QoS in Wireless Networks 12/30/2018

Wireless Network Wireless networks are better than wired networks with regards to ease of installation and flexibility But they suffer from lower bandwidth, higher delays and higher bit error Thus providing QoS over such a network is quite challenging and requires additional measures 12/30/2018

IEEE 802.11 network Most widely used WLAN Uses a shared medium Low medium utilization Risk of collision No service differentiation between types of traffic Has two access methods (MAC) Distributed Coordinator Function (DCF) Point Coordinator Function (PCF) 12/30/2018

DCF Uses a CSMA/CA algorithm in MAC Before a data frame is sent, the station senses the medium If it is idle for at least DCF interframe (DIFS) amount of time, the frame is transmitted Otherwise a backoff time B (measured in time slots) is chosen randomly in the interval [0, CW) 12/30/2018

DCF (cont.) After medium is detected idle for at least DIFS, the backoff timer is decremented and frame is transmitted when it reaches zero If medium becomes busy during count down, backoff timer is paused and restarted when medium is idle for DIFS period If there is a collision, CW is doubled according to 12/30/2018

DCF (cont.) Where i = number of retransmissions k= constant defining minimum CW A new backoff time is then chosen and the backoff process starts over. 12/30/2018

DCF Timing diagram Src Dest Others 12/30/2018 Data Ack Next MPDU DIFS Src Data SIFS Dest Ack DIFS Contention Window Others Next MPDU Defer Access Backoff after Defer 12/30/2018

DCF Example cw = 31 data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31 B2 = 10 12/30/2018

PCF (Point Coordination Function) Contention-free frame transfer Single Point Coordinator (PC) controls access to the medium. AP acts as PC PC transmits beacon packet when medium is free for PIFS time period PCF has higher priority than the DCF (PIFS < DIFS) During PCF mode, PC polls each station for data After a transmission of a MPDU, move on to the next station 12/30/2018

PCF operation TBTT : Target Beacon Transmission Time Carried in the beacon Source : “IEEE 802.11e Wireless LAN for Quality of Service” – Mangold et. al., Proc. European Wireless 2002 12/30/2018

IEEE 802.11 superframe Source : “Quality of Service Schemes for IEEE 802.11 Wireless LANs – An Evaluation – A. Lindgreen et. al., Kluwer Academic Publisher, 2003. 12/30/2018

Enhanced Distributed Channel Access (EDCA) This is HCF Contention Based channel access Provides service differentiation Traffic can be classified into 8 different classes Each station has 4 access categories to provide service differentiation 12/30/2018

Access Category (AC) Access category (AC) as a virtual DCF 4 ACs implemented within a QSTA to support 8 user priorities Multiple ACs contend independently The winning AC transmits frames AC0 AC1 AC2 AC3 A B A B A B A B B I a B I a B I B I F F F a F a O c S O c O c O c [ S S S [ k [ [ k [ [ k [ [ k o 1 o 2 o 3 o ] ] f f ] 1 f 2 3 ] f ] ] f f f ] ] f Virtual Collision Handler Transmission Attempt 12/30/2018

Differentiated Channel Access Each AC contends with AIFS[AC] (instead of DIFS) and CWmin[AC], CWmax[AC] (instead of CWmin, CWmax) 12/30/2018

Backoff Procedure Similar to 802.11 Backoff time chosen between [0, CW[AC]]. On collision CW[AC] = (CW[AC]+1) *2 -1 12/30/2018

Priority to AC Mapping Priority Access Category (AC) Designation (Informative) Best Effort 1 2 3 Video Probe 4 Video 5 6 Voice 7 12/30/2018

HCF Controlled Channel Access(HCCA) Manages access to wireless medium using HC which has a higher medium access priority (than EDCA) than non-AP STAs. Two primary differences between PCF and HCCA Frame exchange can happen both in CP and CFP period HC grants a polled TXOP with duration specified in a QoS CF Poll frame 12/30/2018

HCF Controlled Channel Access(HCCA) HC may function as PC that uses CFP for polled data (this mode can be used by both 802.11 and 802.11e STAs) HC may also send QoS CF polls But not mandatory since it can send those in CP also 12/30/2018

IEEE802.11e superframe Source : “IEEE 802.11e Wireless LAN for Quality of Service” – Mangold et al., Proc. European Wireless 2002. 12/30/2018

Block Acknowledgement Improves channel efficiency by aggregating several Ack into one frame A bitmap is used to ack a set of MPDUs Immediate block ack BlockAckReq is immediately responded with BlockAck Delayed block ack Receiver responds with an ACK to BlockAckReq Then the receiver would send the BlockAck in the next TXOP 12/30/2018

