spring 2006ucsc cmpe2571 cmpe 257: wireless networking set mac-1: medium access control protocols

Spring 2006 UCSC CMPE257 1

CMPE 257: Wireless Networking

SET MAC-1:

Medium Access Control Protocols


Topics Contention-based channel access

IEEE 802.11 as the main example, plus others Basics and focus on algorithms in class;

readings for more details on protocol and mechanisms

Scheduled channel access Directional antennas

Contention-based and scheduled MACs Multiple channels Modeling and performance analysis


Channel Access Schemes Contention based schemes

ALOHA, CSMA/CA (FAMA, MACA, MACAW, IEEE 802.11) : with/without RTS/CTS handshakes.

Difficulties: Throughput, fairness, multipoint communication support, QoS, synchronization.

Scheduled schemes FDMA/TDMA/CDMA in multi-hop networks:

graph coloring problem — UxDMA. Topology independent scheduling Topology-dependent scheduling Difficulties: Efficiency vs consistent state

information for scheduling; synchronization


IEEE 802.11 Standard PHY/MAC standard for wireless LANs

First standardized in 1997 Meet great success starting in 1999

Several working groups IEEE 802.11a: Operates in the 5GHz band; provides up

to 54 Mbps; uses OFDM 802.11b: Operates in 2.4 GHz band; provides 1,2, 5.5

and (nearly always) 11 Mbps; uses high rate DSSS 802.11g: Operates in 2.4GHz band; provides up to 54

Mbps; uses OFDM 802.11e: Quality of service (QoS) enhancement (still

active) 802.11i: Security enhancement 802.11s: Mesh-networking support


Requirements

Single MAC to support multiple PHYs Handle ``hidden terminal’’ problem Provisions time-bounded service

PHYs: Direct sequence Frequency hopping Infrared (never implemented)


Architecture

Infrastructure mode Basic Service Set (BSS) Access Point (AP) and stations (STA) take

different roles Distribution system (DS) interconnect multiple

BSSs to form a single network (not specified in the standard).

Ad hoc mode Independent Basic Service Set (IBSS) Single-hop (the standard makes this

assumption either explicitly or implicitly)


Access Points Stations select an AP and “associate” with it Support roaming (not part of the standard) Provide other functions

time synchronization (beaconing) power management support point coordination function (PCF)

Traffic typically (but not always) flows through AP

direct communication possible (in IEEE 802.11e)


802.11 Protocol Entities

MAC Sublayer

PLCP Sublayer

PMD Sublayer

MAC LayerManagement

PHY LayerManagement

StationManagement

LLC

MAC

PHY


802.11 Protocol Architecture MAC Entity

basic access mechanism fragmentation encryption

MAC Layer Management Entity synchronization power management roaming MAC MIB

Physical Layer Convergence Protocol (PLCP) PHY-specific, supports common PHY SAP provides Clear Channel Assessment (CCA) signal (carrier

sense)


802.11MAC in Detail Channel access mechanism

Distributed Coordination Function (DCF)•Carrier sense multiple access (CSMA) with immediate

MAC-level ACK•RTS/CTS exchange (optional)

Point Coordination Function (PCF)•Polled access through AP and distributed access•Contention-free period (CFP) and contention period

(CP)•Seldom implemented in practice

Synchronization and power management


CSMA/CA in 802.11DIFS

Contention Window

Slot time

Defer Access

Backoff-Window Next Frame

Select Slot and Decrement Backoff as long as medium is idle.

SIFS

PIFSDIFS

Free access when medium

is free longer than DIFS

Busy Medium

Channel priorities based on “free channel intervals”:1. SIFS (Short InterFrame Spacing): Used to give nodes in an ongoing

dialogue the chance to transmit. Example is RTS-CTS handshake, ACK to data, or consecutive data packets.

2. DIFS (DCF InterFrame Spacing ): Any station may attempt to transmit after DIFS elapses.

3. PIFS (PCF InterFrame Spacing): Base station can send poll or beacon if no station accesses channel after PIFS interval.

4. EIFS (Extended InterFrame Spacing): Longer than DIFS and used for base station to report a bad frame


CSMA/CA in 802.11DIFS

Contention Window

Slot time

Defer Access

Backoff-Window Next Frame

Select Slot and Decrement Backoff as long as medium is idle.

SIFS

PIFSDIFS

Free access when medium

is free longer than DIFS

Busy Medium

Reduce collision probability where mostly needed. Stations are waiting for medium to become free. Select Random Backoff after a Defer, resolving contention to avoid

collisions. Backoff algorithm stable at high loads.

