Data Link Layer

Moving Frames

Link Layer

• Protocols: ethernet, 802.11 wireless, 802.5 Token Ring and PPP

• Has node-to-node job of moving network layer datagrams over a single link in the path

• Protocol implemented in adapter/NIC

Possible services

• Framing: encapsulate network datagram into frame

• Link Access: Medium Access Control protocol describes who gets access when

• Reliable delivery: wireless links need this

• Flow control: response to limited buffering

• Error detection: done in hardware

• Half-duplex and full-duplex

Error Detection

• Parity check: use one bit; detects odd number of errors (even number undetected)

• Two dimensional parity: can detect and correct any one error

Error Detection

Cyclic Redundancy Check

• CRC codes also known as polynomial codes

• Agree on generator, G

• For given data D, choose r additional bits such that the resulting d+r bit pattern is exactly divisible by G

Multiple Access Protocols

• Broadcast link: more than one node on the (shared) channel wants to use the channel

• Categories:– Channel partitioning– Random access– Taking-turns

• S

Channel Partitioning

• TDM divides time into sections and further divides sections into (N) time slots; channel rate is R bps

• Eliminates collisions and is perfectly fair

• But: a node is limited to an average rate of R/N even if only node transmitting

• But: a node must always wait for its turn, even if only node transmitting

Code Division Multiple Access

• CDMA

• Assign a different code to each transmitting node; encode data bits; receiver can sort out correct transmission

• Used in wireless channels

Random Access

• Transmitting node at full channel rate• If collision, retransmit packet until packet

gets through without collision• Slotted ALOHA

– Node waits for beginning of next slot; transmits packet

– No collision; transmit another packet– Collision; retransmit [randomly] until success

• 1970

8

After all the math, the maximum efficiency of the protocol is 37% of Slots do useful work

In pure ALOHA, efficiency drops to 18%

Carrier Sense Multiple Access

• Listen before sending (carrier sensing)

• If collision, stop sending (collision detection)

• There are a lot of these that have been proposed

Taking-Turns Protocols

• Polling Protocol– Master node– Master polls each node in round-robin fashion– If master fails, the whole channel goes down

• Token-passing protocol– Small, special purpose frame = token– Can send frames if node has the token– Decentralized and highly efficient

MAC addresses

• Not the node, but the (NIC) adapter that has a link-layer address– LAN address– Physical Address– MAC address

• Permanent – burned into ROM• 6 bytes long; 3 for manufacturer, 3 for

adapter – expressed in Hex notation• IEEE manages address space

Addressing

• Frame includes destination MAC address

• Each adapter on LAN sees each frame– If address is a match, adapter sends frame up

the protocol stack in parent node– If address is not a match, adapter discards

frame

• Broadcast address is all ones (FF FF…)

Address Resolution Protocol

• We have both network addresses (IP) and link addresses (MAC); ARP does translation

• On same LAN

• ARP table stored in RAM

• TTL typically 20 minutes

• ARP query sent in broadcast frame; response in a standard frame

A

B

We want to get a frame from A to BIf our link address is 49-BD … we will never get out of 111 subnetWe have to use router’s E6-E9 address first (with IP 222.222)

Dynamic Host Configuration Protocol

• DHCP is a client/server protocol– Client is newly arriving host

• DHCP server discovery– UDP packet to port 67– Destination address 255.255 and source 0.0 (this host)– Frame has FF-FF destination address

• DHCP server offer(s)– Includes lease time– Link layer frame containing the IP datagram containing the UDP

segment containing the DHCP offer message is then sent

• DHCP request by client• DHCP ACK to client

Ethernet

• Ethernet has taken over the wired LAN market

• Bob Metcalf’s original bar napkin:

Ethernet Frame

• The sending adapter encapsulates the IP datagram within an ethernet frame and passes the frame to the physical layer…

• Preamble: 8 bytes, seven of 10101010 and one of 10101011

• Destination address: 6 bytes• Source address: 6 bytes (MAC address)• Type: IP, IPX, AppleTalk, ARP• Data: 46-1500 bytes• Cyclic Redundancy Check: 4 bytes

Ethernet

• Uses baseband transmission

• Connectionless service (like IP’s datagram and UDP’s segment)

• Unreliable service: adapter B runs CRC check; if pass frame handed to network layer; if fail, frame is discarded; no message back to adapter A

CSMA/CD

• An adapter may begin to transmit at any time (provided the channel is clear)

• Adapter never transmits when it senses that some other adapter is (carrier sense)

• Adapter aborts transmission as soon as it detects another adapter transmitting (collision detection) – sends 48-bit jam signal

• Before attempting retransmission, adapter waits a random time (typically small) [exponential backoff]

Ethernet Technologies

• Most common are 10BaseT and 100BaseT which use twisted pair copper wire in a star/bus topology

• Two wires for transmit; two for receive

• T is for twisted pair

• Maximum cable length is 100 meters

Hubs

• Physical layer device: simply senses bits and amplifies the signal to all ports

• Extends the collision domain

• Can also extend the reach of a LAN by 100 meters each

• Can only connect similar-speed LANs (10- or 100BaseT)

Switches

• Operate on ethernet frames – layer 2 devices

• Forward frames based on LAN destination addresses

• Switch does not have MAC addresses• Thanks to buffers, can connect differing

technologies (10Base2, 10BaseT, 100BaseT)

• Switches are self-learning

Now have three collision domains, thanks to switch

A to A’ at 10B to B’ at 10C to C’ at 10 for total of 30Mbps

Point-to-Point Protocol

• Used between modem and ISP; still have a lot of that

• Flag field• Address field (only 11111111)• Control field (only 11)• Protocol field (IP, AppleTalk, DECnet)• Information (packet)• Checksum (2 or 4 byte CRC)