popular in IBM environmentsIEEE Standardized as 802.5Operates 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 sendingSignal travels the entire ringSender 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 tokenToken will circle indefinitely until a
station wants to transmit
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MAC Frames
Ring management and control framesBeacon, Ring purge, claim token, report
error
Ring Poll every 7 secondsActive monitor presentStandby monitor presentNAUN notification process
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Active and Standby Monitor
Only 1 Active Monitor per ringAM is the master clock for the ringAM inserts 24-bit delay to
transmissionsAM ensures tokens/frames are presentAM removes circulating framesSMs are ready to take over if AM fails
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Monitor Contention
Ring elects a new Active MonitorInitiated when:
Loss of signal is detectedActive monitor not detectedTime-outs of token timer, NAUN, etc.
Highest MAC address winsEveryone else is Standby Monitor
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Token Ring Insertion Process
Phase 0 - Media Lobe CheckPhase 1 - Physical InsertionPhase 2 - Address VerificationPhase 3 - Participation in Ring PollPhase 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 IEEEEach station assigned a unique 48-bit address
First 24-bits are the OUISecond 24-bits are vendor assigned
Usually set when NIC is manufacturedNon-canonical address format
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Token Ring Address Recognition
Each Frame Contains a Destination AddressAll Stations Receive and Repeat All
TransmissionsStations 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 addressRecipient’s destination
addressCyclic Redundancy in
FCS field
Other stations may change:Frame Status
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High-speed Token Ring
HSTROperates at 100 Mb/s1 Gb/s was being worked onStandardized in IEEE 802.5Some 4/16/100 devices
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Why Token Ring Lost
IBM was the only systems manufacturer that promoted it
CostComplexitySupport throughout the industryOnly one vendor left to develop
product!
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Token Ring - Final Notes
JitterEarly Token ReleaseBackup PathToken Transmission TimerNeeds LLC - we haven’t talked about it
yetnews://comp.dcom.lans.token-ring
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Example Ring: FDDI
Uses Optical Fiber cablingHigh reliability (dual rings)Immune to interferenceStandardized by ANSITransmission rate of 100 Mb/sSimilar to token ring
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FDDI Dual Ring Operation
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Logical Link Control
Standardized by IEEE 802.2Often used for MACs that don’t use type field
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LLC with SNAP
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What else?
ATMWireless (802.11)Fiber ChannelHIPPIToken Bus (802.4)IEEE 802 standards may become free!