Network World

research center:

Wireless / Mobile

Search / DocFinder:
Advanced search
Research Centers
Vendor Solutions
Site Resources
Special Issues

Signature SeriesEnterprise All-Stars
Enterprise All-Stars NEW

You in action
You in action

New Data Center The New Data Center: Wireless & Mobility
Wireless & Mobility NEW

The New Data Center: Server Virtualization
Server Virtualization

Error 404--Not Found

Error 404--Not Found

From RFC 2068 Hypertext Transfer Protocol -- HTTP/1.1:

10.4.5 404 Not Found

The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.

If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.

WMM (Wi-Fi Multimedia)

In late 2005, the IEEE approved the 802.11e wireless QoS standard. Eager to spur interoperable QoS among different vendors' wireless gear, the Wi-Fi Alliance developed a subset of 802.11e called the Wi-Fi Multimedia (WMM) specification. Makers of wireless access points as well as wireless client devices such as laptops, phones and consumer electronics products are incorporating support for WMM into their wares.


WMM is designed to support consumer and corporate applications and works with all three 802.11 wireless physical layer standards - 802.11a, 802.11b and 802.11g. The specification provides basic prioritization of data packets based on four categories - voice, video, best effort and background. In its current form, WMM doesn't provide mechanisms for scheduling or controlling network access to improve QoS, but these enhancements may be added in future releases.

Prioritization is based on the original Carrier Sense Multiple Access/Collision Avoidance Protocol in the initial 802.11 standard. A mechanism called Distributed Coordination Function (DCF) uses a simple listen-before-talk algorithm to minimize the chance of packet collisions caused by more than one device accessing the wireless medium at the same time. A client must wait for a randomly selected time period and then "listen" to find whether any other device is communicating before starting to transmit. The random back-off period gives all devices a fair opportunity to transmit.

However, WMM stipulates different fixed and random wait times for the four prioritization categories to provide more favorable network access for applications that are less tolerant of packet delays. Devices that have less time to wait have a better chance of being able to transmit than those that have a longer wait. In order of highest priority, the access categories are voice, video, best effort and background. These WMM prioritization categories map to Ethernet 802.1d prioritization tags to allow consistent QoS across wireless and wired network segments.

The back-off timing for each access category consists of a fixed period called the Arbitrary Inter-Frame Space Number followed by a random period called the Contention Window (CW), both specified in multiples of the slot time. For 802.11b, one slot is 20 microsec and for 802.11a and 802.11g, a slot is 9 microsec. The CW maintains the DCF random back-off component to help avoid collisions of packets from the same access category. The CW range doubles each time there is a collision and is reset to its minimum value after a successful transmission.

WMM prioritization achieves the goal of minimizing delay in wireless networks for time-sensitive applications. The ability to use wireless networks for data, voice and multimedia applications will continue to drive widespread adoption.

From WMM addresses wireless quality of service, Network World Tech Update, 09/26/05.

Additional resources

Wi-Fi Alliance's WMM page
WMM news and papers.

Wireless / Mobile Research Center
Latest wireless news and resources from Network World.

Add a comment

NOTE: Comments are reviewed by an editor before being posted.

Your rating of this resource (with 5 the best)
1 2 3 4 5


Your user name (what other users will see on the review):

Your real name (for our records only):

Your e-mail (ditto):

Your comments (Use a blank line to separate paragraphs):


Sign up for one of NWW's Wireless & Mobile newsletters.

Wireless in the Enterprise
Cool Tools alert
Wireless News Alert
View all newsletters

Email Address:

Vendor Solutions

White Papers


Research Centers: Applications | Application Development | Applications-Standards | Applications Vendor Solutions | Collaboration | CRM / ERP | Databases | Directories | Grid Computing | Java | Messaging | .Net | RFID | SOAP | Web Services | XML | Convergence & VoIP | Convergence Regulatory | Convergence Services | Convergence Standards | Convergence VoIP Vendor Solutions | Video | IP PBX | SIP | VoIP | VoIP Services | E-Business | DNS | RFID | Supply Chain | Web security LANs & Routers | Acceleration | Gigabit Ethernet | Lans-Standards | Routers | Wireless LANs | Network Management | Application Management | Desktop Management | Management Test Patch Management | Operating Systems | Linux | NetWare | Unix | Windows Outsourcing | Managed Services | Offshoring Security | Firewalls - VPN - Intrusion | Identity management | Patch Management | Microsoft Security | Privacy | Security Standards | Spam & Phishing | Viruses & worms | Web Security | Wireless Security | Servers & Desktop | Backup-Recovery | DataCenter | Desktops | Desktop Management | Grid | Servers | Server Blades | Servers Desktops | Utility Computing | Small & Medium Business | Broadband | Telework | Handhelds & PDAs | Home Networking | Security | Storage | Compliance | Infiniband | Network-Attached Storage | SANs | Storage Management | Storage Virtualization | Virtualization | Vendor News | Bankruptcy | Earnings | Lawsuits | Layoffs | Standards | Start Ups | Vendor Markets | Education | Financial | Healthcare | HIPAA | Manufacturing | Retail | Wide Area Network | Broadband | Carriers | Frame Relay | Metro Ethernet | MPLS | Service providers | Wireless services | Wireless & Mobile | Wireless LANs | PDAs & handhelds | Wireless Security | Wireless Services | Wireless Standards | Wireless Switches | All Company Profiles