MOB 3 media access 20010 lập trình mobi

Mobile Communications   Media Access   Motivation   SDMA, FDMA, TDMA   Aloha   Reservation schemes   Collision avoidance, MACA   Polling   CDMA   SAMA   Comparison Internet & Mobile Communications - 2010 Internet & Mobile Communications - 2010 2 Motivation   Can we apply media access methods from fixed networks?   Example CSMA/CD –  Carrier Sense Multiple Access with Collision Detection –  send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Internet & Mobile Communications - 2010 3 Background on LANs   Shared Access Networks are different –  they assume multiple nodes on the same physical link –  Bus, ring and wireless structures –  Transmission sent by one node is received by all others –  No intermediate switches   Need methods for moderating access (MAC protocols) –  Fairness –  Performance –  How can this be done? Internet & Mobile Communications - 2010 4 Background on LANs   Multiple Access Methods –  Fixed assignment   Partition channel so each node gets a slice of the bandwidth   Essentially circuit switching – thus inefficient   Examples: TDMA, FDMA, CDMA (all used in wireless/ cellular environments) –  Contention-based   Nodes contends equally for bandwidth and recover from collisions   Examples: Aloha, Ethernet –  Token-based or reservation-based   Take turns using the channel   Examples: Token ring Internet & Mobile Communications - 2010 5 Background on LANs   Our Focus is Ethernet –  History   Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s   Roots in Aloha packet-radio network   Standardized by Xerox, DEC, and Intel in 1978   LAN standards define MAC and physical layer connectivity –  IEEE 802.3 (CSMA/CD - Ethernet) standard – originally 2Mbps –  IEEE 802.3u standard for 100Mbps Ethernet –  IEEE 802.3z standard for 1,000Mbps Ethernet –  CSMA/CD: Ethernet’s Media Access Control (MAC) policy   CS = carrier sense –  Send only if medium is idle   MA = multiple access   CD = collision detection –  Stop sending immediately if collision is detected Internet & Mobile Communications - 2010 6 Background : Ethernet Overview   Most popular packet-switched LAN technology   Bandwidths: 10Mbps, 100Mbps, 1Gbps   Max bus length: 2500m –  500m segments with 4 repeaters   Bus and Star topologies are used to connect hosts –  Hosts attach to network via Ethernet transceiver or hub or switch   Detects line state and sends/receives signals –  Hubs are used to facilitate shared connections –  All hosts on an Ethernet are competing for access to the medium   Switches break this model   Problem: Distributed algorithm that provides fair access Internet & Mobile Communications - 2010 7 Dest addr 64 48 32 CRC Preamble Src addr Type Body 16 48 Background: Ethernet Overview (contd.)   Ethernet by definition is a broadcast protocol –  Any signal can be received by all hosts –  Switching enables individual hosts to communicate   Network layer packets are transmitted over an Ethernet by encapsulating   Frame Format Internet & Mobile Communications - 2010 8 Background: Ethernet Frames   Preamble is a sequence of 7 bytes, each set to “10101010” –  Used to synchronize receiver before actual data is sent   Addresses –  unique, 48-bit unicast address assigned to each adapter   example: 8:0:e4:b1:2   Each manufacturer gets their own address range –  broadcast: all 1s –  multicast: first bit is 1   Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to   Body can contain up to 1500 bytes of data Internet & Mobile Communications - 2010 9 Background: Ethernet’s MAC Algorithm   In Aloha, decisions to transmit are made without paying attention to what other nodes might be doing   Ethernet uses CSMA/CD – listens to line before/during sending   If line is idle (no carrier sensed) –  send packet immediately –  upper bound message size of 1500 bytes –  must wait 9.6us between back-to-back frames   If line is busy (carrier sensed) –  wait until idle and transmit packet immediately   called 1-persistent sending   If collision detected –  Stop sending and jam signal –  Try again later Internet & Mobile Communications - 2010 10 Background: State Diagram for CSMA/CD Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yes attempts < 16 attempts == 16 [...]