Computer Networks Data link layer Overview q

Computer Networks Data link layer

Overview q Design issues q Channel allocation q Point-to-point links q Multi access protocols q Local area Networks q Ethernet q Data Link layer Switching q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 2

LANs: channel allocation q Allocation? Who goes next? o Static: o Dynamic: assignment for long duration stations continuously compete Data link layer -- June 2004

LANs: channel allocation q Static channel allocation o Scheme: • Request channel • Data Transfer • Release channel o After allocation: • Channel is private • Can be used for a long time o Division of channel • FDM: Frequency division multiplexing • TDM: time division multiplexing Data link layer -- June 2004

LANs: channel allocation q Static channel allocation o Simple and efficient • Small & fixed number of users • Heavy load Mean delay C Capacity of channel in Mbps o Problems T Mean arrival rate in frames/sec • Loss of bandwidth 1/ – If some users are quiescent – If less users than subchannels • Poor performance T 1 = . C- Mean frame length in bits 1 TFDM = Data link layer -- June 2004 C N - N = N. T

Overview q Channel allocation q Design issues q Point-to-point links q Local area Networks q Data Link layer Switching q Multi access protocols o Aloha o CSMA protocols o CSMA/CD protocols o Collision free o Wavelength division multiple access protocols o Wireless LAN protocols q Ethernet q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 6

LANs: MA Protocols Multiple Access protocols q University of Hawaii Aloha system o Central computer system o Terminals spread over 4 islands o Communication: FM radio o No station to station communication o Shared channel for communication from terminals to computer system o Transmission strategy: • Terminal sends data as data comes available • Collision possible • Retransmission if no ack received Data link layer -- June 2004

LANs: MA Protocols q Pure Aloha: frames transmitted at arbitrary times tem s sy n tio ten n Co 2 frames sent at same time collision both frames destroyed Data link layer -- June 2004

LANs: MA Protocols q Pure Aloha: channel efficiency o Vulnerable period = 2 x packet time o Packet time = time required to transmit 1 frame Data link layer -- June 2004

LANs: MA Protocols q Aloha: channel efficiency o Pure: max 18% o Slotted (time divided in slots; start sending at start of slot) Data link layer -- June 2004

LANs: CSMA protocols Carrier Sense Multiple Access CSMA q Aloha type system + ability to test for a carrier i. e. a transmission q Persistent <> Nonpersistent o 1 -persistent • If channel is idle, a frame is transmitted • If channel is busy, the channel is continuously checked o Nonpersistent • If channel is idle, a frame is transmitted • If channel is busy, a random time is waited before channel is sensed again o p-persistent (slotted channel only) • If channel is idle a frame is transmitted with probability p • If channel is busy, next slot is sensed again Data link layer -- June 2004

LANs: CSMA protocols Carrier Sense Multiple Access CSMA q Performance Data link layer -- June 2004

LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect q Strategy o Try to detect collisions asap o Listen while transmitting o If collision is detected, abort transmission q Channel model Data link layer -- June 2004

LANs: CSMA/CD protocols Carrier Sense Multiple Access Collision Detect q Contention period o Worst case scenario o Detection = analog process Data link layer -- June 2004

LANs: collision free protocols q Problem? o 20 km, 100 Mbps o Very long, high bandwidth protocols = 100 sec or 10. 000 bits o 1 km, 10 Mbps o 1 km, 10 Gbps = 5 sec or 50 bits = 5 sec or 50. 000 bits q Bit-Map protocol o Each contention period has N slots (N = #stations) o Station k is assigned slot k; is used to indicate if station k has to send data o Data transmission proceed without collisions Data link layer -- June 2004

LANs: WDMA protocols Wavelength division multiple access q Approach: o Divide channel into subchannels (FDM, …) o Allocate them as needed q 2 channels/station o Narrow: used by other stations to signal the station: o Wide: used by station to output data frames Data link layer -- June 2004

