QoS in 802.11

QoS in 802.11

Reference : IEEE 802.11e: QoS Provisioning At

The MAC Layer & A survey of quality of service in IEEE 802.11 networks

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Outline IEEE 802.11 MAC IEEE 802.11 service differentiation mechanisms IEEE 802.11e Enhanced DCF Admission control and bandwidth reservation Distributed admission control for EDCF 802.11e Direct link protocol 802.11e Group acknowledgment. Architecture of the wireless Internet Support for full mobility QOS AND MOBILITY MANAGEMENT IN

HYBRID WIRELESS NETWORKS Integration of WLAN and 3G wireless networks

IEEE 802.11 MAC (1/3) Each superframe consists of a cont

ention- free period (CFP) for PCF and a contention period (CP) for DCF.

802.11 medium access control (MAC) Distributed coordination function (DCF)

Use carrier sense multiple access with collision avoidance

Use exponential backoff Point coordination function (PCF)

IEEE 802.11 MAC (2/3) NAV : network allocation vector

The time must elapse until current transmission session is complete.

RTS/CTS : request to send/clear to send Solve the hidden terminal and capture effect

problems CW : contention window

Incremented exponentially with an increasing number of attempts to retransmit the frame.

IEEE 802.11 MAC (3/3)

MPDU : MAC protocol data unit Contain header, payload, CRC

Backoff time slot is chosen randomly in the interval [0,CW).

Fragment : If it exceed Frag_threshold Advantage : if an error occurs during its tr

ansmission, a station does not have to wait to back off.

IEEE 802.11 MAC (DCF) A station with a frame to transmit monitors the ch

annel activities until an idle period equal to a distributed interframe space (DIFS) is detected.

After sensing an idle DIFS, the station waits for a random backoff interval before transmitting.

The station transmits its frame when the backoff time reaches zero.

After the destination station successfully receives the frame, it transmits an acknowledgment frame (ACK) following a short interframe space (SIFS) time.

DCF (Cont.) The hidden node problem may happen:

transmissions of a station cannot be detected using carrier sense by a second station, but interfere with transmission from the second station to a third station.

To reduce the hidden station problem, an optional four-way data transmission mechanism, RTS/CTS, is also defined in DCF.

RTS and CTS frames reserve the channel for the data frame transmission that follows.

All four frames (RTS, CTS, data, ACK) are separated by an SIFS time.

IEEE 802.11 MAC (PCF) It logically sits on top of the DCF and performs p

olling, enabling polled stations to transmit without contending for the channel.

Transmits a beacon frame to initiate a CFP (i.e., to initiate a superframe).

After a SIFS time, the PC sends a poll frame to a station to ask to transmit a frame.

After receiving the poll frame from the PC, the station with a frame to transmit may choose to transmit a frame after a SIFS time.

PCF (Cont.) The PC waits a PIFS interval following the ACK fr

ame before polling another station or terminating the CFP by transmitting a CF-End frame.

If the PC receives no response from the polled station for a PIFS interval, the PC can poll the next station or terminate the CFP by transmitting a CF-End frame.

The PCF cannot provide good QoS support since it lacks an admission function to control channel access from stations.

IEEE 802.11 service differentiation mechanisms

IEEE 802.11e Enhanced DCF

The EDCF is based on differentiating priorities at which traffic is to be delivered and works with four access categories (ACs).

The EDCF supports eight different priorities, which are further mapped into four ACs.

ACs are achieved by differentiating the arbitration interframe space (AIFS), initial window size, and maximum window size.

Access categories

IEEE 802.11e EDCF (Cont.)

For 0 ≤ i < j ≤ 3, we have CWmin[i] ≥ CWmin[j], CWmax[i] ≥ CWmax[j], and AIFS[i] ≥ AIFS[j], and at least one of the above inequalities must be “not equal to.”

If there is more than one queue finishing the backoff at the same time, it is called an internal collision.

EDCF timing diagram

P-DCF & DWDQ P-DCF : Persistent Factor DCF

Each flow stops the backoff and starts transmission only if ( r > P ) in the current slot time.

DWDQ : Distributed Weighted Fair Queue1. CW of any traffic flow is adjusted based betwe

en the actual and expected throughputs.2. Li′ = Ri /Wi

Li is smaller than those of others, it will decrease its CW.

DFS & DDRR DFS : Distributed Fair Scheduling

The backoff interval (BI) based on the packet length and traffic class, and the station with smaller BI transmits first.

