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THE MEDIUM ACCESS SUBLAYER

THE MEDIUM ACCESS SUBLAYER. 조 충 호. 고려대학교 전산학과 네트워크 연구실. The Channel Allocation Problem. Static Channel Allocation

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THE MEDIUM ACCESS SUBLAYER

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  1. THE MEDIUM ACCESS SUBLAYER 조 충 호 고려대학교 전산학과 네트워크 연구실

  2. The Channel Allocation Problem • Static Channel Allocation • Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) have been used traditionally to handle many users (N) of a single channel. The telephone company has always used these methods with great success. A reason for their success is that telephone calls have a steady flow of information with a predictable number of users. • But when the information flows in bursts and/or the number of users can fluctuate dramatically, these methods lead to terrible underusage of the channel bandwidth. • Why? • Mean time delay in Static FDM , • C:channel capacity, l / m: frame arrival/service rate • divide 1 channel up into N independent subchannels

  3. Dynamic Channel Allocation • Ok, so traditional methods don't work so well with computer traffic. What do we do about it. We use a more dynamic approach to channel allocation that is much more like a free-for-all. Here are some terms we need in the discussion: • Station Model • The network will contain N independent stations with lambda rate of frames. In a certain time period, delta t, the probability of a station generating a frame is lambda * (delta t). • Single Channel Assumption • There is only one channel so stations can't signal others in any way except on the one common channel they all are using • Collision Assumption • When two stations send at the same time the frames are garbled. • Time can be continuous or divided into intervals called slots. • Carrier Sense / No carrier Sense • Stations can either tell if the channel is in use or not.

  4. Medium Access Control Protocols • key issue : channel 할당 문제 • Multiaccess problem (multiaccess or multiple access protocols) • Multiaccess/broadcast media - satellite, ground radio, twisted pair, coaxial cable, optical fibre • Multiaccess - any user(station, node) can send messages through the medium • Broadcast - any user can receive messages through the medium • Advantages of multiaccess/broadcast network : • eliminates complex topological design and routing problem • achieves statistical load sharing • allows flexibility in future networking growth and changes • can accommodate mobile users

  5. FDMA TDMA SDMA CDMA Fixed MAC Aloha - pure, slotted, controlled Carrier sensing - with or w/o CD Random Access Dynamic Conflict Free Access Reservation Polling - roll call, hub(token) The channel allocation problem • Multiaccess problem - medium access control - coordination of channel access among the competing users • Classification

  6. Medium Access Control Techniques • Round Robin each station in turn is given the opportunity to transmit. during that opportunity, the station may transmit if the stations something to send, when it is finished, relinquishes its turn, and the right to transmit passes to the next station in logical sequence. • Reservation Time on the medium is divided into slots, much as with synchronous TDM, if station wishing to transmit reserves future slots for its use. • Contention For burst traffic, contention techniques are usually appropriate. With these techniques, no control is exercised to determine whose turn it is; its advantage is that the are simple to implement and, nder light to moderate load, efficient. If request size<< channel capacity, then the problem is not so significant.

  7. Random access protocols • ALOHA • Aloha was developed in Hawaii and used with radio broadcasts for LAN's. It was very simple. Everyone just sent frames at will. If a collision occurred it was detected and that frame was sent again after waiting a random amount of time. How efficient is this? Lets look at some math. . .

  8. Pure ALOHA- unslotted Aloha 1. Protocol • Ready users, users having packets to send, transmit their packets. • When two or more packet transmission overlap in time, a collision occurs and all the packets involved in the collision are destroyed.(non-capture) • When a packet collides with other packets, it is retransmitted after a random time. They detect the packet collision through the feedback channel. - explicit or implicit ACK

  9. new packets(S) channel traffic(G) Success(S) retransmission delay collision retransmission A B vulnerable period =2(A, B collide) 2. Analysis(Throughput) - Assumptions • Infinite population of users generate (new) packets according to a Poisson process with rate S packets/slot. • packet size is fixed • slot packet transmission time. • Channel traffic(offered load) = new transmission + retransmission. • channel traffic is also a Poisson process with rate G packets/slot. - This is obviously not exact; however, if the retransmission delay is ‘sufficiently’ large the assumption becomes reasonable. • A packet transmission is successful if no other packet begins transmitting its vulnerable period

