TECHNISCHE UNIVERSITÄT ILMENAU Integrated Hard and Software Systems http://www.tu-ilmenau.de/ihs Media Access Schemes Motivation limits of CSMA/CD hidden and exposed terminals near-far problem SDMA, FDMA, TDMA overview TDMA Aloha, slotted Aloha Demand Assigned Multiple Access (DAMA) Multiple Access with Collision Avoidance: MACA, Polling, etc. CDMA theory and practice SAMA (Spread Aloha Multiple Access) Comparison
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Media Access Schemes - tu- · PDF fileMACA, Polling, etc. CDMA theory and practice ... Also severe problem for CDMA-networks ... Access methods SDMA/FDMA/TDMA SDMA
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TECHNISCHE UNIVERSITÄTILMENAU
Inte
grat
ed H
ard
and
Softw
are
Syst
ems
http
://w
ww
.tu-il
men
au.d
e/ih
sMedia Access Schemes
Motivationlimits of CSMA/CDhidden and exposed terminalsnear-far problem
SDMA, FDMA, TDMA overviewTDMA
Aloha, slotted AlohaDemand Assigned Multiple Access (DAMA)Multiple Access with Collision Avoidance: MACA, Polling, etc.
CDMA theory and practiceSAMA (Spread Aloha Multiple Access)Comparison
Wireless Internet 2Andreas Mitschele-Thiel 6-Apr-06
Media Access: Motivation
The problem: multiple users compete for a common, shared resource (medium)
Can we apply media access methods from fixed networks?
Example: CSMA/CDCarrier Sense Multiple Access with Collision Detection (IEEE 802.3)send as soon as the medium is free (carrier sensing – CS)listen to the medium, if a collision occurs stop transmission and jam (collision detection – CD)
Problems in wireless networkssignal strength decreases (at least) proportional to the square of the distancethe sender would apply CS and CD, but the collisions happen at the receiverit might be the case that a sender cannot “hear” the collision, i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is “hidden”
Wireless Internet 3Andreas Mitschele-Thiel 6-Apr-06
Hidden terminalsA sends to B, C cannot receive A C wants to send to B, C senses a “free” medium -> CS failscollision at B, A cannot receive C -> CD failsA is “hidden” for C
Exposed terminals
B sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryC is “exposed” to B
Motivation - hidden and exposed terminals
BA C
BA C
Wireless Internet 4Andreas Mitschele-Thiel 6-Apr-06
Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC cannot receive A
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!
Motivation - near and far terminals
A B C
Wireless Internet 5Andreas Mitschele-Thiel 6-Apr-06
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 receiverpermanent (e.g., radio broadcast), slow hopping (e.g. GSM), fasthopping (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 previously are now used to control medium access!
Wireless Internet 6Andreas Mitschele-Thiel 6-Apr-06
Communication link types
Each terminal needs an uplink and a downlink channel
Types of communication links:
Simplex unidirectional link transmission
Half DuplexBi-directional (but not simultaneous)
Duplex simultaneous bi-directional link transmission, two types:
Frequency division duplexing (FDD)Time division duplexing (TDD)
downlink
uplink
Wireless Internet 7Andreas Mitschele-Thiel 6-Apr-06
Duplex modes
Frequency Division Duplex (FDD)
Separate frequency bands for up- and downlink
+ separation of uplink and downlink interference
- no support for asymmetric traffic
Examples: UMTS, GSM, IS-95, AMPS
Fd
Fu
TdTu
TdTu
Time Division Duplex (TDD)
Separation of up- and downlink traffic on time axis
+ support for asymmetric traffic
- mix of uplink and downlink interference on single band
Examples: DECT, WLAN, UMTS (TDD)
Wireless Internet 8Andreas Mitschele-Thiel 6-Apr-06
FDD/FDMA - general scheme, example GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz
Wireless Internet 9Andreas Mitschele-Thiel 6-Apr-06
TDD/TDMA - general scheme, example DECT
1 2 3 11 12 1 2 3 11 12
tdownlink uplink
417 µs
Wireless Internet 10Andreas Mitschele-Thiel 6-Apr-06
Mechanismrandom, distributed (no central arbiter), time-multiplexSlotted Aloha additionally uses time-slots, sending must always start at slot boundaries
Aloha
Slotted Aloha
TDMA: Aloha/slotted aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t
Wireless Internet 11Andreas Mitschele-Thiel 6-Apr-06
TDMA: Demand Assigned Multiple Access (DAMA)
Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution of packet arrivals and packet lengths)
