- Slides: 42
Medium access control By Saumya Chaturvedi
PRIMARY MEDIUM ACCESS CONTROL (MAC) ATTRIBUTES • Collision avoidance – Basic task of a MAC protocol • Energy efficiency – One of the most important attributes for sensor networks, since most nodes are battery powered • Scalability and adaptively – Network size, node density and topology change
Other MAC Attributes • Channel utilization – How well is the channel used? Also called bandwidth utilization or channel capacity • Latency – Delay from sender to receiver; single hop or multi-hop • Throughput – The amount of data transferred from sender to receiver in unit time • Fairness – Can nodes share the channel equally?
MEDIUM ACCESS CONTROL (MAC) • The role of medium access control (MAC) – Controls when and how each node can transmit in the wireless channel • Why do we need MAC? – Wireless channel is a shared medium – Radios transmitting in the same frequency band interfere with each other – collisions – Other shared medium examples: Ethernet
ALOHA CHANNEL ALLOCATION
In pure ALOHA, frames are transmitted at completely arbitrary times.
PROCEDURE FOR ALOHA PROTOCOL
Aloha/slotted • Mechanism aloha – random, distributed (no central arbiter), timemultiplex collision – Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries sender A • sender Aloha B sender C t collision sender A sender B C • sender Slotted Aloha t
1 -persistent CSMA Station listens. If channel idle, it transmits. If collision, wait a random time and try again. If channel busy, wait until idle. If station wants to send AND do send. channel == idle then Success here depends on transmission time - how long after the channel is sensed as idle will it stay idle (there might in fact be someone else's request on the way. )
P-Persistent CSMA Slotted channel used When station become ready to send, it sense the channel, if it is idle , transmit with probability p or defer with probability q=1 -p and defer until next slot If that slot is idle either transmit or defer with probability p or q
Basic Bit map collision free protocol N station with unique address 0 to N-1 Propagation delay is negligible Each contention period consist of exactly N slots, and station send bit 1 during its slot. No other station is allowed to transmit the bit during assigned slot After N slots passed each station has complete knowledge which station wish to transmit Efficiency is d/(d+N)
The basic bit-map protocol, Collision-Free Protocols
Motivation Can we apply media access methods from fixed networks? FOR WIRED NETWORK: Signal strength in entire medium is almost same On collision every one knows Interested in collision at receiver rather than at sender Example CSMA/CD: send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802. 3) • Problems in wireless networks • • • – – signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that have hidden terminal, far and near problem Un-necessary delay in case of hidden terminal problem ( C want to send another mobile (other than A or B) while B is sending to A, C wait unnecessarily)
• Motivation - hidden and exposed Hidden terminals – A sends to B, C cannot receive A – C wants to send to B, C senses a “free” medium (CS fails) – collision at B, A cannot receive the collision (CD fails) – A is “hidden” for C • Exposed terminals A B C – B sends to A, C wants to send to another terminal (not A or B) – C has to wait, CS signals a medium in use – but A is outside the radio range of C, therefore waiting is not necessary – C is “exposed” to B
Motivation near and far terminals • Terminals A and B send, C receives – signal strength decreases proportional to the square of the distance – the signal of terminal B therefore drowns out A’s signal – C cannot receive A A B C • If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer • Also severe problem for CDMA-networks - precise power control needed!
Access methods SDMA/FDMA/TDMA • SDMA (Space Division Multiple Access) – segment space into sectors, use directed antennas – cell structure • FDMA (Frequency Division Multiple Access) – 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) – assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time • The multiplexing schemes presented in chapter 2 are now used to control medium access!
FREQUENCY DIVISION DUPLEX (FDD) • Simultaneous access of medium from base station to mobile station • Up-link frequency: from Mobile station to base station 890. 2 to 915 MHZ • Uplink frequency for any of n channel is fu=890 MHZ+n*0. 2 MHZ • Down-link frequency: from Mobile station to base station 935. 2 to 960 MHZ • Down-link frequency for any of n channel is fu=935 MHZ+n*0. 2 MHZ • Each up-link and down-link have 124 channel each with size of 200 KHZ each. Prof. Dr. -Ing. Jochen Schiller, http: //www. jochenschiller. de/ MC SS 05
FDD/FDMA - general scheme, example GSM f 960 MHz 935. 2 MHz 124 200 k. Hz 1 20 MHz 915 MHz 890. 2 MHz 124 1 t
TDM FOR DUPLEX 12 slot for each direction, total 24 slots for both Each slot have time 417 Micro second Pattern is repeated after 10 millisecond. Guarantee access of medium Perfectly opted for fixed rate and symmetric connection • It is too static and inflexible. • Inefficient for busty data or asymmetric connection • • •
TDD/TDMA - GENERAL SCHEME, EXAMPLE DECT 417 µs 1 2 3 11 12 1 2 3 downlink Prof. Dr. -Ing. Jochen Schiller, http: //www. jochenschiller. de/ MC SS 05 uplink 11 12 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% – – a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links • Examples for reservation algorithms: – Explicit Reservation according to Roberts (Reservation-ALOHA) Combination of reservation mechanism and fixed TDM Have reservation period followed by transmission period Collision can occur during reservation period not during transmission period. Follow explicit reservation policy Different mobile station on earth can reserve the slot for mobile satellite. – Reservation-TDMA: Combination of reserved ALOHA and best effort.