  10. Pr[a packet tx. is successful] = Pr [no other packet tx. occurs within its vulnerable period] = S = mean arrival/success rate per frame time, G = mean tx. attempts per frame time Using S = GP0

  11. Slotted Aloha • channel time is divided into contiguous slots • packets are transmitted within a slot • Throughput • Stability problem of Aloha In order to simplify the discussion, we make the following assumptions. • Finite population of size N • Slotted channel. A B vulnerable period = 1

  12. Carrier Sensing Protocols • Carrier Sense Multiple Access(CSMA)- Listen before talk 1. If channel is idle, transmit 2. Otherwise, do one of the followings : • defer transmission and try again after a random delay.- non-persistent CSMA(NP-CSMA) • wait until channel becomes idle and transmit.- 1-persistent CSMA (1P-CSMA) Non- persistent : Transmit If Idle ,otherwise, Delay, Try Again ready 1-Persistent : P-persistent : Transmit As Soon as channel goes idle Transmit As son as channel goes idle with probability P If collision, Back Off and Try Again otherwise, Delay One Slot, Repeat process Channel Busy Time

  13. Contention slots t0 FRAME FRAME FRAME FRMAE Transmission period Contention period Idle period Time Carrier Sense Multiple Access with Collision Detection(CSMA/CD) • Listen before transmission & listen while transmitting • if a collision is detected abort its transmission and transmit a jamming signal for a duration of g slots. it then performs the same algorithm after a random retransmission delay • Transmission of jamming signal ensures all the users involved in the collision detect the occurrence of the collision. CSMA/CD can be in one of three states : contention, transmission, or idle.

  14. Channel utilization of RAP

  15. Frames 8 contention slots 8 contention slots 1 d 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 1 1 1 1 3 7 1 1 1 5 1 2 Central station Central station Collision free protocol • A Bit-map protocol • reservation transmission followed by packet transmission. • Station j announce the fact that it has a frame to send by inserting a 1bit into slot j. • After all N slots have passed by, each station has complete knowledge of which stations wish to transmit. • At that point, they begin transmitting in numerical order eg. N = 8 users • Polling Users are polled sequentially by the central processor. When polled a user.

  16. Wavelength Division Multiplexing Access Protocol • the spectrum is divided up into channels • each station is assigned 2 channels • each channel is divided up into groups of time slots • m control channels, n+1 data channels

  17. Range of A’s transmitter C A RTS B D C A CTS B D E E (a) (b) Range of B’s transmitter A sending an RTS to B B sending an CTS to A RTS: request to Send CTS:Clear to Send Wireless LAN • CSMA(초기) • MACA(Multiple Access With Collision Avoidance)-IEEE802.11(Karn 1990) • 1) RTS(A->B), CTS(B->A), DATA(A->B) • MACAW (MACA without DLC ACK - TL ACK)

  18. TDM frame Channel 959.8MHz 124 Base to module ... 935.4MHz 2 935.2MHz 1 914.8MHz 124 Module to base ... 890.4MHz time 2 890.2MHz 1 Digital Cellular Radio • GSM(Global System for Mobile Communication) • European digital system : IS-54(USA), JDC(Japan) • 900Mhz band - max 200 full-duplex channels(each 200kHz) • TDM • GSM uses 124 frequency channels, each of which has 8-slots • each slot consists of a 148 bit data frame

  19. GSM Framing Structure

  20. Wireless Data Communication System • 기존셀룰러 시스템 이용한 무선 데이터 시스템 • Cellular Digital Packet Data(CDPD) • 기존셀룰러 시스템에 무선데이터 통신장비를 추가로 설치해 음성채널의 휴지기를 이용해 데이터를 전송하는 방식- SK telecom • Celluar Data Inc.(CDI) • 셀룰러 시스팀을 이용하는 방법은 CDPD와 동일하나 음성채널과 음성채널 사이에 간섭을 방지하기 위해서 설정된 가드밴드를 활용해 데이터를 전송하는 방식 • 전용망을 이용한 전용패킷 무선데이터방식 • 미국의 ARDIS(DataTAC), 스웨덴의 Mobitex, 일본의 Teleterminal • 국내 현황: • Motorola의 DataTAC 시스팀- Dacom • Ericsson의 Mobitex시스팀 - KT