Exponential Backoff window increases for retransmissions. Backoff timer elapses only when medium is idle.

Implement different fixed priority levels. To allow immediate responses and PCF coexistence.


CSMA/CA + ACK

• Defer access based on Carrier Sense.– CCA from PHY and Virtual Carrier Sense state.

• Direct access when medium is sensed free longer than DIFS, otherwise defer and backoff.

• Receiver of directed frames to return an ACK immediately when CRC correct.

– When no ACK received then retransmit frame after a random backoff (up to maximum limit).

Ack

Data

Next MPDU

Src

Dest

Other

Contention Window

Defer Access Backoff after Defer

DIFS

SIFS

DIFS


RTS/CTS Based Access

• Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Net Allocation Vector (NAV)Vector (NAV).

• Defer on either NAV or "CCA" indicating Medium BusyMedium Busy.

• Use of RTS / CTS is optional but must be implemented.

• Use is controlled by a RTS_ThresholdRTS_Threshold parameter per station. – To limit overhead for short frames.

RTS

CTS Ack

Data

NAV Next MPDU

Src

Dest

Other

CW

Defer Access Backoff after Defer

NAV

(RTS)

(CTS)

DIFS


Frame Format

MAC Header format differs per type(control, management, or data frames).

The subtype indicates RTS, CTS, ACK, etc. To/From DS bits indicate that frame comes from/goes to intercell

distribution system More Frag bit indicates that more fragments for frame follow Retry bit states that frame is a retransmission Pwr Mgt bit is used by station to put nodes in/out of sleep state More data bit states that sender has more frames for receiver WEP states if wired equivalent privacy algorithm was used to encrypt

frame O bit states that frames with bit set must be processed in order.

FrameControl

DurationID

Addr 1 Addr 2 Addr 3 Addr 4SequenceControl

CRCFrameBody

2 2 6 6 6 62 0-2312 4

802.11 MAC Header

Bytes:

ProtocolVersion

Type SubTypeToDS

RetryPwrMgt

MoreData

WEP O

Frame Control Field

Bits: 2 4 1 1 1 1 1 1 1 1

DSFrom More

Frag

2


Frame Format

Duration states how long frame and ack will occupy channel. Present in control frames and used for NAV

Four addresses are in standard IEEE 802 format Sequence control is used to number frames; 16 bits

identify frame, and 4 the fragment in a frame. Control frames have only 1 or 2 addresses, no data

and no sequence control fields.

FrameControl

Duration Addr 1 Addr 2 Addr 3 Addr 4SequenceControl

CRCPayload

2 2 6 6 6 62 0-2312 4802.11 MAC Header

Bytes:


Address Field Description

Addr 1 = All stations filter on this address. Addr 2 = Transmitter Address (TA)

Identifies transmitter to address the ACK frame to. Addr 3 = Dependent on To and From DS bits. Addr 4 = Only needed to identify the original

source of WDS ((Wireless Distribution System)Wireless Distribution System) frames.

To DS From DS Address 1 Address 2 Address 3 Address 40 0 DA SA BSSID N/A0 1 DA BSSID SA N/A1 0 BSSID SA DA N/A1 1 RA TA DA SA


Comments on CSMA/CA IEEE 802.11 cannot avoid collisions of data packets

(see [FAMA97]). CSMA/CA works fine when hidden terminals are very

few. For most single-hop wireless LANs, RTS/CTS is not

useful (turned off by default in practice) Spatial reuse is reduced in multi-hop networks Why ACK each frame? Why “sender initiated”? Where does the use of “free channel” intervals come

from? Is RTS-CTS inherently good or bad? Is synchronization in MANETs important?


Synchronization in 802.11 Timing Synchronization Function

(TSF) Used for Power Management

Beacons sent around well known intervals All station timers in BSS are synchronized

Used for Point Coordination Timing TSF Timer used to predict start of Contention

Free burst Used for hop timing for frequency hoping

(FH) PHY TSF timer used to time dwell interval All Stations are synchronized, so they hop at

same time.


All stations maintain a local timer. TSF goals:

Keep timers from all stations in synch AP to control timing in infrastructure networks Distributed function for Independent BSS

Timing conveyed by periodic beacon transmissions

Beacons contain timestamp for the entire BSS Timestamp from beacons used to calibrate local

clocks Not required to hear every beacon to stay in

synch Beacons contain other management information

also used for Power Management and roaming

Synchronization Approach


Infrastructure Beacon Generation

• Simple for infrastructure networks, because AP is the only one sending beacons.

