Overview

1

• Overview
• Data Link Layer
• Medium Access Control of LANs
• Physical Layer
• Metropolitan Area Networks
• Personal Area Networks
• Quality of Services
• Security
• Applications

2

• User Priority and Access Priority
• The first attempt to deal with LAN QoS appears in
  the original version of IEEE 802.1D, which is a
  specification operating at the MAC level. It
  deals with the interconnection of LANs with the
  same MAC protocol as well as with different MAC
  protocols.
• The bridge is able to pass parameters from
  incoming port to the outgoing port. Two of these
  parameters are user_priority and access_priority
  .
• The user_priority and access_priority parameters
  relate to the problem of how to handle
  priorities. In the case of IEEE 802.3 (Ethernet)
  and 802.11 (wireless LAN), priority is not
  supported. Other 802 LAN types support up to
  eight levels of priority. The user_priority value
  provided to the MAC- layer entity at the incoming
  port is derived from the incoming MAC frame
• The access_priority refers to the priority used
  by a bridge MAC entity to access a LAN for frame
  transmission. We may not want the access_priority
  to be equal to the user_priority for several
  reasons

3

• Related High Level QOS
• IETFs Integrated Services (IntServ) and
  Differentiated Services (DiffServ) architectures
  have been proposed to support guaranteed QoS and
  traffic prioritization respectively above the
  link layer.
• IntServ provides service guarantees to individual
  traffic flows.
• IntServ provides high levels of precision in
  resource allocation, however there are some
  scalability issues since the protocol requires
  maintaining a soft state for each flow at every
  router along the reservation path.
• The DiffServ protocol allocates different
  service levels to different groups of users,
  which means that all traffic is distributed into
  groups or classes with different QoS parameters.
  The state awareness is required only at edge
  routers.
• At the edge router datagrams are classified as
  belonging to the certain service class and then
  conditioned (marked, policed and shaped)
  according to the Service Level Agreement. Once
  the datagram is in the DiffServ domain it gets
  the same treatment from all the routers according
  to the service category.

4

• QOS for Ethernet and WLAN
• Due to the technology advance in Ethernet, which
  provides high enough data rate for most
  applications, low jitter and high
  reliability, QoS issues in Ethernet becomes less
  important. However, in wireless LAN
  environment, the story are quite different.
• When used to support applications with quality of
  service requirements, each 802.11 WLAN may
  provide a single link within an end-to-end QoS
  environment that may be established between and
  managed by higher layer entities.
• The 802.11 QoS facility requires the 802.11 MAC
  sublayer to incorporate features that are not
  traditional for MAC sublayers. In addition, it
  may be necessary for certain higher layer
  management entities to be "WLAN aware", at least
  to the extent of understanding that the available
  bandwidth may be subject to sudden and
  unavoidable changes.
• In order to support both IntServ and DiffServ
  protocols the 802.11e Task Group has proposed a
  number of enhancements to the MAC layer.

5

• Why QOS in WLAN ?
• The term QoS refers to set of qualitative and
  quantitative traffic characteristics such
  throughput, service interval, packet size,
  delay, jitter, priority, which describes a
  traffic flow in support of a specific
  application.
• 802.11 WLANs provide best-effort services similar
  to Ethernet. Wireless systems incur very high
  per-packet overhead with limited bandwidth, so
  the case in Ethernet does not apply to 802.11.
• About 32 of available data rate is consumed by
  packet fragmentation, interframe spacing and
  MAC-level acknowledgment. If RTS / CTS is
  enabled, it can approach 50.
• Heavy traffic load increases collisions and
  backoffs, so frame delivery time to the peer
  station increases exponentially.
• Frequent retransmissions also cause
  unpredictable delays in the order of tens
  to hundreds of milliseconds and transmission
  of pending frames could be blocked.