  21. 무선 데이터 시스템

  22. CDPD • 19.2 kbps rate (effective throughput 11-13 kbps) • Reed-Solomon Forward Error correcting

  23. FDMA, TDMA and CDMA • FDMA • It divides radio channels into a range of radio frequencies and is used in the traditional analog cellular system. With FDMA, only one subscriber is assigned to a channel at a time. • FDMA cellular standards include AMPS (Advanced Mobile Phone Service) and TACS (Total Access Communications System). • TDMA (IS-54) • It divides conventional radio channels into time slots to obtain higher capacity. Its standards include North American Digital Cellular, Global System for GSM (Mobile Communications), and PDC (Personal Digital Cellular). • CDMA (IS-95) • It uses a radically deferent approach. It assigns each subscriber a unique "code" to put multiple users on the same wideband channel at the same time. • The codes, called "pseudo-random code sequences", are used by both the mobile station and the base station to distinguish between conversations.

  24. CDMA(Code Division Multiple Access)(1) • A technique for spread-spectrum multiple access digital communication that creates channels through the use of noise-like carrier waves. • Depending on the level of mobility of the system, it provides 10 to 20 times the capacity of AMPS, and 4 to 7 times the capacity of TDMA. • CDMA is the only one of the three technologies that can efficiently utilize spectrum allocation and offer service to many subscribers without requiring extensive frequency planning. • All CDMA users can share the same frequency channel because their conversations are distinguished only by digital code, while TDMA operators have to coordinate the allocation of channels in each cell in order to avoid interfering with adjacent channels. The average transmitted power required by CDMA is much lower than what is required by analog, FDMA and TDMA technologies.

  25. CDMA(2) • CDMA uses a form of direct sequence. Direct sequence is, in essence, multiplication of a more conventional communication waveform by a pseudonoise (PN) binary sequence in the transmitter. • A second multiplication by a replica of the same sequence in the receiver recovers the original signal. • The noise and interference, being uncorrelated with the PN sequence, become noise-like and increase in bandwidth when they reach the detector.

  26. CDMA(3) • chips : m short intervals in each bit time • chip sequence: unique m-bit code assigned in each station • to transmit a 1 bit , a station sends its chip sequence : 00011011 • to transmit a 0 bit , it sends 1’s complement of its chip sequence : 11100100 • All chips sequence are pairwise orthogonal • normalized inner product of any distinct chip sequence, S.T=0

  27. CDMA(3) Binary Chip sequences for 4 stations Bipolar Chip sequences for 4 stations 6 examples of transmission Recovery of station C’s signal

  28. How Wireless Works • When a cellular mobile is switched on it scans the group of control channels to determine the strongest base station signal. Control channels are only involved in setting up a call and moving it to an unused channel. • When a telephone call is placed (1), signal is sent to the base station (3). • MSC dispatches the request to all BSs in the cellular system. The subscriber's telephone number is then broadcast as a paging message to the forward control channels throughout the cellular system. • The mobile receives the page, and identifies itself through the reverse control channel. The base station of the mobile informs the MSC of the "handshake", and the MSC instructs the base station to move the call to an unused channel (4)..

