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) Multiple Access (MA) 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 station’s 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-off 5. 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 they’re 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: Sender’s source address Recipient’s 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 Gigabit 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: Sender’s source address Recipient’s 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 haven’t 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 don’t 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!