Local Area Networks Lecture 17 Fall 2010 Token Ring and FDDI
IEEE 802.5 Ring LAN Unidirectional ring network 4 Mbps and 16 Mbps on twisted pair Differential Manchester line coding Token passing protocol provides access ß Fairness Access priorities Breaks in ring bring entire network down Reliability by using star topology
Star Topology Ring LAN Stations connected in star fashion to wiring closet Use existing telephone wiring Ring implemented inside equipment box Relays can bypass failed links or stations A Wiring Center E B C D
Token Frame Format Data frame format 1 1 1 6 6 4 1 SD AC FC Destination Source Information FCS ED address address 1 FS Token frame format SD AC ED Starting delimiter J K 0 J K 0 0 0 J, K nondata symbols (line code) J begins as 0 but no transition K begins as 1 but no transition Access control Ending delimiter P P P T M R R R J K 1 J K 1 I E PPP=priority; T=token bit M=monitor bit; RRR=reservation T=0 token; T=1 data I = intermediate-frame bit E = error-detection bit
Data Frame Format Data frame format 1 1 1 6 6 4 1 Destination Source SD AC FC Information FCS ED address address 1 FS Frame control F F Z Z Z Z Z Z FF = frame type; FF=01 data frame FF=00 MAC control frame ZZZZZZ type of MAC control Addressing 48 bit format as in 802.3 Information FCS Length limited by allowable token holding time CCITT-32 CRC Frame status A C x x A C x x A = address-recognized bit xx = undefined C = frame-copied bit
Other Ring Functions Priority Operation PPP provides 8 levels of priority Stations wait for token of equal or lower priority Use RRR bits to bid up priority of next token Ring Maintenance Sending station must remove its frames Error conditions Orphan frames, disappeared token, frame corruption Active monitor station responsible for removing orphans
Ring Latency & Ring Reinsertion M stations b bit delay at each station B=2.5 bits (using Manchester coding) Ring Latency: τ = d/ν + Mb/R seconds τ R = dr/ν + Mb bits Example Case 1: R=4 Mbps, M=20, 100 meter separation Latency = 20x100x4x10 6 /(2x10 8 )+20x2.5=90 bits Case 2: R=16 Mbps, M=80 Latency = 840 bits
(a) Low Latency (90 bit) Ring A A A A t = 0, A begins frame t = 90, return of first bit t = 210, return of header t = 400, last bit enters ring, reinsert token (b) High Latency (840 bit) Ring A A A A t = 0, A begins frame t = 400, transmit last bit t = 840, arrival first frame bit t = 960, reinsert token
Fiber Distributed Data Interface (FDDI) Token ring protocol for LAN/MAN Counter-rotating dual ring topology 100 Mbps on optical fiber Up to 200 km diameter, up to 500 stations Station has 10-bit elastic buffer to absorb timing differences between input & output Max frame 40,000 bits 500 stations @ 200 km gives ring latency of 105,000 bits FDDI has option to operate in multitoken mode
A X B E C D Dual ring becomes a single ring
FDDI Frame Format Data Frame Format 8 1 1 6 6 4 1 Destination Source PRE SD FC Information FCS Address Address ED Preamble 1 FS Frame control CLFFZZZZ C = synch/asynch L = address length (16 or 48 bits) FF = LLC/MAC control/reserved frame type CLFFZZZZ = 10000000 or 11000000 denotes token frame Token Frame Format PRE SD FC ED
Timed Token Operation Two traffic types Synchronous Asynchronous All stations in FDDI ring agree on target token rotation time (TTRT) Station i has S i max time to send synch traffic Token rotation time is less than 2*TTRT if S 1 + S 2 + + S M-1 + S M < TTRT FDDI guarantees access delay to synch traffic Station Operation Maintain Token Rotation Timer (TRT): time since station last received token When token arrives, find Token Holding Time THT = TTRT TRT THT > 0, station can send all synchronous traffic up to S i + THT-S i data traffic THT < 0, station can only send synchronous traffic up to S i As ring activity increases, TRT increases and asynch traffic throttled down
Local Area Networks 802.11 Wireless LAN
Wireless Data Communications Wireless communications compelling Easy, low-cost deployment Mobility & roaming: Access information anywhere Supports personal devices PDAs, laptops, data-cell-phones Supports communicating devices Cameras, location devices, wireless identification ß ß ß Signal strength varies in space & time Signal can be captured by snoopers Spectrum is limited & usually regulated