  29. IVD: Integrated Voice Data, MAN: Metropolitan Area Network MAC: Media Access Control, CSMA/CD: Carrier Sense Multiple Access / Collision Detect LAN Standards(IEEE 802) • Local Area Network (LAN) - Diameter few Km - Data rate = several Mbps - Owned by a single organization Bandwidth is not a scarce resource. protocol simplicity

  30. 802.3 CSMA/CD(Ethernet) • 1-persistent CSMA/CD LAN. • 802.3 Cabling name cable max.segment nodes/seg. advantage 10Base5 thick coax 500m 100 good for backbone 10Base2 thin coax 200m 30 cheapest system 10Base-T twisted pair 100m 1024 easy maintenance 10Base-F fiber optics 2000m 1024 best between buildings 10Base5 10Base2 10BaseT

  31. A B C Tap D Repeater Backbone (a) Linear (b)Spine (c)Tree (d) Segmented Cable topologies (a) room to room, with station tapping onto it at the nearest point (b) a vertical spine runs from the basement to the roof horizontal cables on each floor connected it by special repeaters’ (c) the most general topology(because a network with paths between some pairs of stations would suffer from interface between the two signals) (d) to allow larger networks, multiple cable can be connected by repeaters. * repeater : physical layer device, it receives, amplifies, and retransmits signals in both directions.

  32. Manchester Encoding 802.3 계열 Encoding 방식

  33. Bytes 7 1 2 or 6 2 or 6 2 0 - 1500 0 - 46 4 Destination Source preamble address address data pad checksum Start of frame delimiter Length of data field 802.3 MAC Sublayer Protocol • 802.3 frame format • preamble : each frame starts with a preamble of 7 bytes, each containing the bit pattern 10101010 • a Start of frame byte : containing 10101011 to denote the start of the frame itself. • two addresses : destination, source • length field : how many bytes are present in the data field( 0 - 1500) • pad : data portion of a frame 46bytes, is used to fill out the frame to the minimum size • valid frame size: at least 64 bytes long (from destination address to checksum)

  34. A B A minimum length frame Packet starts at time 0 Packet almost at B at - • 만일 minimum length size보다 더 짧은 frame이 전송 될 경우 collision이 발생 되어도 감지 되지 않고 sender는 성공적으로 전송이 완료 된 것으로 간주 할 것이다. • 이러한 상황이 발생하는 것을 막기 위해서 모든 전송될 frame은 2 이상이어야 한다 • valid frame for 802.3 is at least 64 bytes (why?) • MAC프로토콜은 모든 전송 노드가 충돌을 감지할 수 있도록 보장한다 • 전송노드는 충돌을 감지한 후 모든 노드가 충돌을 감지하도록 짧은 “jamming”신호를 전송한다 • 최소 프레임 길이의 제한은 송, 수신중인 노드가 충돌을 감지하지 못하는 경우를 없게 한다 A B (a) (b) Noise burst gets back to A at A B A B (c) Collision at time (d) : the propagation time for this frame to reach the other end - checksum : error detection

  35. 10BASET 허브 • 모든 스테이션은 UTP로 허브의 하나의 포트와 연결 • Manchester encoding방식을 이용 10Mbps데이터 율 • 최대 링크 거리는 100m • 허브(수동 Repeater) 동작 • 한 입력 링크의 신호는 모든 다른 링크로 리피트 됨 • 만약 두개 이상의 입력이 동시에 발생할 경우에는, 충돌된 신호가 모든 링크에 반복되어 제공됨 • 만약 한 링크의 입력 신호가 검출되면, 모든 링크에게 충돌 신호를 리피트 • 스위치(동적 Repeater)가 하나의 해결책 • 각 링크에 10Mbps의 용량 할당 • 한 링크를 복수의 스테이션이 액세스하면 역시 충돌

  36. IEEE 802.3 Operation: New Data

  37. IEEE 802.3 Operation: Attempt Transmit IEEE 802.3 is 1-persistent -- new packets transmitted as soon as the channel is idle. Packets are backlogged only when a collision occurs Minimum packet length ensures collision detection

  38. IEEE 802.3 Operation: Backlog Truncated binary exponential backoff used procedure BackOff; if attempts =1 then maxBackOff else if attemps <= backOffLimit then maxBackOff maxBackOff x 2; Wait(slotTime x UniformRandom (0,maxBackOff)) end; Parameters for 10BASE5 slotTime 512 bit times backOffLimit 10 Fail after 16 attempts

