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University of Nevada – Reno Computer Science & Engineering Department Fall 2015 CPE 400 / 600 Computer Communication Networks Lecture 22 Prof. Shamik Sengupta Office SEM 204 [email protected] http://www.cse.unr.edu/~shamik/ Chapter 5 Link Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 Thanks and enjoy! JFK/KWR All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Link Layer 5-2 The Data Link Layer Our goals: understand principles behind data link layer services: link layer addressing sharing a broadcast channel: multiple access reliable data transfer error detection, correction Understanding various link layer technologies Ethernet (wired domain) Hubs, Switches, Bridges Differences with Routers Wi-Fi (wireless domain) 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 layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 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! reliable delivery between adjacent nodes Q: why both link-level and end-end reliability? 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 Where is the link layer implemented? in each and every host link layer implemented in “adaptor” (aka network interface card NIC) Ethernet card, 802.11 card implements link, physical layer attaches into host’s system buses combination of hardware, software, firmware host schematic application transport network link cpu memory controller link physical host bus (e.g., PCI) physical transmission network adapter card MAC Addresses There are two types of addresses: 32-bit IP address: network-layer address used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address: function: get frame from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable MAC Addresses MAC (or LAN or physical or Ethernet) address: 48 bit MAC address MAC Addresses Each adapter on LAN has unique MAC address Locally administered Broadcast address = FF-FF-FF-FF-FF-FF 1A-2F-BB-76-09-AD 71-65-F7-2B-08-53 LAN (wired or wireless) = adapter 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 Which ones are globally unique and which ones are locally administered? ARP Link Layer 5-11 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 137.196.7.88 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 Each IP node (host, router) on LAN has ARP table ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address> Timeout: time after which address mapping will be forgotten (Varies from vendor to vendor, device to device) ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23 137.196.7.14 LAN 71-65-F7-2B-08-53 137.196.7.88 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 arp -a arp -s arp -d ARP protocol: Same LAN (network) A wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address dest MAC address = FF-FFFF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator Addressing: routing to another LAN Proxy-ARP: walkthrough: send datagram from A to B via R assume A knows B’s IP address 88-B2-2F-54-1A-0F 74-29-9C-E8-FF-55 A 111.111.111.111 E6-E9-00-17-BB-4B 1A-23-F9-CD-06-9B 222.222.222.220 111.111.111.110 111.111.111.112 R 222.222.222.221 222.222.222.222 B 49-BD-D2-C7-56-2A CC-49-DE-D0-AB-7D two ARP tables in router R, one for each IP network (LAN) Addressing: routing to another LAN A creates IP datagram with IP source A, destination B A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A R 111.111.111.111 74-29-9C-E8-FF-55 B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Link Layer 5-16 Addressing: routing to another LAN frame sent from A to R frame received at R, datagram removed, passed up to IP MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B IP src: 111.111.111.111 IP dest: 222.222.222.222 IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A IP Eth Phy R 111.111.111.111 74-29-9C-E8-FF-55 B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Link Layer 5-17 Addressing: routing to another LAN R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A R 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Link Layer 5-18 Addressing: routing to another LAN R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A R 111.111.111.111 74-29-9C-E8-FF-55 IP Eth Phy B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Link Layer 5-19 Addressing: routing to another LAN R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame contains A-to-B IP datagram MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A IP src: 111.111.111.111 IP dest: 222.222.222.222 IP Eth Phy A R 111.111.111.111 74-29-9C-E8-FF-55 B 222.222.222.222 49-BD-D2-C7-56-2A 222.222.222.220 1A-23-F9-CD-06-9B 111.111.111.112 CC-49-DE-D0-AB-7D 111.111.111.110 E6-E9-00-17-BB-4B 222.222.222.221 88-B2-2F-54-1A-0F Link Layer 5-20 Understanding the competition for medium (channel) access Protocols for Medium Access Control (MAC) Multiple Access Links and Protocols Two types of “links”: broadcast (shared wire or medium) Ethernet 802.11 wireless LAN humans at a cocktail party (shared air, acoustical) point-to-point point-to-point link between switches/Bridges and hosts shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) 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 Ideal Multiple Access Protocol What are the multiple access protocols? 