Layer 2

Datalink / Media Access This layer is responsible for allowing the devices to access the media. Creates connections between devices If more than one device on the physical link: –Requires some kind of address –MAC Address, Ethernet –DLCI, Frame Relay –VPI/VCI, ATM –LABEL, MPLS

Simplest case Point to Point link HDLC – High level data link control The physical might be T1, FiberChannel, SONET … Note the layer 2 data technologies CO-EXIST on Layer 1 technologies: –Ethernet often runs over a T1 line. –PPP over RS232 –Etc.

Bit oriented (Peterson) HDLC, High-Level Data Link Control –Standardized version of SDLC (IBM) Also uses the 0x7E control sequence to delineate beginning and end of frames –If 0x7E appears in the body of the message, special provisions must be made via an “escape sequence”

Byte Oriented PPP, Point to Point Protocol –Follow on from SLIP –Simple method of placing IP in Layer2 –Byte oriented –Protocol field determines what’s in the payload. Byte oriented means the frame frame contains an integer number of bytes. LCP/NCP are sister protocols used for setting up PPP session –Negotiate Frame size, IP address, etc.

PPP Frame Format (TCP/IP, Stevens)

Typical layer 2 requirements CRC Cylical Redundancy Check –Checks to make sure there are no errors in the frame. May include FEC Forward Error Correction –Can detect and correct effort Flag telling which Layer 3 protocol should process the frame –Ethernet can carry several protocols simultaneously (IP, IPX, NetBEUI, etc) Sequence numbers –So frames can be ordered and missing ones resent

CRC vs. FEC Simple parity case is similar to (CRC) –00,01,10,11 possible patterns to be sent –000, 011,101,110 actually sent (bit 3 is parity) –If receiver sees 111, it knows there’s a problem Error correction codes are more complicated –When problem is seen, error can be used to determine proper sequence that was sent –Will be discussed later this course

From: Peterson and Davie If ACK not received in time, frame is retransmitted.

Flow control Need to make sure packets are getting to where they are being sent. General concepts: –If message gets there, send another message –If they are getting lost, try again –If trying again and again doesn’t solve the problem Give up and notify upper protocol layer Continuing to dump duplicate messages degrades the network performance dramatically. –Most protocols “time-out”

Data Ack Data t frame t prop  = t prop t frame = Distance/Speed of Signal Frame size /Bit rate = Distance  Bit rate Frame size  Speed of Signal = 1 2  + 1 U= 2t prop +t frame t frame U  Light in vacuum = 300 m/  s Light in fiber = 200 m/  s Electricity = 250 m/  s Stop and wait flow control

Data Ack t frame t prop U= Nt frame 2t prop +t frame = N 2  +1 1 if N>2  +1 Window based flow control

Sharing a Medium

Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times.

Pure ALOHA (2) Vulnerable period for the shaded frame.

Pure ALOHA (3) Throughput versus offered traffic for ALOHA systems. Best case 18% utilization pure Aloha, 36.8% for slotted Aloha. Slotted requires a clock source, only transmit at frame boundaries

Protocols that listen before transmitting Station listens to network, if busy –Wait till net available then transmit: if collision then, Back off for some time then send again if channel available. If the stations always retransmits when the network becomes available – CSMA 1 persistant If the station gradually becomes less aggressive about siezing the network when it’s busy: –CSMA nonpersistent If the station attempts to retransmit with some probability p less than 1 –CSMA p-persistent

Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

Collision-Free Protocols The basic bit-map protocol. –Station asserts the bit in it’s slot if it wants to transmit. –Stations then transmit in turn. –Reservation based, no chance of collision

Collision-Free Protocols (2) Binary countdown protocol More efficient than bit-map protocol –Bits in the stations addresses determine when they access the channel –Reduces overhead compared to bit-map

Limited-Contention Protocols Acquisition probability for a symmetric contention channel.

Ethernet overview

Channel Capacity (C) Bandwidth, Bit Rate, SNR, and BER related Channel Capacity defines relationship C = Maximum reliable bit rate C = W*Log 2 (1 + SNR) bps Bandwidth sets the maximum Baud rate

Channel Capacity (C) Bandwidth, Bit Rate, SNR, and BER related Channel Capacity defines relationship C = Maximum reliable bit rate C = W*Log 2 (1 + SNR) bps Bandwidth sets the maximum Baud rate SNR sets the maximum number of different symbols (the "M" in M-ary)

Normalized Propagation Delay NPD = End-to-End Propagation Delay Average time to inject a Packet NPD > 1 implies “High Speed Network” 1 or more packets can simultaneously be in transit NPD < 1 implies “Slow Speed Network” Packet front end hits far side before back end transmitted TransmitterReceiver High Speed Low Speed

Types of Traffic... Computer Data Bursty Highly sensitive to errors Not as time sensitive as voice or video Interactive Voice/Video Fixed Rate (if not compressed) *Not sensitive to errors Fixed or Variable Rate (if compressed) *Sensitive to errors Time Sensitive

IEEE Ethernet Based on late 1970’s technology Covers OSI Layers 1 & 2 10 Mbps Line Speed Logical Bus Designed to move Computer Data

Serial Bit Stream: NRZ Coding time +1 volts 0 T 00 Logic One Logic Zero Called ‘Non Return to Zero’ because voltage never dwells on zero volts. T Coax Cable

Ethernet Uses Manchester Coding time +1 volts 0 T 00 Logic One Logic Zero All symbols have a transition in the middle.