• Beacons scheduled at beacon intervals.

• Transmission may be delayed by CSMA deferral.– subsequent transmissions at expected beacon interval, not relative to last

beacon transmission

– next beacon sent at Target Beacon Transmission Time (TBTT)

• Timestamp contains timer value at transmit time.

Time Axis

Beacon Interval

X X X X

"Actual time" stamp in Beacon

BeaconBeacon Busy MediumBusy Medium


TSF in Ad Hoc NetworksBeacon generation in IBSS is distributed and peer-to-

peerprocess aimed at beacon avoidance first, and adopting

the most current time second1) Each TBTT, node computes a random delay uniformly

distributed in [0, (2aCWmin)(aSlotTime)], e.g., 31x20us for DSSS

2) Node waits for random delay period

3) Node cancels its pending beacon and remaining random delay if a beacon arrives before timer expires

4) If random delay timer expires, node transmits beacon with a timestamp equal to its TSF timer value

5) When receiving a beacon, node sets its TSF equal to the time stamp of the beacon if the value of the timestamp is more recent than the TSF value.


Problems, Issues?1) There is no need to adopt the “latest” time in order to synchronize

nodes for the purpose of channel access activities.2) Beacon avoidance in the presence of many nodes may lead to

network being out of synchronization (TSF timer values at nodes differ by more than a threshold delta)

a) Fastest-station asynchronism: Clock of fastest node is ahead of clocks of rest of nodes by more than delta

b) Global asynchronism: A percentage of the n(n-1)/2 pairs of nodes are out of synchronization.

TSF in Ad Hoc Networks

timeasynchronism

L HH

E(H)/[E(H)+E(L)] = (1-p)^taup = prob. that node sends beacon successfullytau = delta/(diff. clock accuracy x beacon interval length)


Solutions?

Do not require clocks to always move forward

Have larger windows for beacon contention

Give priority of beacon transmission to nodes with most recent clock value.

…others? [HW]


Scheduled Access Problem description:

Given a set of contenders Mi of an entity i in contention context t, how does i determine whether itself is the winner during t ?

Topology dependence: In unit disc graph model, two-hop

neighbor information required to resolve contentions.

In ad hoc networks, two-hop neighbors are acquired by each node broadcasting its one-hop neighbor set.


Goals Safety: In our case, this amounts to collision avoidance

(avoid the hidden terminal problem) under a given set of assumptions.

Liveness: No node is precluded from accessing the channel for an indefinite period of time

Fairness: The channel is assigned to competing entities in proportion to their needs

Timeliness: Through additional mechanisms, a node can be granted guarantees on how long it needs to wait between consecutive channel access times.

Efficiency: Attain the highest possible maximum throughput Locality and distributed operation: No need for global

network information. Simplicity: The algorithm(s) used for scheduling are simple

enough to verify


Neighbor-Aware Contention Resolution (NCR)

In each contention context (time slot t ): Compute priorities

i is the winner for channel access if:

}{,)( iMkktkRandp itk

tj

tii ppMj ,

a bc

d

e

Contention Floor

69

5

4

2


Channel Access Probability: Dependent on the number of

contenders in the neighborhood. Channel access probability:

Bandwidth allocation general formula to i

}{iMk k

ii

iI

Iq


NAMA: Node Activation Multiple Access (Broadcast)

Channel is time-slotted. Transmissions are broadcasts via

omni-directional antenna: all one-hop neighbors can receive the packet from a node.

The contenders of a node for channel access are neighbors within two hops because of direct and hidden terminal contentions.


Algorithm


Illustration of NAMA

A B

C D E

F

HG

8

6

5

3 4

1

29


How Do We Share Context? Scheduling requires all nodes in two-hop

neighborhood of each receiver to have consistent state.

Neighbor protocol must disseminate such state

How fast can it be done? How should it be integrated with time sync

and the frame structure of the protocol? Each time slot has part of it dedicated to context? Part of the frame dedicated to context sharing?


Other Channel Access Protocols Other protocols using omni-directional

antennas: LAMA: Link Activation Multiple Access PAMA: Pair-wise Activation Multiple Access

Protocols that work when uni-directional links exist. Node A can receive node B’ s

transmission but B cannot receive A’ s. Protocols using directional antenna

systems.


Channel Access Probability Analysis of NAMA

The channel access probability for a single node i is given by

We are interested in average probability of channel access in multi-hop ad hoc networks.