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 1 3 4 1 3 4 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 FDM cable frequency bands “Taking Turns” MAC protocols Polling: master node “invites” slave nodes to transmit in turn typically used with “dumb” slave devices concerns: 1. 2. 3. polling overhead latency single point of failure (master) data poll master data slaves “Taking Turns” MAC protocols Token ring: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) T (nothing to send) T data Concerns with Ideal protocols Conservative Too much overhead wasted Not flexible, dynamic If one user has nothing to send that “slot” is wasted Internet is all about dynamic…why not make MAC protocol dynamic in nature? MAC: 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: Ethernet (IEEE 802.3) Wi-Fi (IEEE 802.11) Based on the principle of reducing collisions! 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! CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, collision detection 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 Ethernet Ethernet “dominant” wired LAN technology: cheap $20 for NIC first widely used LAN technology simpler, cheaper kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch Ethernet: physical topology bus: popular through mid 90s all nodes in same collision domain • can collide with each other star: prevails today active switch in center each “spoke” runs a (separate) Ethernet protocol • nodes do not collide with each other switch star bus: coaxial cable Link Layer 5-35 Ethernet frame structure sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame type dest. source preamble address address data (payload) CRC preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Link Layer 5-36 Ethernet frame structure (more) addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame type: indicates higher layer protocol IPV4, IPV6, ARP etc. CRC: cyclic redundancy check at receiver error detected: frame is dropped type dest. source preamble address address data (payload) CRC Link Layer 5-37 University of Nevada – Reno Computer Science & Engineering Department Fall 2015 CPE 400 / 600 Computer Communication Networks Lecture 23 Prof. Shamik Sengupta Office SEM 204 [email protected] http://www.cse.unr.edu/~shamik/ Ethernet: Unreliable, connectionless connectionless: No handshaking between sending and receiving NICs unreliable: receiving NIC doesn’t send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps (missing datagrams) gaps will be filled if app is using TCP otherwise, app will see gaps Ethernet’s MAC protocol: unslotted CSMA/CD Ethernet CSMA/CD algorithm 1. NIC receives datagram from network 4. If NIC detects another transmission layer, creates frame while transmitting, aborts, send a jam signal and prepare for retransmission 2. If NIC senses channel idle, starts frame transmission. 5. After collision, NIC enters If NIC senses channel busy, waits exponential backoff: after mth until channel idle, then transmits collision, NIC chooses K at random from {0,1,2,…,2m-1}. NIC waits K 3. If NIC transmits entire frame without slot times, returns to Step 2 detecting another transmission, NIC (1 slot = 512 bit times) is done with frame ! Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: .1 microsec for 10 Mbps 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} Overview of physical devices in the LAN Hubs Switches Bridges 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 Switch link-layer device: smarter than hubs, take active role store, forward Ethernet frames examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches typically do not need to be configured Switch: allows multiple simultaneous transmissions A hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions C’ B 1 2 6 5 4 C each link is its own collision domain Full duplex switching: A-to-A’ and Bto-B’ simultaneously, without collisions not possible with “dumb” hub 3 B’ A’ switch with six interfaces (1,2,3,4,5,6) Switch Table Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? A C’ B A: each switch maintains a switch table 1 2 6 5 4 C each entry in the table holds: MAC address of host interface to reach host time stamp 3 B’ A’ Switch: self-learning Q: how are entries created, maintained in switch table? A A A’ C’ B switch learns which hosts can be reached through which interfaces 1 2 6 when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table 5 3 4 C B’ MAC addr