6

• Why QOS in WLAN ?
• The quality of most multimedia services involving
  voice and video transmission deteriorate
  dramatically if delay increases above certain
  level. This is a major stumbling block preventing
  802.11 to enter the markets such as Consumers
  Electronics or Medical Systems.
• When used to support applications with quality of
  service requirements, each 802.11 WLAN may
  provide a single link within an end-to-end QoS
  environment that may be established between and
  managed by higher layer entities.
• The 802.11 QoS facility requires the 802.11 MAC
  sublayer to incorporate features that are not
  traditional for MAC sublayers. It may be
  necessary for certain higher layer management
  entities to be "WLAN aware", at least to the
  extent of understanding that the available
  bandwidth may be subject to sudden and
  unavoidable changes.
• In order to support both IntServ and DiffServ
  protocols the 802.11e Task Group has proposed a
  number of enhancements to the MAC layer.

7

• 802.11 MAC without QOS
• The main purpose of the 802.11 MAC layer is to
  provide reliable data services to the user of the
  MAC and to control fair access to the shared
  wireless medium.
• We consider an infrastructure Basic Service Set
  (BSS) composed of an Access Point (AP) and a
  number of Stations (STAs) associated with the AP.
• To provide reliable data services the 802.11
  standard defines a frame exchange protocol. The
  minimum exchange sequence consists of two frames
  a frame sent from source to the destination and
  an Acknowledgment (ACK) sent from destination to
  source if the frame is received successfully.
• For every frame received at the MAC the Frame
  Check Sequence (IEEE 32bit CRC) is checked. If
  the source does not receive the expected ACK or
  the FCS fails, the frame is re-transmitted. This
  mechanism consumes some of the available
  bandwidth, but it allows coping with error
  conditions such as interference in the wireless
  medium.

8

• 802.11 MAC without QOS
• In general it is more efficient to ensure data
  integrity at the link layer than leaving it up to
  higher layer protocols, which rely on mechanisms
  such as timeouts measured in seconds instead of
  milliseconds at the link layer.
• In addition to this basic frame exchange
  sequence, an optional Request-To-Send /
  Clear-To-Send (RTS/CTS) mechanism is defined to
  increase the robustness of the protocol and
  address problems such as the hidden node.
• A STA sends a RTS frame to the destination before
  transmitting any MAC Service Data Unit (MSDU).
  Both RTS and CTS frames contain duration
  information about the length of the MSDU / ACK
  transmission. Based on this information all
  surrounding STAs can update an internal timer
  called Network Allocation Vector (NAV) and defer
  any transmission until this timer expires. Even
  if a hidden STA will be able to receive the CTS
  response and update its NAV accordingly. This
  mechanism protects transmission between STAs
  against unexpected transmissions from hidden
  STAs, and it is also used by the 802.11g
  amendment to protect unexpected Txs from legacy
  equipment.

9
Example DCF frame exchange sequence
10

• PCF for controlled access
• PCF is an optional channel access function in the
  802.11 standard, which was designed to
  support time bounded services. Contention free
  access to the wireless medium is controlled by a
  Point Coordinator (PC) collocated with the AP.
• DTIM beacon frames are used by the PC to indicate
  the start of a CFP. 802.11 defines two periods
  between two consecutive Delivery Traffic
  Indication Message (DTIM) beacon frames
  Contention Free Period (CFP) and Contention
  Period (CP).
• In general beacon frames are sent periodically by
  the AP, although it can be delayed by a busy
  wireless medium, and they carry synchronisation
  and network (BSS) information. During CP all the
  STAs contend for the wireless medium using DCF.

11

• PCF for controlled access
• During CFP the AP schedules transmissions to
  and/or from individual STA using a polling
  mechanism. There is no contention between STAs
  during CFP.
• CFP starts when the AP obtains access to the
  wireless medium using PCF Interframe Space (PIFS)
  timing at Target Beacon Transmission Time (TBTT).
  
• PIFS is shorter than DIFS, but longer than SIFS.
  Therefore PCF gets higher priority of access than
  DCF, but does not interrupt any ongoing DCF
  transmissions.
• Once PCF obtains access to the wireless medium
  SIFS timing is used for frames exchanges during
  CFP.