LANs: WDMA protocols Wavelength division multiple access q 2 transmitters & 2 receivers for each station: o o Fixed-wavelength receiver for its own control channel Tunable transmitter for sending on other control channels Fixed wavelength transmitter for its own data channel Tunable receiver for other data channels q Support for 3 traffic classes: o Constant data rate connection oriented traffic o Variable data rate connection oriented traffic o Datagram traffic Data link layer -- June 2004

LANs: WDMA protocols Wavelength division multiple access q Scenario for datagram from A to B: o A: tunes on data channel of B o A: waits for status slot & selects free slot on control channel of B (e. g. slot 4) o A: sends on control channel of B, slot 4: data on data channel of A, slot 3 Collision possible! o B: tunes on data channel of A o B: accepts data on slot 3 B not ready to accept data Data link layer -- June 2004

LANs: WDMA protocols Wavelength division multiple access q Scenario for variable data rate connection from A to B: o A: tunes on data channel of B o A: waits for status slot & selects free slot on control channel of B (e. g. slot 4) o A: sends on control channel of B, slot 4: connection request o B: announces assignment of slot 4 to A in status slot of its data channel o A wants to send data: • A: sends on control channel of B, slot 4: data on data channel of A, slot 3 • B: tunes on data channel of A • B: accepts data on slot 3 Data link layer -- June 2004

LANs: wireless protocols q Common configuration for a wireless LAN o Base stations (access points) wired together o Notebooks with radio transmitter/receiver o A receiver within range of 2 active transmitters receives a garbled signal o Not all stations are in range of one another o CSMA does not work: interference at receiver is important not at sender o Example … Data link layer -- June 2004

LANs: wireless protocols q A sends to B q B sends to A q If C senses medium, it will q If C senses the medium, it not hear A q If C transmits to B, it will garble the signal at B may falsely conclude it cannot send to D Hidden station problem Exposed station problem Data link layer -- June 2004 21

LANs: wireless protocols Multiple Access with Collision Avoidance MACA q A wants to send to B q A sends RTS (request to send) frame (short frame) q B replies with CTS (clear to with length of data frame send) frame, containing same length Data link layer -- June 2004 22

LANs: wireless protocols Multiple Access with Collision Avoidance MACA A wants to send to B q A sends RTS (request to send) frame (short frame) with length of data frame q q Stations hearing RTS q Stations hearing CTS q should remain silent to not interfere with CTS B replies with CTS (clear to send) frame, containing same length should remain silent to not interfere with data frame Collisions possible! Wait random time Binary exponential backoff! Data link layer -- June 2004 23

LANs: wireless protocols MACAW = MACA for Wireless q Optimisations for MACA o Introduce ACK from receiver of data frame to sender: detect loss in DL iso network/transport layer o Add CSMA: avoid sending a RTS by a station close to a station sending to the same destination o Run binary exponential backoff for each destination iso each station o Exchange of information between stations about congestion Data link layer -- June 2004

Overview q Design issues q Channel allocation q Point-to-point links q Multi access protocols q Local area Networks q Ethernet q Data Link layer Switching q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 25

LANs: IEEE 802. 3 or Ethernet q Overview o 1 -persistent CSMA/CD • • When a station wants to transmit, it listens to the cable If idle, it transmits immediately If busy, it waits until the cable goes idle If collision, it waits a random time o History • • Real start: Aloha 3 Mbps experiment at Xerox ethernet Agreement between Intel, DEC, Xerox Base for IEEE 802. 3 Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q Cabling Name Cable Max Length Nodes/ segment Advantages 10 Base 5 Thick coax 500 m 10 Base 2 Thin coax 200 m 30 Cheapest system 10 Base-T Twisted pair 100 m 1024 Easy maintenance 10 Base-F Fiber optics 2000 m Data link layer -- June 2004 100 ? ? 1024 Best between buildings