interface A Source: A Dest: A’ 1 A’ TTL 60 Switch table (initially empty) Self-learning, forwarding: example frame destination unknown: flood Source: A Dest: A’ A A A’ C’ B A6A’ 5 destination A location known: selective send 1 2 4 C A’ A B’ MAC addr interface A A’ 1 4 3 A’ TTL 60 60 Switch table (initially empty) Interconnecting switches switches can be connected together S4 S1 S3 S2 A B C F D E I G H Q: sending from A to G - how does S1 know to forward frame destined to G via S4 and S3? A: self learning! (works exactly the same as in single-switch case!) Institutional network mail server to external network router web server Link Layer 5-50 Switches vs. routers datagram both are store-and-forward: frame routers: network-layer devices application transport network link physical examine network-layer headers switches: link-layer devices frame link physical switch examine link-layer headers both have forwarding tables: routers: compute tables using routing algorithms, IP addresses switches: learn forwarding table using flooding, learning, MAC addresses network datagram link frame physical application transport network link physical Link Layer 5-51 Introduction to Bridges Many times it is necessary to connect a local area network to another local area network The LANs might be of different types E.g., Ethernet LAN, token ring LAN etc. Switches are not sufficient then! Local area network to local area network connections are often performed with a bridge A bridge interconnecting two dissimilar LANs Bridges A bridge can be used to connect two different LANs, such as a CSMA/CD LAN and a token ring LAN A bridge can also be used to connect two similar LANs, such as two CSMA/CD LANs Just like switch functionality Bridges (2) Just like switch, a bridge does not need programming but observes all traffic and builds routing tables from this observation This observation is called backward learning. Each bridge has two connections (ports) and there is a routing table associated with each port. A bridge observes each frame that arrives at a port, extracts the source address from the frame, and places that address in the port’s routing table. Bridges (3) More importantly, A bridge can also convert one frame format to another The bridge removes the headers and trailers from one frame format and inserts (encapsulates) the headers and trailers for the second frame format Data Communications and Computer Networks Chapter 8 Encapsulation University of Nevada – Reno Computer Science & Engineering Department Fall 2015 CPE 400 / 600 Computer Communication Networks Lecture 24 Prof. Shamik Sengupta Office SEM 204 [email protected] http://www.cse.unr.edu/~shamik/ VLANS Link Layer 5-59 VLANs: motivation consider: Computer Science Electrical Engineering Computer Engineering CS user moves office to EE, but wants connect to CS switch? single broadcast domain: all layer-2 broadcast traffic must cross entire LAN ARP, DHCP, unknown location of destination MAC address security/privacy, efficiency issues Link Layer 5-60 VLANs port-based VLAN: switch ports grouped (by switch management software) so that single physical switch Virtual Local Area Network switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over a single physical LAN infrastructure 1 7 9 15 2 8 10 16 … … Electrical Engineering (VLAN ports 1-8) Computer Science (VLAN ports 9-15) … operates as multiple virtual switches 1 7 9 15 2 8 10 16 … Electrical Engineering (VLAN ports 1-8) … Computer Science (VLAN ports 9-16) Link Layer 5-61 Port-based VLAN traffic isolation: frames to/from ports 1-8 can only reach ports 1-8 router can also define VLAN based on MAC addresses of endpoints, rather than switch port dynamic membership: ports can be dynamically assigned among VLANs 1 7 9 15 2 8 10 16 … Electrical Engineering (VLAN ports 1-8) … Computer Science (VLAN ports 9-15) forwarding between VLANS: done via routing (just as with separate switches) in practice vendors sell combined switches plus routers Link Layer 5-62 VLAN-aware switches Commercial 8-port VLAN-aware switches ($30-$60) Link Layer 5-63 VLANS spanning multiple switches 1 7 9 15 1 3 5 7 2 8 10 16 2 4 6 8 … Electrical Engineering (VLAN ports 1-8) … Computer Science (VLAN ports 9-15) Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN trunk port: carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches can’t be ordinary 802.1 frames (must carry VLAN ID info) 802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports Link Layer 5-64 802.1Q VLAN frame format type preamble dest. address source address data (payload) CRC 802.1 frame type preamble dest. address source address data (payload) 2-byte Tag Protocol Identifier (value: 0x8100) CRC 802.1Q frame Recomputed CRC Tag Control Information (3 bit priority field like IP TOS, 1 bit drop eligible indicator, 12 bit VLAN ID field) Link