12
Example PCF frame exchange sequence
13

• PCF frame exchange sequence
• Polling is started by sending a CFPoll frame to
  one of the pollable STAs. If the PC itself has
  pending transmission, it could use a data frame
  piggybacking a CF-Poll frame. The polled STA can
  respond with a DataCF-ACK frame, or with a
  CF-ACK frame only if there is no pending
  transmission in the STA.
• Once the frame exchange sequence with one STA is
  completed, the PC then sends CF-Poll to another
  STA in its list of pollable STAs.
• When the PC has finished polling all pollable
  STAs or the CFP duration has expired, the PC
  broadcasts a CF-End frame to announce the end of
  the CFP.
• The NAV of all STAs are set to maximum at TBTT to
  protect the CFP from unwanted transmissions. Then
  the AP broadcasts the actual CFP duration in the
  beacon, and the NAV are updated accordingly.
• At the end of the CFP, all STAs reset their NAV
  to zero when either they have received a CF-End
  frame, or the CFP duration expires. From now on
  until the next DTIM beacon, all STAs contend for
  the wireless medium using DCF.

14

• QoS limitation of the original MAC
• DCF does not have any provision to support QoS.
  All data traffic is treated in a best effort
  manner. All STAs in the BSS contend for the
  wireless medium with the same priority. It causes
  asymmetric throughput between uplink and
  downlink.
• No differentiation between data flows to support
  traffic with QoS requirements. When the number of
  STAs in a BSS increases, probability of
  collisions becomes higher and results in frequent
  retransmissions.
• PCF features unpredictable beacon delays
  resulting in significantly shortened CFP, and
  unknown transmission duration of polled STA
  making it very difficult for the PC to predict
  and control the polling schedule for the
  remainder of the CFP.
• No management interface defined to setup and
  control PCF operations. Therefore it is
  impossible to setup a PCF policy according to the
  requirements of higher layer protocols. Also
  there is no mechanism for STAs to communicate QoS
  requirements to the AP, which is essential for
  optimizing the performance of the polling
  algorithm in the PC.

15

• 802.11e QoS features
• Bandwidth and latency cannot be guaranteed in a
  wireless LAN system, especially in
  unlicensed spectrum.
• We can always provide mechanisms, which would
  allow maximizing the probability that certain
  traffic classes will get adequate QoS in a
  properly controlled environment.
• IEEE 802.11e defines a superset of features
  specified in the 1999 edition of IEEE 802.11.
  These enhancements distinguish QoS enhanced
  stations (QSTAs) from non-QoS STAs (STAs), and
  QoS enhanced access point (QAP) from non-QoS
  access point (AP). These features are
  collectively termed QoS facility.
• Two main functional blocks are defined in
  802.11e, Channel Access Functions and Traffic
  Specification (TSPEC) management. TSPEC
  management provides the link between the Channel
  Access Functions and higher layer QoS protocols
  such as IntServ or DiffServ. Optional features
  such as Block Acknowledgement (BA), Direct Link
  Protocol (DLP), Automatic Power Save Delivery
  (APSD), and Local Multicast service class are not
  directly related to QoS but improve the
  efficiency of the 802.11 MAC in general.

16

• Channel Access Functions
• QoS facility defines a new coordination function
  called Hybrid Coordination Function (HCF) used
  only in QoS enhance BSS (QBSS).
• HCF has two modes of operation Enhanced
  Distributed Channel Access (EDCA) is a
  contention-based channel access function that
  operates concurrently with HCF Controlled Channel
  Access (HCCA) based on a polling mechanism which
  is controlled by the Hybrid Co ordinator (HC).
• The HC is co-located with the QAP. Both access
  functions enhance or extend functionality of the
  original access methods DCF and PCF.
• EDCA has been designed for support of prioritized
  traffic similar to DiffServ, whereas HCCA
  supports parameterized traffic similar to IntServ.

