# transferring datagram from one node data-link layer has responsibility of to adjacent node over a link 5-1 TDTS06 Computer networks

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3 Internet checksum (review) Goal: detect errors (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Sender: treat segment as seq of 16-bit integers checksum: addition (1 s complement sum) of segment contents sender puts value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? 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 wraparound sum checksum D. 2 r R = remainder[ G ] Checksumming: Cyclic Redundancy Check Common CRC polynomials (G) view data bits, D, as a binary number choose r+1 bit pattern (generator polynomial), 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 ( WiFi, ATM) CRC-8-ATM: x 8 + x 2 + x + 1 CRC-10: x 10 + x 9 + x 5 + x 4 + x + 1 CRC-12: x 12 + x 11 + x 3 + x 2 + x + 1 (telecom systems) CRC-16-IBM: x 16 + x 15 + x (xmodem, USB) CRC-16-CCITT: x 16 + x 12 + x (X.25, V.41, PPP, Bluetooth) CRC-32-IEEE 802.3: x 32 + x 26 + x 23 + x 22 + x 16 + x x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1 (Ethernet)

4 Link types 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) (our focus!) old-fashioned Ethernet upstream Hybrid Fibre Coax (HFC) wireless LAN Shared medium single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time Medium Access Control (MAC) 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 shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) MAC Protocols: a taxonomy 1a. Channel Partitioning MAC protocols: TDMA Three broad classes: 1. Channel Partitioning divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use 2. Random Access channel not divided, allow collisions recover from collisions 3. Taking turns (Round Robin) nodes take turns, but nodes with more to send can take longer turns 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

5 1b. Channel Partitioning MAC protocols: FDMA 2. Random Access Protocols 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 frequency bands FDM cable 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 Example random access: Slotted ALOHA Slotted ALOHA Assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot if no collision: node can send new frame in next slot if collision: node retransmits frame in each subsequent slot with prob. p until success 5-19 Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization 5-20

6 Slotted Aloha efficiency Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send) suppose: N nodes with many frames to send, each transmits in slot with probability p prob that given node has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 max efficiency: find p* that maximizes Np(1-p) N-1 for many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives: Max efficiency = 1/e =.37 At best: channel! used for useful transmissions 37 % of time! 5-21 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [t 0-1,t 0 ]. P(no other node transmits in [t 0,t 0 +1] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) choosing optimum p and then letting n -> infty... = 1/(2e) =.18 even worse than slotted Aloha! 5-23 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t 0 collides with other frames sent in [t 0-1,t 0 +1] 5-22 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don t interrupt others!

7 CSMA collisions CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: received signal strength overwhelmed by local transmission strength human analogy: the polite conversationalist CSMA/CD collision detection Taking Turns MAC protocols Polling: master node invites slave nodes to transmit in turn typically used with dumb slave devices concerns: polling overhead latency slaves data data poll master single point of failure (master)

10 Ethernet Frame Structure Ethernet CSMA/CD algorithm Remember: Give peas a chance 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle + wait IFG (interframe gap), then transmits 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 46 bits 5. After aborting, NIC enters exponential backoff: after mth collision, NIC chooses K at random from {0,1,2,,2 m -1}. NIC waits K 512 bit times, returns to Step TDTS06 Lecture 7: Link layer I 5-38 Ethernet s CSMA/CD (more) CSMA/CD efficiency Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time:.1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec See/interact with Java applet on AWL Web site: highly recommended! Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten collisions, choose K from {0,1,2,3,4,,1023} T prop = max prop delay between 2 nodes in LAN t trans = time to transmit max-size frame efficiency efficiency goes to 1 as t prop goes to 0 as t trans goes to infinity better performance than ALOHA: and simple, cheap, decentralized! = t 1 prop/t trans

11 CSMA/CD efficiency, cont d 5-41 Hubs physical-layer ( dumb ) repeaters: bits coming in one link go out all other links at same rate all nodes connected to hub can collide with one another no frame buffering no CSMA/CD at hub: host NICs detect collisions twisted pair hub Ethernet Standards: Link & Physical Layers application transport network link physical 100BASE-TX 100BASE-T4 MAC protocol and frame format 100BASE-T2 100BASE-FX 100BASE-SX 100BASE-BX copper (twisted pair) physical layer fiber physical layer Supplement Year Description 802.3a BASE-2 thin Ethernet 802.3c Mbps repeater specification 802.3d 1987 Fiber Optic Inter Repeater Link 802.3i BASE-T twisted pair 802.3j BASE-F fiber optic 802.3u BASE-T Fast Ethernet and auto negotiation 802.3x 1997 Full duplex standard 802.3z BASE-X Gigabit Ethernet SX, LX, CX 802.3ab BASE-T Gigabit Ethernet over twisted pair 802.3ac 1998 Frame size extension to 1522 bytes for VLAN tag ad 2000 Link TDTS06 aggregation Lecture 7: Link for layer parallel I links 5-42 Switch link-layer device: smarter than hubs, takes active role transparent plug-and-play, selflearning switching: A-to-A and B- to-b simultaneously, without collisions not possible with dumb hub C B 6 5 A A B switch with six interfaces (1,2,3,4,5,6) C

12 Switch Table Q: how does switch know that A reachable via interface 4, B reachable via interface 5? A: each switch has a switch table, each entry: (MAC address of host, interface to reach host, time stamp) looks like a routing table! Q: how are entries created, maintained in switch table? something like a routing protocol? C B 6 5 A A B switch with six interfaces (1,2,3,4,5,6) C Switch: self-learning switch learns which hosts can be reached through which interfaces when frame received, switch learns location of sender: incoming LAN segment records sender/location pair in switch table C B MAC addr interface TTL 6 A A A A A Source: A Dest: A B C Switch table (initially empty) Self-learning, forwarding: example C A A A Source: A Dest: A B Interconnecting: Institutional network Switch frame destination unknown: flood destination A location known: selective send B A 6 A A A 4 A C to external network router S 4 mail server web server IP subnet MAC addr interface TTL A 1 60 A 4 60 Switch table (initially empty) A S 1 S 2 S 3 F TDTS06 Lecture 7: Link layer I

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