Layer 5-65 Link layer in wireless Link Layer 5-66 link access in wireless domain # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! computer networks: laptops, palmtops, PDAs, Smart Phones promise anytime wireless Internet access! It has really been a wireless revolution decade…with more to come Wireless is no longer a luxury but a necessity WLAN Market: WiFi Worldwide WLAN Infrastructure Shipments (Source: Gartner) Forecast Sales of Wi-Fi Equipment (Source: InfoTech Trends) 5 7 4 Millions of Units 6 $-bil 3 2 5 4 3 2 1 1 2001 2002 2003 2004 2005 20 01 20 02 20 03 20 04 20 05 20 06 20 07 0 0 WLAN growing exponentially Source: Pyramid Research Source: AirTight Networks IEEE 802.11 Wireless LAN 802.11b 802.11g 2.4 GHz unlicensed spectrum up to 11 Mbps 2.4 GHz range up to 54 Mbps 802.11a 5 GHz range up to 54 Mbps 802.11n: multiple antennae 2.4 / 5 GHz range up to 200 Mbps What else? 802.11 ac – builds on 802.n – provides 80-160MHz channels 802.11ad – 60GHz mmwave spectrum 802.11af – Super Wi-Fi all use CSMA/CA for multiple access all have infrastructure and ad-hoc network versions 802.11 LAN architecture wireless host communicates Internet AP hub, switch or router BSS 1 AP BSS 2 with base station base station = access point (AP) Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only Basic Service Set (BSS) BSS Extended Service Set (ESS) BSS’s with wired Distribution System (DS) BSS BSS 802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 13 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address selects AP to associate with will typically run DHCP to get IP address in AP’s subnet IEEE 802.11: multiple access problem 802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node Certain differences from Ethernet LAN in wired domain 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • Signal strength falls off rapidly with distance • Signal strength may weaken due to obstacles • Medium “air” shared among many users (not just WiFi users) can’t detect all collisions in any case: hidden terminal problem “Open” Wireless Medium Wireless interference S1 R1 S2 R1 Hidden terminal S1 R1 S2 Goal: CSMA/C(ollision)A(voidance) How does the medium access work in WLAN? Contention Based Distributed Coordination Function (DCF) Contention Free Point Coordination Function (PCF) Access methods DCF CSMA/CA (mandatory) • collision avoidance via exponential backoff • Minimum distance (IFS) between consecutive packets • ACK packet for acknowledgements (not for broadcasts) DCF with RTS/CTS (optional) • Distributed Foundation Wireless MAC • avoids hidden terminal problem PCF (optional) • access point polls terminals according to a list 802.11 – MAC DIFS = SIFS + (2 * Slot time) Priorities defined through different inter frame spaces SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response PIFS (PCF IFS) • medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service, competing stations DIFS DIFS medium busy PIFS SIFS access if medium is free DIFS contention next frame t WLAN access scheme details Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly automatic retransmission of data packets in case of transmission errors DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention Contention for channel When the other stations find the channel idle, they would like to transmit their own packets Contention for channel If all the waiting stations attempt at once, this will surely result in collision Some CA scheme is necessary Backoff intervals can be used to reduce collision probability DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention Backoff Interval When transmitting a packet, choose a backoff interval in the range [0,cw] cw is contention window Count down the backoff interval when medium is idle Count-down is suspended if medium becomes busy When backoff interval reaches 0, transmit packet B1 = 25 B1 = 5 wait data data B2 = 20 Assume cw = 31 wait B2 = 15 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 Backoff Interval The time spent counting down backoff intervals is a part of MAC overhead Choosing a large cw leads to large backoff intervals and can result in larger overhead Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously) Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed IEEE 802.11 DCF: contention window cw is chosen dynamically depending on collision occurrence Follows Binary exponential backoff algorithm Binary Exponential Backoff (BEB) in DCF Even before the first collision, nodes follow BEB Initial backoff interval (before 1st collision) [0,7] If still packets collide, double the collision interval [0,15], [0,31] and so on… Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! Collision Avoidance: RTS-CTS exchange A AP B reservation collision DATA (A) time defer Numerical Problems Practice Wi-Fi Hidden Node Problem Wi-Fi with RTS/CTS