Chapter 5: The Data Link Layer Our goals: understand principles behind data link layer 1. services: � error detection, correction � sharing a broadcast channel: multiple access � link layer addressing � reliable data transfer, flow control: done! 2. instantiation and implementation of various link layer technologies 5: DataLink Layer 5-1
Link Layer � 5.1 Introduction and � 5.6 Link-layer switches services � 5.7 PPP � 5.2 Error detection � 5.8 Link virtualization: and correction ATM, MPLS � 5.3Multiple access protocols � 5.4 Link-layer Addressing � 5.5 Ethernet 5: DataLink Layer 5-2
Link Layer: Introduction Some terminology: � hosts and routers are nodes � communication channels that connect adjacent nodes along communication path are links � wired links � wireless links � LANs � layer-2 packet is a frame � encapsulates layer-3 datagram data-link layer has responsibility of transferring datagrams from one node to adjacent node over a link 5: DataLink Layer 5-3
Link layer: context � Datagrams are routed � transportation analogy (Network layer) between � Take a trip from Princeton to source, destination, and Lausanne: routers over one or more links . • limo: Princeton to JFK � Each link is governed by its • plane: JFK to Geneva own protocol : • train: Geneva to Lausanne � e.g., Ethernet on first link, � tourist = datagram frame relay on � highway, air corridor, train intermediate links, 802.11 on last link tracks = communication links � Each link protocol provides � transportation mode = link different services layer protocol � e.g., may or may not � travel agent = routing provide RDT over link algorithm 5: DataLink Layer 5-4
Link Layer Services � framing, link access: � encapsulate datagram into frame, adding header, trailer � channel access if shared medium � “MAC” addresses used in frame headers to identify source, dest • different from IP address! all frames must have MAC addresses – non-routed frames don’t need IP addresses � reliable delivery between adjacent nodes � we learned how to do this already (chapter 3)! � seldom used on low bit-error link (fiber, some twisted pair) � wireless links: high error rates • Q: why both link-level and end-end reliability? 5: DataLink Layer 5-5
Link Layer Services (more) � flow control: � pacing between adjacent sending and receiving nodes � error detection : � errors caused by signal attenuation, noise. � receiver detects presence of errors: • signals sender for retransmission or drops frame � error correction: � receiver identifies and corrects bit error(s) without resorting to retransmission � half-duplex and full-duplex � with half duplex, nodes at both ends of link can transmit, but not at same time 5: DataLink Layer 5-6
Where is the link layer implemented? � in each and every host � link layer implemented in “adaptor” (aka network host schematic interface card NIC) application transport � Ethernet card, PCMCIA cpu memory network link card, 802.11 card � implements link , physical host bus layers controller (e.g., PCI) link � attaches into host’s physical physical system buses transmission � combination of network adapter hardware, software, card firmware 5: DataLink Layer 5-7
Adaptors Communicating datagram datagram controller controller sending host receiving host datagram frame � sending side: � receiving side � encapsulates datagram in � looks for errors, rdt, flow frame control, etc � extracts datagram, passes � adds error checking bits, to upper layer at receiving rdt, flow control, etc. side 5: DataLink Layer 5-8
Link Layer � 5.1 Introduction and � 5.6 Link-layer switches services � 5.7 PPP � 5.2 Error detection � 5.8 Link Virtualization: and correction ATM. MPLS � 5.3Multiple access protocols � 5.4 Link-layer Addressing � 5.5 Ethernet 5: DataLink Layer 5-9
Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction otherwise 5: DataLink Layer 5-10
Parity Checking Two Dimensional Bit Parity : Single Bit Parity: Detect and correct single bit errors Detect single bit errors 0 0 5: DataLink Layer 5-11
Internet checksum (review) Goal: detect “errors” (e.g., flipped bits) in transmitted packet used at transport layer (Layer 4) only Sender: Receiver: � compute checksum of � treat segment contents as received segment sequence of 16-bit integers � check if computed checksum equals checksum field value: � checksum: addition (1’s complement sum) of � NO - error detected segment contents � YES - no error detected. � sender puts checksum But maybe errors value into UDP checksum nonetheless? field 5: DataLink Layer 5-12
Checksumming: Cyclic Redundancy Check � view data bits, D, as a binary number � choose r+1 bit pattern (generator), G � goal: choose r CRC bits, R, such that � <D,R> exactly divisible by G (modulo 2) � receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! � can detect all burst errors less than r+1 bits � widely used in practice (Ethernet, 802.11 WiFi, ATM) 5: DataLink Layer 5-13
CRC Example Want: D . 2 r XOR R = nG equivalently: D . 2 r = nG XOR R equivalently: if we divide D . 2 r by G, want remainder R D . 2 r R = remainder[ ] G 5: DataLink Layer 5-14
Link Layer � 5.1 Introduction and � 5.6 Link-layer switches services � 5.7 PPP � 5.2 Error detection � 5.8 Link Virtualization: and correction ATM, MPLS � 5.3Multiple access protocols � 5.4 Link-layer Addressing � 5.5 Ethernet 5: DataLink Layer 5-15
Multiple Access Links and Protocols Two types of “links”: � point-to-point � PPP for dial-up access � point-to-point link between Ethernet switch and host � broadcast (shared wire or medium) � old-fashioned Ethernet � upstream HFC � 802.11 wireless LAN humans at a shared wire (e.g., cocktail party shared RF shared RF cabled Ethernet) (shared air, acoustical) (e.g., 802.11 WiFi) (satellite) 5: DataLink Layer 5-16
Multiple Access protocols � single shared broadcast channel � two or more simultaneous transmissions by nodes: interference � collision if node receives two or more signals at the same time multiple access protocol � distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit � communication about channel sharing must use channel itself! � no out-of-band channel for coordination 5: DataLink Layer 5-17
Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: � no special node to coordinate transmissions � no synchronization of clocks, slots 4. simple 5: DataLink Layer 5-18
MAC Protocols: a taxonomy Three broad classes: � Channel Partitioning � divide channel into smaller “pieces” (time slots, frequency, code) � allocate piece to node for exclusive use � Random Access � channel not divided, allow collisions � “recover” from collisions � “Taking turns” � nodes take turns, but nodes with more to send can take longer turns 5: DataLink Layer 5-19
Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access � access to channel in "rounds" � each station gets fixed length slot (length = pkt trans time) in each round � unused slots go idle � example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame 3 3 4 4 1 1 5: DataLink Layer 5-20
Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access � channel spectrum divided into frequency bands � each station assigned fixed frequency band � unused transmission time in frequency bands go idle � example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle t i m e frequency bands FDM cable 5: DataLink Layer 5-21
Random Access Protocols � When node has packet to send � transmit at full channel data rate R. � no a priori coordination among nodes � two or more transmitting nodes ➜ “collision”, � random access MAC protocol specifies: � how to detect collisions � how to recover from collisions (e.g., via delayed retransmissions) � Examples of random access MAC protocols: � slotted ALOHA � ALOHA � CSMA, CSMA/CD, CSMA/CA 5: DataLink Layer 5-22
Slotted ALOHA Assumptions: Operation: � all frames same size � when node obtains fresh frame, transmits in next � time divided into equal slot size slots (time to transmit 1 frame) � if no collision: node can send new frame in next � nodes start to transmit slot only slot beginning � if collision: node � nodes are synchronized retransmits frame in � if 2 or more nodes each subsequent slot transmit in slot, all with prob. p until nodes detect collision success 5: DataLink Layer 5-23
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