}{iMk k

ii

i

I

Iq


Ad Hoc Network Settings Equal transmission range; Each node knows its one- and two-

hop neighbors — Mi . Nodes are uniformly distributed on

an infinite plane with density . A node may have different numbers

of neighbors in one-hop and two-hop.


Counting One-Hop Neighbors The prob of having k nodes in an

area of size S is a Poisson distribution:

Average one-hop neighbors is:

Note: the mean of r.v. with Poisson dist is2rS


Counting Two-hop Neighbors Two nodes become two-hop

neighbors if they share at least one one-hop neighbor. Average number in B(t):


Counting Two-hop Neighbors Probability of becoming two-hop: Prob of a node staying at tr is 2t. Summation of nodes in ring (r,2r)

times the corresponding prob of becoming two-hop --- number of two-hop neighbors:


Total One- and Two-hop Neighbors

Sum:

This is average number of one-hop and two-hop neighbors.


Average Probability of Channel Access

Apply Poisson distribution with the mean (number of one- and two-hop neighbors)


Plotting Channel Access Probability


Delay per Node Delay is related with the probability

of channel access and the load at each node.

Channel access probability can be different at each node.

Delay is considered per node.


Packet Arrival and Serving: M/G/1 with server vacation: Poisson arrival

(exponential arrival interval), service time distribution (any), single server.

FIFO service strategy: head-of-line packet waits for geometric distributed period Yi with parameter 1-qi T qi is the channel access probability of node i.


Service Time:

Service time: Xi = Yi + 1. The mean and second moment of

service time:

Server vacation: V=1,


Delay in The System Pollaczek-Kinchin formula:

Take in Xi and Vi :

Delay in the system: (q>)


Plotting System Delay


System Throughput Multi-hop networks have concurrent

transmissions >1. The system can carry as many

packets at a time as all nodes can be activate.

Simple!


Comparisons with CSMA CSMA/CA by Analysis

Different slotting: NAMA long slots CSMA CSMA/CA short slots

CSMA(CA) assumptions: Heavy load (always have packets

waiting) Channel access regulated by back-off

probability p’ in each slot. Convert the load to comparable one

in NAMA.


Convert Load in CSMA(CA) to the Load in NAMA

Each attempt to access channel is a packet arrival p’.

Packet duration is geometric with average 1/q.

Two state Markov chain to compute the load.


NAMA Load Relation:

i=b is the load for each node. qm is the channel access probability

of each node.


Protocol Throughput Comparison


Simulations

Two scenarios: Fully connected: 2, 5, 10, 20 nodes. Multi-hop network:

100 nodes randomly placed in 1000x1000 area.

Transmission range: 100, 200, 300, 400.

Compare with UxDMA:


Fully Connected Network (Throughput)


Fully Connected Network (Delay)


Multi-hop Network (Throughput)


Multi-hop Network (Delay)


Conclusions NCR ensures collision-free transmissions. Only two-hop topology information is

needed. HAMA performs better than static

scheduling algorithms (UxDMA). HAMA performs better than contention-

based protocols. The use of directional antennas can

improve performance further. (Next topic)


Comments

Scheduled-access protocols are evaluated in static environments and what about their performance in mobile networks?

Neighbor protocol will also have impact on the performance of these protocols

Need comprehensive comparison of contention-based and scheduled access protocols.


Beyond Elections and Graph Coloring…

Elections and graph coloring are not enough to support all traffic or be efficient (example is a string of nodes).

How do we support traffic with deadlines? Reservations

How do we combine reservations & elections? CATA as a “pre-example”

What if we can do more than transmit or receive one packet at a time? Network coding Channel division Multi-user detection


Synchronization and Power Management

Synchronization Finding and staying with a WLAN Synchronization functions

TSF Timer, Beacon Generation

Power Management Goal is sleeping without missing messages Power Management functions

periodic sleep, frame buffering, Traffic Indication Map


Power Management Mobile devices are battery powered.

Power management is important for mobility. Current LAN protocols assume stations are

always ready to receive. Idle receive state dominates LAN adapter power

consumption over time. How can we power off during idle periods, yet

maintain an active session? 802.11 Power Management Protocol goals:

Allow transceiver to be off as much as possible Transparent to existing protocols Flexible to support different applications

possible to trade off throughput for battery life


Power Management in 802.11

Two power modes: active and power saving (PS).

Protocols for Infrastructure, i.e., network with an

access point (AP) Ad hoc networks.


