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Quality of Service is an important aspect of wireless communication given the lower bandwidths offered by such environments. Stuart Mark reports on standardisation efforts by the IEEE 802.11e Task Group 802.11e Wireless QoS The IEEE 802.11 standard intends to provide a common method by which ethernet frames can be transported over wireless media. The idea is that anyone who uses a wireless NIC, for example, in the office can use that same NIC to access wireless network services in an airport lounge, hotel room or at home, free from the constraints of proprietary wireless solutions. 802.11 consists of a number of Task Groups (TGs), each focusing on a particular aspect of the standard and denoted by a single-letter suffix. 802.11a and b cover wireless networks in the 5GHz and 2.4GHz license exempt ranges respectively. TGa is intended for higher bandwidth, Metropolitan and, perhaps, longer range uses, proposing to provide bandwidths of between 6 and 54Mbps. TGb will provide a standard for the home or small office network and will run at up to 3Mbps using Frequency Hopping Spread Spectrum or up to 11Mbps using Direct Sequence Spread Spectrum. The Wireless Domain The IEEE have defined a framework for wireless architecture. Wireless Stations such as PCs, Laptops, PDAs mobile phones and potentially domestic appliances are grouped in a local environment known as a Basic Service Set (BSS), the equivalent of a wired LAN or workgroup. They can communicate in one of two modes. Ad-Hoc network mode allows all stations to talk directly with each other, much like peer-to-peer networking across a shared ethernet segment. Infrastructure network Mode requires all stations to communicate via an Access Point (AP) which is a specialised station in the BSS, more comparable to a switched medium. The AP will also communicate with the APs of other BSSs across a Distributed System to form an Extended Service Set as well as providing connections from each BSS to a Bridge Portal providing connections to wire based LAN types such as 802.3. It is envisaged that most corporate and public implementations of 802.11 will use Infrastructure network mode while home networks will use Ad-hoc mode. Layer two transport will be handled by MAC Service Data Units (MSDUs) loosely based on ethernet MAC frames. QoS in the Ether It is clear that some form of QoS will be required if wireless is to become a valid media for streaming content-delivery applications, not only because of transmission speed, but also due to the fact that, in Infrastructure mode, end station communication will occur via the AP. Because the AP is used by all stations in a BSS, and for inter-BSS traffic, it will become a bottleneck. The IEEE have recognised this with the formation of the 802.11e WG. The remit of the group is to provide MAC extensions to 802.11 that will accommodate both QoS and Security capabilities. Such is the depth of technological complexity associated with each sphere, 802.11e was split into two subgroups last September to allow members of the team to be exclusively assigned to their specific area of expertise. Because the requirement for QoS came after the initial scoping of the 802.11 standard, the Working Group are faced with the problem of adding the extra media capability without altering the existing MAC standard, or at least keeping alterations to a minimum. This is necessary in order to retain as much backward compatibility with previous design decisions as possible. The problem has been addressed by using a form of relative priority marking, very similar to that employed by 802.1p. In the same way that 802.1p can insert a ‘tag’ into a packet between layers two and three, 802.11e will also insert a ‘tag’ into MSDUs to provide the same 8 levels of priority marking. Priorities range from 0 to 7, the higher number providing the higher priority. However this is only the basic mechanism behind what is a very complex, wireless QoS. 802.11 defines two access methods onto the Wireless Medium, both of which must be accommodated by the QoS solution. Distributed Co-ordinated Function (DCF) is based on the Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method already used by ethernet. Stations on a wireless medium contend for a transmission opportunity which is assigned through the use of backoff and idle timers. DCF will run in Ad-Hoc and Network Infrastructure modes, the latter requiring an AP in each BSS and using centralised contention. Point Co-ordination Function (PCF) gives contention control to Point Co-ordinator (expected to be co-located with the AP). which will provide contention-free access to the wireless medium. PCF is used only on Network Infrastructure mode. Support of wireless QoS requires alterations to original framework components such as Enhanced Wireless Stations, Enhanced APs (EAP) and Enhanced Point Co-ordinators (EPC) which support the QoS MAC sublayer. As well as being able to differentiate between eight priority levels, EAPs also have four transmission queues for outbound data, particularly useful when centralised contention is used. These components are grouped in a QoS BSS (QBSS) On top of this, two types of QoS can be used, Parameterised and Prioritised. Each performs QoS in a different context, the former only across station to station wireless links while the latter supports QoS across the QBSS. A Traffic Category Identifier is carried in each MSDU. In Prioritised QoS, this is aligned closely to priority while Parameterised Traffic Categories can be affected by a number of media related factors such as maximum delay, variance and constant vs. variable bit rate. Each form of QoS will handle up to 8 categories. There are also four different levels of QoS. Level 0 specifies no QoS; Level 1 provides Prioritised QoS over DCF; Level 2 provides Prioritised QoS over DCF or PCF; Level 3 provides Prioritised QoS over DCF and Parameterised QoS over PCF. Expedited transmission of high priority MSDUs will be provided at a physical MAC level for DCF access using statistical probability. This means that the contention timers used in DCF access will be manipulated at the wireless stations and APs to provide favourable transmission opportunities for urgent data. PCF access transmission may be achieved by making use of its non-contention nature but this is still in the early stages of development. Different Standards The 802.11 standard, including QoS, is still a long way from completion and, because of its complexity, changes to the baseline proposal documents are issued by the IEEE with brisk regularity. There are also a number of other wireless technologies in existence or development, some of which are even considered to be standards by certain sections of the IT community. Solutions such as SWAP providing up to 2Mbps in the home, OpenAir which gives 1.6Mbps per channel and HIPERLAN Type1, the ETSI standard offering up to 23.5Mbps bandwidth and QoS support are available now. There are also a number or parallel development efforts including 10Mbps SWAP, 54Mbps HIPERLAN Type 2 (which is being co-ordinated with 802.11a as they both use the 5Ghz band), 1Mbs Bluetooth on 2.4GHz and another IEEE standard, 802.16 WirelessHUMAN! It is apparent that the wireless standard is still up for grabs but the IEEE’s association with the ETSI’s HIPERLAN could turn out to be a prudent one as these are the highest capacity technologies currently under development, the IEEE also having defined a specification under 802.11a that will use Orthogonal Frequency Division Multiplexing to match HIPERLANs Type 2 54Mbps. While not exactly around the corner, the days of sofa surfing and personal networking are approaching fast, their advance helped by the bounds in wireless evolution.
http://grouper.ieee.org/groups/802/11/ http://www.etsi.org http://www.bluetooth.com
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This site was last updated 04/25/07