Immediate block ack Source : IEEE 802.11e standard document 12/30/2018

Delayed block ack Source : IEEE 802.11e standard document 12/30/2018

Distributed Fair Scheduling (DFS) Based on SCFQ (Actually based on WFQ) Uses a distributed approach for determining the smallest finish tag using backoff interval mechanism of 802.11 Backoff interval is chosen such that it is proportional to the finish tag of packet to be transmitted So packets with smaller finish tag will be assigned smaller backoff interval 12/30/2018

Distributed Fair Scheduling (DFS) On arrival a packet is stamped with start tag And the finish tag is Equivalently fki is the real time when the packet reaches head of the queue 12/30/2018

Distributed Fair Scheduling (DFS) So the finish number of a packet is given by A back off interval is chosen so that packets with smaller finish number gets lower backoff. Hence the back off is taken as 12/30/2018

Distributed Fair Scheduling (cont.) is a random variable uniformly distribute between [0.9,1.1] to inject randomness into the backoff mechanism Backoff interval is inversely proportional to weight assigned to a node. Thus node with higher weight is given a higher priority (because of smaller backoff interval) 12/30/2018

DFS (cont’d) For flows with small weight, the duration of the backoff interval can become quite large Leads to long duration of idle time when nodes are counting down An exponential mapping scheme is proposed to address this 12/30/2018

DFS (cont’d) When weights of flows are small, backoff interval is reduced A larger range of linear backoff interval is compressed into smaller exponential range. Hence the likelihood of collisions can increase with exponential case. For example, for threshold=80, K1=80, K2=0.002, (990) = (1000) = 147. 12/30/2018

Wireless Token Ring Protocol Wireless Token Ring Protocol (WTRP) can support QoS in terms of bounded latency and reserved bandwidth Efficient, since it reduces the number of retransmissions Fair in the sense that every station takes a turn to transmit and gives up its right to transmit (by releasing the token) until the next round Can be implemented on top of 802.11 12/30/2018

WTRP (cont.) Successor and predecessor fields of each node in the ring define the ring and the transmission order Station receives token from predecessor, transmits data and passes the token to the successor. Sequence number is used to detect any nodes that are part of the ring, but not in the range of a node 12/30/2018

WTRP (cont.) F seq=2 A Seq=3 unknown seq=4 seq=5 D seq = 1 A B C D E F Transmission range of E seq = 1 F seq=2 A Seq=3 unknown seq=4 seq=5 D Connectivity table of E 12/30/2018

WTRP (cont.) Implicit acknowledgement is used to monitor successful transmission of token Timer is used to guard against loss of token (successor might have moved out of range) Using connectivity table, the ring can be reformed when a node moves out of range By controlling the token holding time and token rotation time delay of packets can be bounded. 12/30/2018

Blackburst Devised with a view to minimizing delay for real-time traffic Stations are assigned priority When a high priority station wants to send a frame Senses the medium to see if it is idle for tmed time period and then sends its frame The data nodes (low priority station) use tlong > tmed to sense medium for access If medium is busy, station waits until channel has been idle for a tmed and then enters a black burst contention period The station sends a black burst by jamming the channel for a period of time 12/30/2018

Blackburst The length of the black burst is proportional to the amount of time the station has been waiting to access the medium (calculated as a number of black slots) After transmitting black burst, the station listens to the medium for a short period of time tobs (less than a black slot) to see if some other station is sending a longer black burst (hence has waited longer) If the medium is idle, then station sends its frame Otherwise it waits until the medium becomes idle again and enters another black burst contention 12/30/2018

Blackburst After successful transmission of a frame, the station schedules the next access instant tsch seconds in the future. This has the nice feature that real-time flows will synchronize and share the medium in a TDM fashion Unless there is a transmission by low priority station when a high priority station accesses the medium, very little blackbursting needs to be done once stations have synchronized Low priority stations use ordinary DCF access mechanism 12/30/2018

Timing diagram Source: Sobrinho J.L., Krishnakumar A.S., “Quality of Service in ad hoc carrier sense multiple access networks” – IEEE Journal on Selected Areas in Communications 17(8) (1999), pp. 1353-1368. 12/30/2018

References Schiller J., “Mobile Communications” - Addison Wesley, 2000. Benvensite M., et. al., “EDCF proposed draft text” – IEEE working document 802.11-01/131r1 (2001) Vaidya N.H., et. al., “Distributed Fair Scheduling in a wireless LAN” – Sixth International Conference on Mobile Computing and Networking, Boston 2000. Ergen M., et. al., “Wireless Token Ring Protocol” –Proceedings of 8th International Symposium on Computer and Communication 2003. Lindgren A., et. al., “Quality of Service Schemes for IEEE 802.11 Wireless LANs – An Evaluation” – Mobile Networks and Applications vol. 8, pp 223-235, Kluwer Academic Publishers, 2003. 12/30/2018