LANs: IEEE 802. 3 or Ethernet Cabling q Thick Ethernet o Vampire taps, 2. 5 m apart o Segments up to 500 m + repeaters o Transceiver at tap q Twisted pair o Cables to central hub o Net = box o Easy maintenance q Thin Ethernet o Industry standard BNC connectors o Easier to install, more reliable, cheaper o Up to 200 m, 30 systems per segment o Transceiver on controller board q Fiber o Expensive due to cost of connectors o Excellent noise immunity o Preference for connections between buildings Data link layer -- June 2004 28

LANs: IEEE 802. 3 or Ethernet q MAC frame o Preamble: 7 bytes with 101010 o Start of frame delimiter: 101011 o Address: • For 10 Mbps: 6 byte addresses • Assigned by IEEE; globally unique • All 1 s: broadcast o Pad: to ensure minimum length of 64 bytes for frame; minimum frame must take 51. 2 sec Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q MAC frame: 2 definitions o DIX ethernet (original proposal of DEC, Intel, Xerox) o IEEE 802. 3 q Differences: o Type length: All defined types > 1500 o SOF: for compatibility with 802. 4 & 802. 5 Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q Binary exponential Backoff Algorithm o Randomisation in case of collisions? o After collision time is divided in discrete slots of 51. 2 sec o After 1 collision: station waits 0. . 1 slots before trying to send again o After 2 collisions: station waits 0. . 3 slots before … o After i collisions: station waits 0. . 2 i – 1 slots before … o After 10 collisions: interval is frozen at 1024 slots o After 16 collisions: failure reported o Why not always 1024 slots? o Fair? Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q Performance o Assumptions • Heavy load, k stations always ready to transmit • Constant retransmission probability (not exponential Backoff) o Channel efficiency P = P + 5. 4 P 1 B L 1 + 5. 4 c F Average length of collision period Data link layer -- June 2004 Time to transmit a frame B Bandwidth L Cable length F Frame length c Signal propagation speed

LANs: IEEE 802. 3 or Ethernet q Performance Why so poor? Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q From hubs to switches: o Collision domain = box hub = card #collisions reduced = line no collisions o Internal forwarding? Data link layer -- June 2004

LANs: IEEE 802. 3 or Ethernet q Fast Ethernet: 10 Mbps 100 Mbps q Implications on performance? P = P + 5. 4 q B + o L o F 1 1 + 5. 4 B L c F same efficiency: Twisted pair cabling: 100 m * 2 <> 2500 m for coax cable Data link layer -- June 2004

Fast Ethernet q Faster 802. 3, no other changes o Need for backward compatibility with existing LANs o New protocol unforeseen problems? o Get it done before technology changes q Simple basic idea: o Reduce bit time from 100 nsec to 10 nsec o Allow only hubs/switches q Cabling o 100 Base-T 4 o 100 Base-TX o 100 Base-F UTP 3, 25 Mhz signaling 4 twisted pairs (host to hub, hub to host, 2 switchable) Ternary signals, 3 wires 27 symbols 4 bits per cycle UTP 5, 125 Mhz signaling 2 twisted pairs Coding scheme: 4 B/5 B 4 bits in 5 clock periods Data link layer -- June 2004

Gigabit Ethernet q Goal of standards committee o 10 times faster 1000 Mbps or 1 Gbps o Remain backward compatible! Same frame format q Implications on performance? Same minimum/maximum frame size P = P + 5. 4 q B + o L o F 1 Same addressing scheme B L 1 + 5. 4 c F same efficiency: 2500 m for Ethernet 25 m for gigabit ethernet Data link layer -- June 2004

Gigabit Ethernet q Solution? Limitation on configurations o Only point-to-point line o Use hubs or switches Data link layer -- June 2004