17

• Channel Access Functions
• The basic concept of these channel access
  functions is the Transmission Opportunity (TXOP).
  A TXOP is a bounded time interval in which the
  QSTA is allowed to transmit a series of frames.
• A TXOP is defined by the start time and a maximum
  duration. If a TXOP is obtained using the
  contention-based channel access, it is called an
  EDCA-TXOP.
• If a TXOP is granted through HCCA, it is called a
  HCCA (polled) TXOP. The duration of the EDCA-TXOP
  is controlled by the QAP and is distributed to
  non-AP QSTAs in the beacon frames along with
  other EDCA related parameters.
• The duration of a HCCA (polled) TXOP is passed to
  the non-AP QSTA directly by the HC as part of a
  QoS CF-Poll frame, which grants the HCCA (polled)
  TXOP.

18
Relationship between Channel Access Mechanisms
19

• Channel Access Functions
• EDCA is used only during CP, while HCCA can
  theoretically operate during both CFP and CP.
  However the 802.11e standard recommends using
  HCCA during CP only, and discourages its use
  during CFP.
• This is mainly due to the complexity in
  implementing polling used CF-Poll and QoS CF-Poll
  at the same time.
• Multicast and broadcast frames are delivered by
  the QAP during either CP or CFP under EDCA or PCF
  respectively.
• In 802.11e MAC-level Acknowledgment (ACK) has
  become optional. This means that when the No
  ACK policy is used, the MAC would not send an
  ACK when it has correctly received a frame. This
  improves the overall MAC efficiency for time
  sensitive traffic, such as VoIP.
• The No ACK option also introduces more
  stringent real-time constraints since if an ACK
  is not expected, then the next frame for
  transmission has to be ready within SIFS time
  from the end of the last transmission. Designers
  should bear this in mind when architecting an
  802.11e system.

20

• EDCA for support of prioritized traffic
• EDCA enhances the original DCF to provide
  prioritized QoS, i.e. QoS based on priority of
  access, and it supports priority based
  best-effort service such as DiffServ.
• Prioritized QoS is realized through the
  introduction of four Access Categories (AC),
  which provide delivery of frames associated with
  user priorities as defined in IEEE 802.1D.
• Each AC has its own transmit queue and its own
  set of AC parameters. The differentiation in
  priority between AC is realized by setting
  different values for the AC parameters. The most
  important of which are listed below
• - Arbitrary Inter-frame Space Number (AIFSN). It
  is the minimum time interval between the wireless
  medium becoming idle and the start of
  transmission of a frame.
• - Contention Window (CW). A random number is
  drawn from this interval, or window, for the
  backoff mechanism.
• - TXOP Limit. The maximum duration for which a
  QSTA can transmit after obtaining a TXOP.

21

• 802.11e Operation
• When data arrives at the MAC-UNITDATA SAP, the
  802.11e MAC first classifies the data with the
  appropriate AC, and then pushes the newly
  arrived MSDU into the appropriate AC transmit
  queue.
• MSDUs from different ACs contend for EDCA-TXOP
  internally within the QSTA.
• The internal contention algorithm calculates the
  backoff, independently for each AC, based on
  AIFSN, CW, and a random number. The backoff
  procedure is similar to that in DCF, and the AC
  with the smallest backoff wins the internal
  contention.
• The winning AC would then contend externally for
  the wireless medium. The external contention
  algorithm has not changed significantly compared
  to DCF, except that in DCF the deferral and
  backoff were constant for a particular PHY.
  802.11e has changed the deferral and backoff to
  be variable, and the values are set according to
  the appropriate AC.

22
Implementation of the external contention
algorithm
When the medium is detected to transit from busy
to idle, the channel is monitored for SIFS time.
At the end of SIFS and if the channel is still
idle, a slot counter is started to count the
number of slots from zero. At the end of each
slot, the slot counter is incremented. If a
transmit request has been made, the slot counter
is compared with the programmed number of backoff
slots. If the slot counter is equal or greater
than the number of backoff slots, then an
EDCA-TXOP has been obtained and transmission of
the frame starts. If the medium is detected to
transit from idle to busy at any time during the
SIFS and contention period then the counting of
slots is suspended.
23
Implementation of the external contention
algorithm

• With proper tuning of AC parameters, traffic
  performance from different ACs can be optimized
  as well as achieving prioritization of traffic.
• It requires a central coordinator (QAP) to
  maintain a common set of AC parameters to
  guarantee fairness of access for all QSTA within
  the QBSS.
• In order to address the asymmetry between uplink
  (QSTA to QAP) and the much heavier downlink (QAP
  to QSTA) traffic, a separate set of EDCA
  parameters is defined for the QAP only, which
  takes this asymmetry into account.

