Wireless bridges and repeaters can eliminate wires and phone lines. Blanket outdoor coverage might be provided inexpensively and without wires. It’s sorta like Christmas.

The Wireless Distribution System (WDS) pdf is described by Intersil. It’s largely reiterated below (in html). Network authority Tim Higgins has the definitive explanation of wireless bridging and wireless repeating using the $100 D-Link 900AP+.

BuffaloTech’s AirStation 802.11g Router, the WBR-G54, will do repeating at 54 Mbps using the 802.11g “standard”. Tim Higgins reports their WDS (Wireless Distribution System) implementation is incompatible with the WDS feature on Agere (Lucent) 802.11b based products.

“But it appears from the interface that you can set two (or more) G54’s so that they repeat (allow clients to connect and also connect wirelessly to each other), or just form a wireless bridge (i.e. with no clients allowed to connect)”.

A 54 Mbps repeater for $130 is a pretty good deal by anyone’s standard.

While a wireless bridge can link two lans, repeaters combine an Access Point and a wireless bridge (”repeating” a distant “hot spot” locally on the same local channel). Repeaters used to cost $500 and up. Now they’re practically free.

A $100 Wireless Ethernet Bridge like the Linksys WET11, D-LinkAir DWL-810 or Hawking WB320 can connect a video game, printer, computer or small LAN over a wireless link but they can’t, themselves, be an Access Point. A repeater can.

Variations of The Wireless Distribution System are used by the $900 Orinoco/Proxim AP-2500, the $100 D-Link’s DWL-900AP+ and the 802.11g BuffaloTech’s AirStation WBR-G54 ($130). Apparently the Linksys WAP51AB Access Point (with both Atheros 802.11a and Broadcom 802.11b chips), will also do bridging with the latest firmware. Bridging with Access Point devices is trickier than with devices designed for “wireless bridging” like the WET-11.

Keep in mind that when using APs to bridge, both the client and the AP source device must be identical. Devices designed as “wireless bridges”, like the WET-11, can work with many different APs.

The AP-2500 adds gateway functionality to the dual-slot AP-2000 Access Point, including bandwidth management, home page redirection and Dynamic Address Translation (DAT). DAT overrules all user specific IP and proxy settings in the wireless client to ensure that the user can easily access the hot spot network without changing computer settings.

WDS provides a wireless backbone and a local “hot spot” - simultaneously. Large, open areas like public parks could benefit.

If the structure utilizing an access point is too large to cover using one AP, a second AP would be used as a wireless repeater. The WDS feature uses a single radio in the access point both for the wireless cell and the connection to another access point (backbone connection).

Wireless Bridge
The first use for AP-to-AP communication is the creation of a wireless bridge between two wired networks. AP1 can be configured to forward all data to AP2 and vice versa. Both APs apply the same wireless parameters, thus enabling the two to communicate. As a bridge, both APs learn the network devices which are connected to their respective Ethernet ports to limit the amount of data forwarded.

Data destined for stations that are known to reside on the peer Ethernet, multicast data, or unknown destination must be forwarded to the peer AP. The fact that the data is being bridged is completely transparent; i.e., the data received by the LAN station is identical to data that would be received if both LAN stations had been connected to the same wire.

Wireless Repeater
The WDS has forwarding functionality for extending the range of an AP. By setting up an extended service set (ESS) between access points 1 and 2 and manually configuring the WDS peers, stations can intersect with any AP of this ESS and move between the coverage of both APs while the higher layer network connection remains intact. This is similar to the mobility provided in an ESS environment with a wired distribution system. This means that the WDS link is used as the distribution system.

As in the bridging set-up, you can add several WDS links for extending greater area coverage. One drawback is the possibility that loops in the network can be created. It is imperative for the AP to communicate with other APs to handle network loops. This is accomplished by way of a spanning tree algorithm. Another drawback of this functionality is the degradation of the performance by half as the data is transmitted twice in order

The repeater function is configured in the web server. To enable the repeater mode, you would configure two APs with each other’s MAC address. The MAC address you give both APs has an identical SSID and ACL list (just as if they were wire connected). You can use up to eight APs in the WDS mode, but because their functionality resides on the same channel as both a WDS unit and an AP, throughput will go down.

WDS and Security
As previously discussed, the AP supports WDS links for establishing links between APs. These links should use the same security methods as regularly associated stations. If the privacy method in use is an open system or WEP, then the WDS links can be set-up accordingly. For WEP, both peers should know the secret WEP key.

Spanning Tree Algorithm
One drawback of transparent bridging lies in network topologies with loops due to multiple bridges. Data could potentially be forwarded and duplicated endlessly. This can be prevented by discovering other bridges in the network topology and establishing forwarding rules such that one bridge forwards data between network segments. This is done by using the spanning tree algorithm (STP).

The spanning tree algorithm makes it possible to handle more than two LAN segments. It is possible to have multiple segments form a large network by point-to-multipoint connections. This implies that the AP is able to set-up multiple WDS links. In the APDK 1.2 and 2.0 versions, there is a maximum of 8 WDS links.

If you want to use the AP only as a wireless bridge, you can deny regular wireless stations to join with the bridging APs and thereby avoid pollution of the wireless medium with undesired data traffic, e.g., by using the access control list (ACL list).

Finally, a wireless bridge is commonly used to connect the LANs of different sites from one and the same company. The distance between those sites normally exceed the maximum range of the AP. For deploying the AP as a wireless bridge, it is necessary to be able to adjust transmission settings like the transmit power and maximum delay for increasing the outdoor range of the AP. The APDK 1.2 and 2.0 can be used for this, but the sample firmware and web server do not expose this functioanlity. The customer can easily develop this in the open source code of these products.