BIi = ρi × scaling × factor × Li /ϕi DDRR : Distributed Deficit Round Robin

In order to minimize the collision between stations with the same deficit counter, randomization of IFSi,j will be further adopted if a backoff scheme is eliminated.

Admission control and bandwidth reservation

Distributed admission control for EDCF (1/4)

The QoS parameter set element (QPSE) includes CWmin[i], CWmax[i], AIFS[i], TXOPLimit[i], TXOPBudget[i], Load[i], and SurplusFactor[i] for (i = 0,…3)

SurplusFactor[i] : the ratio of over-theair bandwidth reserved to the bandwidth of the transported frames required for successful transmission

TXOPBudget[i] : the additional amount of time available during the next beacon interval

ATL[i] is the maximum amount time that may be spent on transmissions of AC I per beacon interval.

Distributed admission control for EDCF (2/4)

TXOPLimit[i] : the time limit on TXOPs Load[i] : the amount of time used during the pr

evious beacon interval The QAP shall maintain a set of counters TxTim

e[i], which shall be set to zero immediately following transmission of a beacon.

For each data frame transmission , the QAP shall add to the TxTime counter.

TXOPBudget[i] =Max(ATL[i] – TxTime[i]*SurplusFactor[i],0).

Distributed admission control for EDCF (3/4)

Each QSTA has to maintain the following variables for each AC: TxCounter[i], TxUsed[i],TxLimit[i], TxRemainder[i], and TxMemory[i].

TxUsed[i] counts the amount of time occupied on air by transmissions.

TxCounter[i] counts successful transmissions. TxRemainder[i] = TxLimit[i] – TxUsed[i];

Distributed admission control for EDCF (4/4) If TXOPBudget[i] = 0

–TxMemory[i] shall be set to zero for new QSTAs and all other QSTAs TxMemory[i] remains unchanged.

If the TXOPBudget[i] >0–TxMemory[i] = f *TxMemory[i] + (1 – f) *

(TxCounter[i]*SurplusFactor[i] +TXOPBudget[i])

–TxCounter[i] = 0–TxLimit[i] = TxMemory[i] + TxRemainder[i]

802.11e Direct link protocol

Direct Link Protocol (DLP) allows QSTAs to transmit frames directly to other QSTAs.

A direct link can be built by the following sequences:1. QSTA-1 sends a DLP-request frame to QAP2. QAP forward DLP-request to QSTA-23. QSTA-2 send response frame to QAP4. QAP forward DLP-response to QAP-15. Frame can be sent from QSTA-1 to QSTA-2 and QSTA-2 t

o QSTA-1 After DLPIdleTimeout, frames with destination Q

STA-2 shall be sent via the QAP

802.11e Group acknowledgment

In order to reduce the acknowledgment overhead, a new mechanism, called group acknowledgment (GA)

GA allows a group of frames to be transmitted before any acknowledgment.

After sending a burst of frames, the sender sends a group acknowledgment request (GroupAckReq) frame, and the receiver must respond by sending the group acknowledgment (GroupAck) frame, in which the correctly received frames’ information is included.

Group acknowledgment

Architecture of the wireless Internet

Support for full mobility

This roaming capability is achieved through MSs’ beacon scanning in a channel sweep.

Recent efforts have been made to extend 802.11 WLANs into outdoor cellular networks to provide fully mobile broadband service with ubiquitous coverage and high-speed connectivity.

QOS AND MOBILITY MANAGEMENT INHYBRID WIRELESS NETWORKS

Roaming and horizontal handoff among 802.11 WLANs, supporting QoS anytime, anywhere, and by any media requires seamless vertical handoffs between different wireless networks.

QOS AND MOBILITY MANAGEMENT INHYBRID WIRELESS NETWORKS (Cont.)

Integration of WLAN and MANET: routing within MANETs is handled by the Optimi

zed Link State Routing (OLSR) protocol, and handoff between WLANs and MANETs is supported through automatic mode detection and node switching capabilities of the mobiles.

Integration of WLAN and Bluetooth: evaluation of the interference between IEEE 802.

11 and Bluetooth.

Integration of WLAN and 3G wireless networks

A mobile node can maintain two connections in parallel (i.e., data connection through WLAN and voice connection through UMTS).

With the decreasing size of cells in next-generation multimedia-enabled wireless networks, the number of handoffs during a call’s lifetime increases.