Ethernet Uses Manchester Coding time +1 volts 0 T High Pass Filters Emphasize Change

High Pass Filter Output time +1 0

Rectify (Absolute Value) time +1 0 T Result always has pulses T seconds apart. Useful for receiver synchronization.

What is CSMA/CD? Polite Conversation –One node active at a time –No deliberate interruptions –Collisions sometimes occur after a break

802.3 Back-Off Algorithm choose random number 1st Collision0, 1 2nd Collision0, 1, 2, 3 3rd Collision0, 1,..., 6, 7 4th Collision0, 1,..., 14, 15 10th Collision0, 1,..., 1022, th Collision0, 1,..., 1022, th CollisionPunt Wait (Random Number* ) seconds

802.3 Flow Chart Packet to Send? No Yes Set Collision Counter = 0 Traffic on Network? Yes No Send Packet Collision? No Jam Yes Bump Collision Counter by +1 16th Collision? Drop Packet. Notify Higher Layer Yes Back-Off No

Major Drawbacks of CSMA/CD... Worst case waiting time equals infinity (No guaranteed Bandwidth) No Priorities These make Ethernet the worst LAN protocol to use for Multimedia Traffic

802.3 Packet Format PreSFD Destination Address Source Address LenCRCData + Padding Bytes:

Preamble time +1 volts 0 T 0 0 Logic One Logic Zero Series of pulses generated at receiver T seconds apart & in middle of each symbol.

Transmitting a File Broken into smaller packets Initial packets from Layer 5 Open Logical Connection Packets from Layer 7 “Data” Contains Layer 7 traffic “Data” Contains Layer 3-5 info Packets from Layer 4 Acknowledgements Final packets from Layer 5 Close Logical Connection

10Base5 & 10Base2 (Obsolete) PC Printer Logical & Physical Bus All nodes monitor traffic 3 Nodes share 10 Mbps Coax Cable

10BaseT & Shared Hub PC Hub Logical Bus & Physical Star Shared hub (OSI Level 1) copies input bits to all outputs. All nodes monitor traffic. 4 nodes share 10 Mbps. Twisted Pair

10BaseT & Switched Hub PC Switched Hub Logical Bus & Physical Star Switched Hub (OSI Level 1 & 2) copies packet to proper output. Only the destination monitors traffic.

10BaseT & Switched Hub PC Switched Hub Logical Bus & Physical Star This example system can move up to 20 Mbps

10BaseT & Switched Hub PC Switched Hub Logical Bus & Physical Star Each node shares 10 Mbps with the Switched Hub.

10BaseT & Switched Hub PC Switched Hub Using Half Duplex 10BaseT, a collision occurs if PC & Switched Hub simultaneously transmit. reception is screwed up

IEEE 802.3u 100 Mbps Fast Ethernet Preserves CSMA/CD Preserves Packet Format Maximum End-to-End Lengths (a.k.a. Collision Domain) reduced to keep Normalized Propagation Delay low Sales are pretty good

Full Duplex System PC Switched Hub Most 1 Gbps (& many 100 Mbps) systems are Full Duplex. NIC’s are designed to simultaneously transmit & receive. Line no longer shared. No Collisions. No need for CSMA/CD.

1995 Two 100 Mbps ‘Ethernets’ introduced Version A –CSMA/CD MAC, Ethernet Frames Version B –Demand Priority MAC, Ethernet Frames IEEE said Version A is Ethernet –IEEE 802.3u Fast Ethernet IEEE said Version B is not Ethernet –IEEE VG-AnyLAN is currently Dead RIP

IEEE 802.3z 1 Gbps Ethernet Uses an extended version of CSMA/CD, including ‘Frame Bursting’ Best performance uses full duplex connections & switched hubs –CSMA/CD included so it can be called Ethernet Collision Domain same as Fast Ethernet Preserves Packet Format Good Sales More on GigE later

IEEE 802.1p Priority Tags 8 priorities MAC protocols remain unchanged Used by 802.1p enabled switches –Allows interactive voice or video to receive preferential treatment on an Ethernet LAN

IEEE Token Ring Based on early 1980’s technology Covers OSI Layers 1 & 2 4 or 16 Mbps Line Speed Logical Ring A ‘Token’ is passed around the ring Node must have the Token to transmit Guaranteed Bandwidth Has Priorities

802.5 Token Format SDACED Bytes: Starting Delimiter: Token/Frame starts here Access Control: Indicates whether Token or Frame, Priority Ending Delimiter: End of Token/Frame

802.5 Packet Format SDAC Destination Address Source Address CRCData Bytes: > FC Modified Token a.k.a Starting Frame Delimiter EDFS Frame Control: Ring Status Frame Status: Receiver indicates whether received OK

IEEE Token Ring Technically Superior to Ethernet 2nd most widely used LAN protocol Similar evolution to Ethernet –Logical & Physical Ring –Logical Ring, Shared Physical Star –Logical Ring, Switched Physical Star 100 Mbps products available since ’98 Sales sharply declining. Heading for LAN graveyard. RIP

Layer 2 Switching Why?? Bridges Spanning Tree Algorithm