Ad Hoc Communications A C B D Temporary association of group of stations Within range of each other Need to exchange information E.g. Presentation in meeting, or distributed computer game, or both
Infrastructure Network Server Portal Distribution System Portal Gateway to the Internet AP1 AP2 A1 B1 BSS A A2 B2 BSS B Permanent Access Points provide access to Internet
Hidden Terminal Problem (a) A Data Frame C A transmits data frame B C senses medium, station A is hidden from C (b) Data Frame B Data Frame C A C transmits data frame & collides with A at B New MAC: CSMA with Collision Avoidance
CSMA with Collision Avoidance (a) RTS B A requests to send C (b) CTS B CTS A C B announces A ok to send (c) Data Frame B A sends C remains quiet
IEEE 802.11 Wireless LAN Stimulated by availability of unlicensed spectrum U.S. Industrial, Scientific, Medical (ISM) bands 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz Targeted wireless LANs @ 20 Mbps MAC for high speed wireless LAN Ad Hoc & Infrastructure networks Variety of physical layers
802.11 Definitions Basic Service Set (BSS) Group of stations that coordinate their access using a given instance of MAC Located in a Basic Service Area (BSA) Stations in BSS can communicate with each other Distinct collocated BSS s can coexist Extended Service Set (ESS) Multiple BSSs interconnected by Distribution System (DS) Each BSS is like a cell and stations in BSS communicate with an Access Point (AP) Portals attached to DS provide access to Internet
Infrastructure Network Server Portal Distribution System Portal Gateway to the Internet AP1 AP2 A1 B1 BSS A A2 BSS B B2
Distribution Services Stations within BSS can communicate directly with each other DS provides distribution services: Transfer MAC SDUs between APs in ESS Transfer MSDUs between portals & BSSs in ESS Transfer MSDUs between stations in same BSS Multicast, broadcast, or stations s preference ESS looks like single BSS to LLC layer
Infrastructure Services Select AP and establish associationwith AP Then can send/receive frames via AP & DS Reassociation service to move from one AP to another AP Dissociation service to terminate association Authentication service to establish identity of other stations Privacy service to keep contents secret
IEEE 802.11 MAC MAC sublayer responsibilities Channel access PDU addressing, formatting, error checking Fragmentation & reassembly of MAC SDUs MAC security service options Authentication & privacy MAC management services Roaming within ESS Power management
MAC Services Contention Service: Best effort Contention-Free Service: time-bounded transfer MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs) Contentionfree service MSDUs MSDUs Contention service Point coordination function Distribution coordination function (CSMA-CA) MAC Physical
Distributed Coordination Function (DCF) DIFS Contention window DIFS Busy medium PIFS SIFS Next frame Defer access Wait for reattempt time Time DCF provides basic access service Asynchronous best-effort data transfer All stations contend for access to medium CSMA-CA Ready stations wait for completion of transmission All stations must wait Interframe Space (IFS)
Priorities through Interframe Spacing DIFS Contention window DIFS Busy medium PIFS SIFS Next frame Defer access Wait for reattempt time Time High-Priority frames wait Short IFS (SIFS) Typically to complete exchange in progress ACKs, CTS, data frames of segmented MSDU, etc. PCF IFS (PIFS) to initiate Contention-Free Periods DCF IFS (DIFS) to transmit data & MPDUs
Contention & Backoff Behavior If channel is still idle after DIFS period, ready station can transmit an initial MPDU If channel becomes busy before DIFS, then station must schedule backoff time for reattempt Backoff period is integer # of idle contention time slots Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires Station can contend when backoff timer expires A station that completes a frame transmission is not allowed to transmit immediately Must first perform a backoff procedure
(a) RTS A requests to send B C (b) CTS B CTS A C B announces A ok to send (c) Data Frame B A sends C remains quiet (d) ACK B ACK B sends ACK