24

• EDCA Architecture Implementation
• The frame data payload is stored in a pool of
  buffers in RAM. The EDCA higher-level SW queues
  implement the four AC queues as defined in the
  802.11e draft standard.
• The EDCA low level SW queue implements the TX
  Opportunity (EDCA) as defined in the 802.11e
  draft standard 2. It is implemented as a link
  list, and entries in the queue point to TX
  descriptors in the EDCA higher level queues.
• All the entries in the EDCA low level queue form
  a granted or pending TXOP.
• The EDCA internal contention and collision
  algorithm implements all the rules regarding
  internal contention and collision as defined in
  the 802.11e draft standard. It contains a random
  number generator.

25
EDCA overview architecture implementation
By S. Chung, K. Piechota Silicon Software
Systems.
26

• HCCA for support of parameterized traffic
• HCCA is a component of HCF and provides support
  for parameterized QoS. It inherits some of
  the rules of legacy PCF, and it introduces
  many extensions.
• Similar to PCF, HCCA provides polled access to
  the wireless medium. But unlike PCF, QoS polling
  can take place during CP and scheduling of
  packets is based on admitted TSPECs.
• The central concept of HCCA is Controlled Access
  Phase (CAP), which is a bounded time interval and
  formed by concatenating a series of HCCA (polled)
  TXOPs.
• Scheduling of HCCA (polled) TXOP and formation of
  CAP are performed by the HC. Figure 4-4
  illustrates an example frame exchange sequence
  during the CAP.

27
Example CAP timing (2 polled-TXOP, different
QSTAs)
By S. Chung, K. Piechota Silicon Software
Systems.
28
Example CAP timing

• QoS CF-Poll Transmitted by QAP to grant a
  HCCA-TXOP, no data.
• QoS-DataCF-Poll Transmitted by QAP to grant a
  HCCA-TXOP, with data.
• QoS-Null Transmitted by QSTA when it has no more
  data, or it is the last frame of the TXOP.
• QoS-Data QoS data transfer between QAP and QSTA.
  Used by EDCA as well as HCCA.
• QoS CF-Ack Transmitted by QAP in response to
  QoS-Null requesting a TXOP, no data.
• QoS-DataCF-Ack Transmitted by QAP in response to
  QoS-Null requesting a TXOP, with data.
• QoS CF-AckCF-Poll, QoS-DataCF-AckCF-Poll
  Generally not used.

29

• Example CAP timing
• The HC gains access to the wireless medium based
  on timing information stored in three MIB
  variables dot11HCCWmin, dot11HCCWmax and
  dot11HCAIFSN.
• The default values of these MIB variables give
  PIFS timing, which is shorter than AIFS or DIFS.
  This gives the HC the highest priority over all
  non-AP QSTAs in accessing the wireless medium.
• 802.11e introduces a number of new QoS Data frame
  subtypes. For HCCA (polled) TXOP, the QoS CF-Poll
  frames is used to grant the TXOP, and then data
  transfer commences using QoS Data frames.
• QoS-Null frames can be used to terminate a HCCA
  (polled) TXOP by a non-AP QSTA if it does not
  have any data to send, or the data transfer has
  completed.

30

• Example CAP timing
• Many different types of QoS Data frames and their
  associated usage rules increase the efficiency of
  the 802.11e MAC, although it also increases the
  complexity of the HCCA scheduler.
• According to the 802.11e standard there can be up
  to eight uplink or sidelink traffic streams and
  the same number of downlink traffic streams
  within a non-AP QSTA.
• Each uplink or sidelink traffic stream has its
  own transmit queue, which means that any non-AP
  QSTAs can provide parameterized QoS services for
  up to eight traffic flows. In a QAP the number of
  supported flows is not limited by the standard
  but by available resources such as memory.