Gigabit Ethernet q Full-duplex operation mode switches o Collisions impossible: CSMA/CD not used o Maximum cable length determined by signal strength q Half -duplex operation mode hubs o Collisions possible: CSMA/CD required o Cable length not to be reduced: 200 m! o Solutions: • Carrier extension: hardware padding of frames to minimum of 512 bytes • Frame bursting: transmit concatenated sequence of frames in a single transmission Data link layer -- June 2004

Gigabit Ethernet q Need for flow control o 1 ms 1. 000 bits or 1953 frames of minimal length q Control frames o Type = 0 x 8808 q PAUSE control frame o Wait for x time units o Time unit = 512 nsec Data link layer -- June 2004

Gigabit Ethernet q 10 -gigabit Ethernet o IEEE 802. 3 ae: standard approved in 2002 Data link layer -- June 2004

Overview q Design issues q Channel allocation q Point-to-point links q Multi access protocols q Local area Networks q Ethernet q Data Link layer Switching q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 42

LANs: 802 q LLC: Logical Link Control o Services: ~ data link o Header: ~ HDLC o Identical for all LANs q MAC: Medium Access Control o Access to medium specific layer o Service • Datagram with some ack Data link layer -- June 2004 43

Overview q Design issues q Channel allocation q Point-to-point links q Multi access protocols q Local area Networks q Ethernet q Data Link layer Switching q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 44

Wireless LANs – 802. 11 q 2 modes: o In the presence of a wired base station – access point o In the absence of a base station – ad hoc networking q Make 802. 11 compatible with ethernet Data link layer -- June 2004

Wireless LANs – 802. 11 s M bp 1 o r 2 or t-r a ng er ad io in 2. 4 GH z. I SM ba nd Physical Layer Sh q Too slow!! Data link layer -- June 2004

Wireless LANs – 802. 11 q 802. 11 a - OFDM Orthogonal Frequency Division Multiplexing o 54 Mbps in 5 GHz ISM band o Different frequencies used (48 data + 4 synchronisation o Form of spread spectrum q 802. 11 b - HR-DSS High rate direct sequence spread spectrum o up to 11 Mbps in 2. 4 GHz ISM band o Supports 1, 2, 5. 5, 11 Mbps q 802. 11 g – OFDM o 54 Mbps in 2. 4 GHz ISM band Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q 2 modes of operation: o DCF – Distributed coordination function • CSMA/CA: physical + virtual channel sensing • 2 methods of operation – Physical channel sensing only – Physical + virtual channel sensing MACAW o PCF – Point coordination function • Base station controls all activity in cell • Polling mode + DCF Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q DCF – Distributed coordination function o Physical channel sensing only • Channel idle: station starts transmission busy: sense till idle • No listening during transmission • Collision: wait random time – binary exponential backoff o Physical + virtual channel sensing MACAW Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q DCF – Distributed coordination function o Physical channel sensing only o Physical + virtual channel sensing MACAW NAV = Network Allocation Vector or kind of virtual channel busy Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q PCF– Point coordination function o Polling: base station broadcasts a beacon frame periodically (10 – 100 times/sec) • Beacon frame contains system parameters • Invites new stations to sign up for polling service o Once signed up for polling at a certain rate guaranteed fraction of bandwidth o Supports battery management: forces a station to go into sleep state until awakened by base station or user; base station will buffer arriving frames Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q PCF & DCF combined o Central control (polling) combided with distributed control by using 4 timers Inter. Frame Spacing o Some dead time after every frame: 4 intervals Data link layer -- June 2004

Wireless LANs – 802. 11 MAC Sublayer Protocol q Transmission errors o Wireless noisy & unreliable o Long frames high chance for errors q Solution: fragmentation (+ Checksum, seq number, stop&wait) Data link layer -- June 2004

Wireless LANs – 802. 11 Frame Structure q 3 classes of frames: data, control, management Data frame Duration time to transmit frame + ack Seq. Supports fragment numbering: 12 bits frame + 4 bits fragment Address 1 destination Address 2 source Address 3 destination base station Address 4 source base station Data link layer -- June 2004