Carrier Sensing in 802.11 Physical Carrier Sensing Analyze all detected frames Monitor relative signal strength from other sources Virtual Carrier Sensing at MAC sublayer Source stations informs other stations of transmission time (in µsec) for an MPDU Carried in Duration field of RTS & CTS Stations adjust Network Allocation Vector to indicate when channel will become idle Channel busy if either sensing is busy
Transmission of MPDU without RTS/CTS DIFS Data Source SIFS ACK Destination DIFS Other NAV Defer Access Wait for Reattempt Time
Transmission of MPDU with RTS/CTS DIFS Source RTS Data SIFS CTS SIFS SIFS Ack Destination NAV (RTS) DIFS Other NAV (CTS) NAV (Data) Defer access
Collisions, Losses & Errors Collision Avoidance When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots) Station transmits frame when backoff timer expires If collision occurs, recompute backoff over interval that is twice as long Receiving stations of error-free frames send ACK Sending station interprets non-arrival of ACK as loss Executes backoff and then retransmits Receiving stations use sequence numbers to identify duplicate frames
Point Coordination Function PCF provides connection-oriented, contention-free service through polling Point coordinator (PC) in AP performs PCF Polling table up to implementor CFP repetition interval Determines frequency with which CFP occurs Initiated by beacon frame transmitted by PC in AP Contains CFP and CP During CFP stations may only transmit to respond to a poll from PC or to send ACK
PCF Frame Transfer TBTT Contention-free repetition interval PIFS B SIFS D1 + Poll SIFS U 1 + ACK SIFS D2+Ack+ Poll SIFS U 2 + ACK SIFS CF End Contention period Reset NAV NAV CF_Max_duration D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame
Frame Types Management frames Station association & disassociation with AP Timing & synchronization Authentication & deauthentication Control frames Handshaking ACKs during data transfer Data frames Data transfer
Frame Structure MAC header (bytes) 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body CRC MAC Header: 30 bytes Frame Body: 0-2312 bytes CRC: CCITT-32 4 bytes CRC over MAC header & frame body
Frame Control (1) MAC header (bytes) 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body CRC 2 2 Protocol version Type 4 1 1 1 1 1 1 1 1 Subtype To DS From DS More frag Retry Pwr mgt More data WEP Rsvd Protocol version = 0 Type: Management (00), Control (01), Data (10) Subtype within frame type Type=00, subtype=association; Type=01, subtype=ack MoreFrag=1 if another fragment of MSDU to follow
Frame Control (2) 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body CRC 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To DS From DS More frag Retry Pwr mgt More data WEP Rsvd To DS From DS 0 0 0 1 Address 1 Destination address Destination address 1 0 BSSID 1 1 Receiver address Address 2 Source address BSSID Source address Transmitter address Address 3 BSSID Source address Destination address Destination address Address 4 N/A N/A N/A Source address Meaning Data frame from station to station within a BSS Data frame exiting the DS Data frame destined for the DS WDS frame being distributed from AP to AP To DS = 1 if frame goes to DS; From DS = 1 if frame exiting DS
Frame Control (3) MAC header (bytes) 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence control Address 4 Frame body CRC 2 2 Protocol version Type 4 1 1 1 1 1 1 1 1 Subtype To DS From DS More frag Retry Pwr mgt More data WEP Rsvd Retry=1 if mgmt/control frame is a retransmission Power Management used to put station in/out of sleep mode More Data =1 to tell station in power-save mode more data buffered for it at AP WEP=1 if frame body encrypted
Physical Layers LLC PDU LLC PLCP preamble PLCP header MAC header MAC SDU PLCP PDU CRC MAC layer Physical layer convergence procedure Physical medium dependent Physica layer 802.11 designed to Support LLC Operate over many physical layers
IEEE 802.11 Physical Layer Options Frequency Band Bit Rate 802.11 2.4 GHz 1-2 Mbps Modulation Scheme Frequency-Hopping Spread Spectrum, Direct Sequence Spread Spectrum 802.11b 2.4 GHz 11 Mbps Complementary Code Keying & QPSK 802.11g 2.4 GHz 54 Mbps Orthogonal Frequency Division Multiplexing & CCK for backward compatibility with 802.11b 802.11a 5-6 GHz 54 Mbps Orthogonal Frequency Division Multiplexing