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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 stations signal
interferes Back off from interference and try again
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Transmission Logic1. 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 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
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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
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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
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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:
Senders source address
Recipients 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:
Senders source address Recipients 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 havent 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 dont 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!