Wireless LANs – 802. 11 Frame Structure Data frame Version Support for different versions of protocol Type Data, control, management Subtype e. g. RTS, CTS To DS Frame to intercell distribution system From DS Frame from intercell distribution system MF More fragments to follow Retry Indicates retransmission of frame Pwr Used by base station to put receiver into sleep state or to wake it up More Sender has additional frames for receiver W Frame body encrypted using WEP (Wired -- June 2004 Data link layer Equivalent Privacy) O Sequence of frames must be processed strictly in order

Wireless LANs – 802. 11 q Services: o 5 distribution services: • • • Association Disassociation Reassociation Distribution Integration o 4 station services: • • Authentication De-authentication Privacy Data delivery Association: Ø Connect to base station Ø Announce identity & capabilities ü Data rates supported ü Need for PCF services ü Power management requirements Determines how to route frames sent to base station Handles translation to format of destination network Data link layer -- June 2004

Overview q Design issues q Channel allocation q Point-to-point links q Multi access protocols q Local area Networks q Ethernet q Data Link layer Switching q Logical link control q Wireless LANs q Broadband wireless Data link layer -- June 2004 57

Broadband wireless q 802. 16 or o Wireless MAN o Wireless local loop q Competition for last mile o Cable o Telephone system Designed for 802. 11 mobile ethernet 802. 16 mobile cable television Data link layer -- June 2004

802. 16 - Broadband wireless q Comparison with 802. 11 o Provide service to buildings and buildings are not mobile o Better radios can be used o Security & privacy are essential and mandatory o Users need more bandwidth operate in 10 – 66 GHz o Waves are strongly absorbed by water error handling more important (error correction) o Support for real-time traffic (telephony, television, …) Data link layer -- June 2004

802. 16 - Broadband wireless q Protocol stack Replaces LLC: aims for integration with Location of main protocols & channel management Ø Connection oriented Ø Controlled by base station Ø Datagram protocols: PPP, IP, Ethernet Ø ATM Encryption Decryption Key management Data link layer -- June 2004

802. 16 - Broadband wireless q Physical layer o Waves travel in straight lines o Multiple antenna’s possible o Signal strength falls off sharply with distance o 3 different modulation schemes Data link layer -- June 2004

802. 16 - Broadband wireless q Physical layer o Efficient use of available bandwidth: 2 schemes • FDD: frequency division duplexing • TDD: time division duplexing • first 2 subframes: upstream & downstream maps » What is in which slot? » Which time slots are free? – – – Base station sends out frames Each frame contains time slots Variable # slots for upstream & downstream traffic Base station maps downstream traffic to slots Upstream slot allocation done by MAC sublayer Error correction Data link layer -- June 2004

802. 16 - Broadband wireless q MAC protocol o MAC frame • Occupy an integral number of physical layer time slots o Allocation of upstream slots • Depends on service requested • Connection-oriented o Services: • • Constant bit rate Real-time variable bit rate Non-real-time variable bit rate Best-efforts rate Data link layer -- June 2004

802. 16 - Broadband wireless q MAC protocol o Services: • Constant bit rate – Slots are reserved when connection is set up • Real-time variable bit rate – Polling at fixed rate to know how much bandwidth is needed • Non-real-time variable bit rate – Polling (not at fixed rate) – No response for k times multicast group • Best-efforts rate – Requests done in time slots available for contention – Success noted in downstream map – No success: retry + binary exponential backoff Data link layer -- June 2004

802. 16 - Broadband wireless q Generic frame q EC: payload encrypted? q Type: frame type q Bandwidth request frame q CI: final CRC present? q EK: which encryption key used q Length q Connection ID Data link layer -- June 2004 65

Overview q Design issues q Point-to-point links q Local area Networks q Data Link layer Switching Data link layer -- June 2004 66

Computer Networks Data link layer -- June 2004