31
HCCA overview architecture implementation
S. Chung, K. Piechota Silicon Software Systems.
32
HCCA overview architecture implementation

• The frame data payload is stored in a pool of
  buffers in RAM.
• The HCCA higher-level SW queues implement the
  TSID queues as defined in the 802.11e draft
  standard.
• The HCCA low-level queue implements the HCCA
  schedule derived by higher layer SW entities.
• Each entry has an associated type, which
  identifies the whether it is a TXOP from QAP to
  QSTA, polled TXOP from QSTA to QAP, or EDCA.
• The HCCA scheduling algorithm translates the HCCA
  schedule from higher layers into a series of
  different types of TXOP to form the HCCA
  low-level queue.

33

• Traffic Specifications
• Traffic Specification (TSPEC) is the traffic
  stream management device which provides the
  management link between higher layer QoS
  protocols with the 802.11e channel access
  functions.
• TSPEC describes characteristics of traffic
  streams, such as data rate, packet size, delay,
  and service interval. TSPEC negotiation between
  peer MAC layers provides the mechanism for
  controlling admission, establishment, adjustment
  and removal of traffic streams.
• Bandwidth access must be controlled to avoid
  traffic congestion, which can lead to breaking
  QoS and drastic degradation of overall
  throughput.
• The 802.11e standard specifies the use of Traffic
  Specification (TSPEC) for such a purpose for both
  EDCA and HCCA.
• QoS Management frames, primitives and procedures
  are defined for TSPEC negotiation, which is
  initiated by the SME of a QSTA, and accepted or
  rejected by the HC. Requested TSPEC is
  communicated to
• the MAC via the MAC Layer Management Entity
  (MLME) SAP. This allows higher layer SW,
  protocols, and application, to allocate resources
  within the MAC layer.

34
Typical TSPEC Negotiation
S. Chung, K. Piechota Silicon Software Systems.
35
802.11e Overview Architecture
S. Chung, K. Piechota Silicon Software Systems.
36

• 802.11e Implementation Issues
• 802.11e significantly increases the complexity of
  the original 802.11 MAC architecture.
  Most of the changes in the MAC architecture
  are logical consequences of introducing HCF with
  two new channel access functions EDCA
  and HCCA.
• Upgrading from the original 802.11 MAC to 802.11e
  requires extensive changes to existing functional
  blocks as well as adding new ones.
• Implementation of 802.11e requires significantly
  increases in memory, particularly RAM. The amount
  of additional RAM is a function of the increase
  in the number of transmit queues.
• In the original 802.11 there are two queues
  broadcast multicast, and unicast. In 802.11e,
  there are at least five broadcast multicast,
  and four AC.
• If HCCA is also implemented, the number of
  additional queues for traffic streams varies
  between 1 to 8 for QSTA, and 1 to any number for
  QAP limited by available memory. Obviously these
  queues and the associated buffers could be
  optimized to reduce the amount of RAM memory
  required, but the increase is still significant.
  This also depends on the existing SW architecture
  of the MAC and the OS.

37
802.11e SW architecture
S. Chung, K. Piechota Silicon Software Systems.
38

• 802.11e Summary
• The aim of the upcoming 802.11e amendment is to
  provide new features for supporting applications
  with QoS requirements. However performance of the
  system depends to a large degree on the
  performance of the many algorithms, such as the
  EDCA internal contention algorithm, and the EDCA
  HCCA scheduler.
• Other algorithms not mentioned include the
  Admission Control algorithm, and Traffic Schedule
  Generation. These algorithms are the subjects of
  extensive research. Their implementation in a
  real system will make the difference between the
  good and the best of class.
• Provisioning QoS in a wireless environment is a
  difficult task, mainly due to the characteristic
  of the wireless medium, overlapping BSSs and
  roaming between them, etc.
• It should not be expected that the upcoming
  802.11e standard could provide QoS equal to wired
  systems.
• It should also be noted that mobility gained from
  wireless LAN far out weights the little loss in
  QoS, and there is a significant market to be
  tapped by the addition of QoS to 802.11.