Reservation can increase efficiency to 80%a sender reserves a future time-slotsending within this reserved time-slot is possible without collisionreservation also causes higher delaystypical scheme for satellite links (long round-trip-times)application to packet data, e.g. in GPRS and UMTS
Examples for reservation algorithms:Explicit Reservation (Reservation-ALOHA)Implicit Reservation (PRMA)Reservation-TDMA
Wireless Internet 12Andreas Mitschele-Thiel 6-Apr-06
ALOHA mode for reservation:competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible)
synchronisation: 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
Wireless Internet 13Andreas Mitschele-Thiel 6-Apr-06
TDMA: DAMA – Packet Reservation (PRMA)
Implicit reservation (PRMA - Packet Reservation MA):a certain number of slots form a frame, frames are repeatedstations compete for empty slots according to the slotted aloha principleonce 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 sendcompetition for this slots starts again as soon as the slot was empty in the last frame
1 2 3 4 5 6 7 8 time-slot
frame2 A C A B A
frame3 A B A F
frame4 A B A F D
frame5 A C E E B A F Dt
frame1 A C D A B A FACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservations
collision at reservation attempts
ACEEBAFD
New successful reservation attempts are in bold letters
Wireless Internet 14Andreas Mitschele-Thiel 6-Apr-06
TDMA: DAMA - Reservation-TDMA
Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slotsevery 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 N * k data-slots
reservationsfor data-slots
other stations can use free data-slotsbased on a round-robin scheme
e.g. N = 6 stations, k = 2 data slots per station
Advantage: (small) guaranteed bandwidth with small latency for each station
Disadvantages: fixed number of stations (mini slots); global coordination
Wireless Internet 15Andreas Mitschele-Thiel 6-Apr-06
TDMA: Multiple Access with Collision Avoidance (MACA)
Motivation: deal with hidden terminals without a base station (central controller)
MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance
RTS (request to send): a sender requests the right to send from a receiver with a short RTS packet before it sends a data packetCTS (clear to send): the receiver grants the right to send as soon as it is ready to receiveall other stations listen to the signal
Wireless Internet 18Andreas Mitschele-Thiel 6-Apr-06
TDMA: Polling mechanisms
If one terminal can be heard by all others, this “central” terminal (e.g. 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, round-robin, random, reservation-based)
Example: Randomly Addressed Pollingbase station signals readiness to all mobile terminalsterminals ready to send can now transmit a random number withoutcollision 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 received random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminalthis cycle starts again after polling all terminals of the list
Application to Bluetooth and 802.11 (possible access function)
Wireless Internet 19Andreas Mitschele-Thiel 6-Apr-06
TDMA: 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 stopsthe base station 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 (AMPS)
Wireless Internet 20Andreas Mitschele-Thiel 6-Apr-06
CDMA access method
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 numberthe receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function
Advantages: all terminals can use the same frequency, less planning neededhuge code space (e.g. 232) compared to frequency spaceinterference (e.g. white noise) is not codedforward error correction and encryption can be easily integrated
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 (power control)
Wireless Internet 21Andreas Mitschele-Thiel 6-Apr-06
CDMA principle
Code 0
Code 1
Code 2
Σ
data 0
data 1
data 2
Code 0
Code 1
Code 2
data 0
data 1
data 2
sender (base station) receiver (terminal)
Transmission viaair interface
Wireless Internet 22Andreas Mitschele-Thiel 6-Apr-06
CDMA by example
Source 2
Source 1
data stream A & B
Code 2
Code 1
spreading
Source 2 spread
Source 1 spread
spreaded signal
Wireless Internet 23Andreas Mitschele-Thiel 6-Apr-06
CDMA by example
Sum of Sources Spread
+
overlay of signals
Sum of Sources Spread + Noise
transmission and distortion (noise and interference)
Despread Source 2
Despread Source 1
decoding and despreading
Wireless Internet 24Andreas Mitschele-Thiel 6-Apr-06
CDMA in theory
Sender A
sends Ad = 1, key Ak = 010011 (i.e. -1 1 -1 -1 1 1)
sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)