  39. 17 14 Logical ring Broadband coaxial cable 20 19 This station not currently in the logical ring 13 11 7 Direction of token motion • A token bus 802.4 Token Bus • Configuration - Users form a logical ring - Token(control frame) regulates the access of the broadcast bus The token is passed around the ring. Once a user receives the token, the user can access the channel up to some maximum time and passes the token to the next user in the ring. - Priority&guaranteed bandwidth. -can be used for time constrained information transfer. - Much more complex than 802.3

  40. Bytes 1 1 1 2 or 6 2 or 6 0 - 8182 4 1 Destination address Source address Data Checksum Frame control End deliminater Start delimiter Preamble The 802.4 frame format 802.4 Frame format - Preamble : to synchronize the receiver’s clock. - Start delimiter, end delimiter : to mark the frame boundaries - Frame control : to distinguish data frames from control frames it carries the frame’s priority.

  41. 802.5 Token Ring • Configuration Station Ring interface Ring interface 1bit delay Undirectional ring To station From station To station From station (a) A ring network (b) Listen mode (c) Transmit mode • Token operation -Capture the token, transmit one or more frames, remove the • frames(source removal), release the token.

  42. 1 1 1 SD AC ED (a) Token format Bytes 1 1 1 2 or 6 2 or 6 No limit 4 1 1 Destination address Source address SD AC FC Data Checksum ED FS Frame control Access control Starting delimiter Ending delimiter Framestatus (b) Data frame format The Token Ring Mac Sublayer Protocol - Acknowledgement from destination to source can be sent using A and C bits of the frame status(FS) field AC = 00 : destination not present AC = 10 : destination present but frame not accepted AC = 11 : destinaton present and frame copied.

  43. Distributed Queue Dual Bus(DQDB) • IEEE 802.6 MAN의 대표적인 통신구조 • 수십 km - 수백 km의 범위에 적용 • Fiber optic에 기반한 150 Mbps 데이터 전송률 • 단방향의 Dual-bus 토폴로지 • ATM의 셀 크기와 동일한 53byte크기의 셀

  44. Token bus Bridge Ethernet FDDI ring Bridge Bridge Computer Bridge Token ring Ethernet An FDDI ring being used as a backbone to connected LANs and computers. FDDI(Fiber Distributed Data Interface) • A high-performance fiber optic token ring LAN running at 100Mbps. • Covers up to 200km

  45. FDDI • FDDI는 IEEE 802.5을 일부 개선한 형태 • Fiber optic에 기반한 100 Mbps 데이터 전송률 • 신뢰성을 위한 Dual-ring 토폴로지 • Timed Token Ring 프로토콜 • 송신측에 의한 조기의 토큰 해방 • 토큰이 해방되는 시점 • 모든 데이터가 전송되었을 경우(802.5에서는 송신데이터가 모두 되돌아 올 때까지 기다림) • 송신노드에서 busy토큰을 받았을 때 • low-priority (asychronous) and high-priority (synchronous) 패킷을 지원 • 각 노드는 토큰의 도착 간격을 조사 • 낮은 우선순위 트래픽은 토큰간격 시간이 충분히 적을 때 전송 • 높은 우선순위 트래픽은 각토큰 도착시 일부 전송(limited time)

  46. Fast-Ethernet

  47. HIPPI(High Performance Parallel Interface) • 800 Mbps - 1600 Mbps

  48. Logical link control(LLC) • IEEE802.2 • Unacknowledged connectionless service • Connection-mode service • Acknowledged connectionless service Medium access control (MAC) CSMA/CD Token bus Token ring Token ring DQDB IEEE 802.5 FDDI IEEE 802.4 IEEE 802.6 IEEE802.3 Baseband coaxial: 10Mbps (2 versions) Unshileded twisted pair: 1,10Mbps Broadband coaxial: 10Mbps Broadband coaxial: 1.5, 10Mbps carrierband: 1,5,10Mbps Optical fiber: 5,10,20Mbps Shielded twisted pair:4,16 Mbps Unshielded twisted pair:4Mbps Optical fiber or coaxial: 44.736Mbps Physical Optical fiber: 100Mbps 중복

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