Power Management in 802.11

Infrastructure network Node in active mode behaves “normally” and node

in PS wakes up periodically to check for packets from AP.

Node notifies AP when it changes modes. AP transmits beacons every beacon interval and

nodes must monitor those frames. AP sends a traffic indication map (TIM) in each

beacon, which states PS nodes with buffered frames in the AP.

When PS node hears its ID, it remains active for the remaining of the beacon interval.


Power Management Approach Allow idle stations to go to sleep

station’s power save mode stored in AP APs buffer packets for sleeping stations.

AP announces which stations have frames buffered Traffic Indication Map (TIM) sent with every Beacon

Power Saving stations wake up periodically listen for Beacons

TSF assures AP and Power Save stations are synchronized

stations will wake up to hear a Beacon TSF timer keeps running when stations are sleeping synchronization allows extreme low power operation

Independent BSS also have Power Management similar in concept, distributed approach


Infrastructure Power Management

Broadcast frames are also buffered in AP.

all broadcasts/multicasts are buffered broadcasts/multicasts are only sent after

DTIM DTIM interval is a multiple of TIM interval

TIM

TIM-Interval

Time-axis

Busy Medium

AP activity

TIM TIM TIM DTIMDTIM

DTIM interval

BroadcastBroadcast



Broadcast frames are also buffered in AP. all broadcasts/multicasts are buffered broadcasts/multicasts are only sent after DTIM DTIM interval is a multiple of TIM interval

Stations wake up prior to an expected (D)TIM.

TIM

TIM-Interval

Time-axis

Busy Medium

AP activity


DTIM interval

PS Station

BroadcastBroadcast



Broadcast frames are also buffered in AP. all broadcasts/multicasts are buffered broadcasts/multicasts are only sent after DTIM DTIM interval is a multiple of TIM interval

Stations wake up prior to an expected (D)TIM. If TIM indicates frame buffered

station sends PS-Poll and stays awake to receive data else station sleeps again

TIM

TIM-Interval

Time-axis

Busy Medium

Tx operation

AP activity


DTIM interval

PS Station

Broadcast

PS-Poll

Broadcast


References [R01] S. Ramanathan, A unified framework and

algorithm for channel assignment in wireless networks, ACM Wireless Networks, Vol. 5, No. 2, March 1999.

[BG01] Lichun Bao and JJ, A New Approach to Channel Access Scheduling for Ad Hoc Networks, Proc. of The Seventh ACM Annual International Conference on Mobile Computing and networking (MOBICOM), July 16-21, 2001, Rome, Italy.

[BG02] Lichun Bao and JJ, Hybrid Channel Access Scheduling in Ad Hoc Networks, IEEE Tenth International Conference on Network Protocols (ICNP), Paris, France, November 12-15, 2002.


References [IEEE99] IEEE Standard for Wireless LAN Medium Access

Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std 802.11-1999.

[TK84] H. Takagi and L. Kleinrock, Optimal Transmission Range for Randomly Distributed Packet Radio Terminals, IEEE Trans. on Comm., vol. 32, no. 3, pp. 246-57, 1984.

[WV99] L. Wu and P. Varshney, Performance Analysis of CSMA and BTMA Protocols in Multihop Networks (I). Single Channel Case, Information Sciences, Elsevier Sciences Inc., vol. 120, pp. 159-77, 1999.

[WG02] Yu Wang and JJ, Performance of Collision Avoidance Protocols in Single-Channel Ad Hoc Networks, IEEE Intl. Conf. on Network Protocols (ICNP ’02), Paris, France, Nov. 2002.


Acknowledgments

Parts of the presentation are adapted from the following sources:

Mustafa Ergen, UC Berkeley, http://www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt

Greg Ennis, Symbol Technologies, http://grouper.ieee.org/groups/802/11/Tutorial/archit.pdf

Phil Belanger, Aironet and Wim Diepstraten, Lucent Technologies, http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf

Abhishek Karnik, Dr. Ratan Guha, University Of Central Florida, http://www.cs.ucf.edu/courses/cda4527/WLAN/80211.ppt

http://www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt

http://www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt

http://grouper.ieee.org/groups/802/11/Tutorial/archit.pdf

http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf

http://www.cs.ucf.edu/courses/cda4527/WLAN/80211.ppt

http://www.cs.ucf.edu/courses/cda4527/WLAN/80211.ppt

spring 2006ucsc cmpe2571 cmpe 257: wireless networking set mac-1: medium access control protocols

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