... presented in chapter 2 are now used to control medium access! Internet & Mobile Communications - 2010 FDD/FDMA general scheme, example GSM f 960 MHz 935 .2 MHz 124 200 kHz 1 20 MHz 915 MHz 890.2 MHz 124 1 t 21 Internet & Mobile Communications - 2010 TDD/TDMA - general scheme, example DECT 417 µs 1 2 3 11 12 1 2 3 downlink uplink 11 12 t 22 Internet & Mobile Communications - 2010 Aloha/slotted aloha  ... signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) mechanism used, e.g., for CDPD (USA, integrated into AMPS) 32 Internet & Mobile Communications - 2010 Access method CDMA   CDMA (Code Division Multiple Access) –  –  –  –  all terminals send on the same frequency probably at the same time and can use... Internet & Mobile Communications - 2010 Background: Collision Detection contd B A B A time = 0 A B time = Tp time = 2Tp 13 Internet & Mobile Communications - 2010 Background: Exponential Backoff     If a collision is detected, delay and try again Delay time is selected using binary exponential backoff –  –  –  1st time: choose K from [0,1] then delay = K * 51.2µs 2nd time: choose K from [0,1,2 ,3] then... frame reservation ACDABA-F ACDABA-F AC-ABAFA -BAFD 26 ACEEBAFD 1 2 3 4 5 6 7 8 frame1 A C D A B A frame2 A C time-slot F A B A frame3 A frame4 A collision at reservation attempts B A F B A F D frame5 A C E E B A F D t Internet & Mobile Communications - 2010 Access method DAMA: Reservation-TDMA   Reservation Time Division Multiple Access –  –  –  every frame consists of N mini-slots and x data-slots... Internet & Mobile Communications - 2010 Access methods SDMA/FDMA/TDMA   SDMA (Space Division Multiple Access) –  –    FDMA (Frequency Division Multiple Access) –  –    20 assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) ... Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e „0“ 34 Internet & Mobile Communications - 2010 CDMA on signal level I data A 1 0 1 Ad key A key sequence A data ⊕ key 0 1 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1 Ak 1 0 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0 As signal A Real systems use much longer keys resulting in a larger distance between single code words in code space 35 Internet & Mobile Communications - 2010 CDMA on signal... strength at a receiver Advantages:         all terminals can use the same frequency, no planning needed huge code space (e.g 232 ) compared to frequency space interferences (e.g white noise) is not coded forward error correction and encryption can be easily integrated 33 Internet & Mobile Communications - 2010 CDMA in theory   Sender A –  –    Sender B –  –    sends Bd = 0, key Bk = 110101 (assign: „0“=... sending must always start at slot boundaries collision   Aloha sender A sender B sender C   Slotted Aloha collision t sender A sender B 23 sender C Internet & Mobile Communications - 2010 t DAMA - Demand Assigned Multiple Access     Channel efficiency only 18% for Aloha, 36 % for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) Reservation can increase efficiency to 80%... for ACK 30 RTS; CTS time-out ∨ data; NAK ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy Internet & Mobile Communications - 2010 RTS; RxBusy Polling mechanisms   If one terminal can be heard by all others, this “central” terminal (a.k.a base station) can poll all other terminals according to a certain scheme –    Example: Randomly Addressed Polling –  –  –  –  31 now... after polling all terminals of the list Internet & Mobile Communications - 2010 ISMA (Inhibit Sense Multiple Access)   Current state of the medium is signaled via a “busy tone” –  –  –  –  –  the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the . IEEE 802 .3 (CSMA/CD - Ethernet) standard – originally 2Mbps –  IEEE 802.3u standard for 100Mbps Ethernet –  IEEE 802.3z standard for 1,000Mbps Ethernet –  CSMA/CD: Ethernet’s Media Access Control.   SAMA   Comparison Internet & Mobile Communications - 2010 Internet & Mobile Communications - 2010 2 Motivation   Can we apply media access methods from fixed networks?  . competing for access to the medium   Switches break this model   Problem: Distributed algorithm that provides fair access Internet & Mobile Communications - 2010 7 Dest addr 64 48 32 CRC