Access method DAMA: Explicit • Explicit Reservation (Reservation Aloha): Reservation – two modes: • ALOHA mode for reservation: competition for small reservation slots, collisions possible • reserved mode for data transmission within successful reserved slots (no collisions possible) – it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision Aloha reserved Aloha t
PACKET RESERVATION MULTIPLE ACCESS (PRMA) q Slots are reserved implicitly q Certain number of slots are repeated in fixed TDM manner q Satellite broadcast status of each slot to all mobile stations q All the station receive the vector and know which slot is occupied and which is free. q All station wishing to transmit can compete for free slot in ALOHA fashion q Already occupied slots are not touched q Combination of fixed and random TDM schemes with reservation policy. Ensure transmission with guaranteed data rate
Access method DAMA: PRMA – a certain number of slots form a frame, frames are repeated • Implicit reservation (PRMA - Packet Reservation MA): – stations compete for empty slots according to the slotted aloha principle – once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send – competition for this slots starts again as soon as the slot was empty in the last frame reservation ACDABA-F AC-ABAFA---BAFD ACEEBAFD 1 2 3 4 5 6 7 8 frame 1 A C D A B A frame 2 A C time-slot F A B A frame 3 A B A F frame 4 A B A F D frame 5 A C E E B A F D collision at reservation attempts t
Access method DAMA: Reservation. TDMA • Reservation Time Division Multiple Access – every frame consists of N mini-slots and x data-slots – every station has its own mini-slot and can reserve up to k data -slots using this mini-slot (i. e. x = N * k). – other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) N mini-slots reservations for data-slots N * k data-slots e. g. N=6, k=2 other stations can use free data-slots based on a round-robin scheme
MACA - COLLISION AVOIDANCE • MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance – RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet – CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive • Signaling packets contain – sender address – receiver address – packet size • Variants of this method can be found in IEEE 802. 11 as DFWMAC (Distributed Foundation Wireless MAC)
MACA examples • MACA avoids the problem of hidden terminals – A and C want to send to B – A sends RTS first – C waits after receiving CTS from B RTS CTS A CTS B C • MACA avoids the problem of exposed terminals – B wants to send to A, C to another terminal – now C does not have to wait for it cannot receive CTS from A RTS CTS A B C
MACA variant: DFWMAC in IEEE 802. 11 sender receiver idle packet ready to send; RTS Rx. Busy ACK time-out NAK; RTS wait for the right to send time-out; RTS data; ACK RTS; CTS time-out data; NAK CTS; data wait for ACK: positive acknowledgement NAK: negative acknowledgement Rx. Busy: receiver busy RTS; Rx. Busy
• 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 – now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) • Example: Randomly Addressed Polling – base station signals readiness to all mobile terminals – terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) – the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) – the base station acknowledges correct packets and continues polling the next terminal – this cycle starts again after polling all terminals of the list
Access method CDMA • CDMA (Code Division Multiple Access) – all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel – each sender has a unique random number, the sender XORs the signal with this random number – the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function • Disadvantages: – higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) – all signals should have the same 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
CDMA in theory • Sender A – sends Ad = 1, key Ak = 010011 (assign: „ 0“= -1, „ 1“= +1) – sending signal As = Ad * Ak = (-1, +1, -1, +1) • Sender B – sends Bd = 0, key Bk = 110101 (assign: „ 0“= -1, „ 1“= +1) – sending signal Bs = Bd * Bk = (-1, +1, -1) • Both signals superimpose in space – interference neglected (noise etc. ) – As + Bs = (-2, 0, 0, -2, +2, 0) • Receiver wants to receive signal from sender A – apply key Ak bitwise (inner product) • Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 2 + 0 = 6 • result greater than 0, therefore, original bit was „ 1“ – receiving B • Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 - 2 + 0 = -6, i. e. „ 0“
data A CDMA on signal level I 1 0 Ad 1 key A key sequence A data key 0 1 0 1 1 0 0 1 1 1 0 0 0 1 1 0 0 signal A Real systems use much longer keys resulting in a larger distance between single code words in code space. Ak As
CDMA on signal level II signal A As data B key sequence B data key signal B As + B s 1 0 Bd 0 0 1 1 0 1 0 1 1 1 0 0 0 0 1 1 1 Bk Bs
data A CDMA on signal level III 1 0 1 As + B s Ak (As + Bs) * Ak integrator output comparator output 1 0 1 Ad
data B CDMA on signal level IV 1 0 0 As + B s Bk (As + Bs) * Bk integrator output comparator output 1 0 0 Bd
CDMA on signal level V As + B s wrong key K (As + Bs) *K integrator output comparator output (0) ?
SAMA Spread Aloha Multiple Access • Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time • Idea: use spread spectrum with only one single collision 1 0 1 sender A(chipping narrow code sequence) for spreading for all 0 1 1 band sender B senders accessing according to aloha. send for a shorter period Combination of CDMA and TDMA with higher power spread the signal e. g. using the chipping sequence 110101 („CDMA without CD“) t Problem: find a chipping sequence with good characteristics
QUESTIONS 1. Consider the case where a mobile station receives different signals from different base stations and these signals is of different length. Discuss the methods used for selection of base station for your mobile. 2. Consider a mobile system where mobility of mobile station is more. Whether MACA (Multiple Access with Collision Avoidance) will be able to deal the hidden terminal problem occur in this situation? Explain with reason. 3. A Mobile services required to deals supported with arbitrary number of devices ( Mobiles). Discuss how Spread ALOHA Multiple Access SAMA a combination of CDMA and TDMA can be able to provide robust services with better collision rate?
TUTORIAL SHEET NO 2 Submission date: 05 -02 -2006 Submission Mode-Electronics copy only (in a group of 10 students )