WDS and Performance
A normal Bridge link will have the same performance as when using the AP with a client (see Tables 1 and 2). Using the AP both as WDS link and with a client will cause a throughput drop by 50 % when either a wireless client is added or a WDS AP is added.

Interoperability
The WDS feature is not completely specified in IEEE or Wifi standards. Therefore interoperability between 802.11 products of different vendors is not guaranteed.

WDS Configuration
In versions 1.2 and 2.0 of the APDK, the WDS link can be configured by entering the peer Access Point MAC address in each other’s AP configuration application. Further, the settings of the two (or more) APs must be the same (as in the radio channel).

• For making a point-to-point link between two WDS APs, Each AP’s MAC address must be entered in the ACL list of the corresponding AP. As a result, other clients are not allowed on both APs.
• Configuring WDS links and using the repeater mode is described in the APDK end user manual under Configuring WDS Links with an Intersil APDK.

The DWL-900AP+ Access Point, which costs under $100 adds wireles repeating with a simple software upgrade. To quote Tim Higgins;

“The 2.20 firmware, supports wireless routers and access points in the “enhanced 802.11b” D-Link AirPlus line of wireless networking products, as well as the enhanced 802.11b side of the D-Link AirPro line of inexpensive ($200) dual-band products (review) .

The DWL-900AP+, can now work as an access point, a bridge between access points on two different networks, a hub between multiple wireless networks, and a wireless (and driverless) client. As a repeater, it increases the overall coverage by a reported 50% although you can’t daisy chain one DWL-900AP+ after another to make an unlimited range wireless LAN — it’s only one hop.

Previously, “repeater” funtionality for Wi-Fi networks was only available in products like the $500 Intel 2011 or the $999 Cisco 1200“.

Tom’s Hardware Guide explains more about D-Link’s DWL-900AP+ repeater mode. The 900AP+ is reportedly implementing the Wireless Distribution System (WDS).

D-Links says their DWL-900AP+, when set to Client or Repeater mode, can ONLY connect to their DWL-900AP+, DWL-100AP, DI-614+, DI-764, and DI-754 (with DWL-650+ adapter). But Tom’s Hardware says it works with other clients and Access Points not necessarily D-Link’s AirPlus and AirPro product lines. The Orinoco/Proxim AP2000 can use the Wireless Distribution System for repeating using their twin PCMIA slots.

Tom’s Hardware reviewer Tim Higgins was able to get the 900AP+ to successfully repeat using a D-Link DI-614+ wireless router, and SMC 2655W 802.11b Access Point, using a D-Link DWL-650+ and ORiNOCO Gold PC client cards.

Limitations of this repeating mode in the 900+ include:

1. A significant (over 50%) throughput reduction. Most of the reduction comes from using the 900AP+’s single radio for two simultaneous tranfers, one from the client to 900AP+, the other from the 900AP+ to main AP. The rest is probably processing overhead.
2. Inability to set the 900AP+ to a separate channel when it’s in repeating mode. Since there’s only one radio, and it has to communicate back to the main AP, the 900AP+ will override its channel setting and automatically set itself to the main AP’s channel.
3. You can’t chain 900AP+s together to even further extend your WLAN’s range. Although you can have multiple 900AP+’s repeating to a single main AP (I haven’t tried this), they all would use the same channel.
4. Once you set the 900AP+ to repeating mode, you lose the ability to access its built-in admin function via wireless connection.

While no more than 1 repeater may be chained (practically), others have chained multiple DWL-900s together after an access point.

Wireless Parks would benefit from repeaters because it eliminates the need to run phone lines (and DSL) to remote “hot spots”.

Every community has their own Waterfront Park. Stick a couple of APs on either side of a bridge. A directional panel could shoot a signal along a linear park or roadway to the next bridge. A bi-directional antenna on the 2nd bridge might extend coverage, continuously, for a mile or more. One access point and one repeater on either side would require only a single phone line. A wireless Birdcam might be handy for zoos or wild life safaris.

Network management software packages for wireless networks include: Proxim’s Network Management software, Wavelink’s Mobile Manager with cross-platform support, Cisco IOS Software, NetNearU’s Runtime Engine with NetNearU’s Routing Access Point, Cirond’s roaming clients and U-Roam’s management software. Gateway boxes from ReefEdge, Nomadix and Bluesocket can provide access control and security.

Host AP Mode
A “wireless DSL” backbone might use the 802.11g “standard”. Or not.

Broadcom’s 54g chips are incorporated in Belkin, Netgear, Buffalo and Linksys gear. Unfortunately, Broadcom’s chips don’t utilize Host AP Mode, a Linux driver for wireless LAN cards, commonly used in “free” public wireless networks.

The Wireless Distribution System, as implemented in the Host AP mode uses Intersil chips since only they have the necessary firmware.

Intersil’s Prism GT, a 54Mbps 802.11g chip, is used in D-Link’s products including the 802.11g “AirPlus Xtreme G”, a $120 (802.11g) Access Point with detachable antennas and a companion $65 PCMCIA card. Intersil Prism2/2.5/3 chipsets support Host AP mode but whether their new 802.11g chips will support HostAP is unclear.

Wouldn’t it be nice if a central tower could deliver 54 Mbps “wireless DSL” backbones to a dozen, $120 Wi-Fi repeaters. Maybe it’s possible.

HostAP can use a 200mW Intersil chip (like Senao or Zcomax cards use), and provide “hot spot” management functions in the host computer. Host AP, running on Linux, enables a cheap computer to act like an access point just by adding a PC Card. It allows you to provide “hot spot” access at low cost along with automatic redirection to a splash page. Personal Telco implements Host AP mode on their small, inexpensive Clone Army Box. HostAP Archives have more information.

Intersil’s 802.11g, Prism GT builds in two mandatory modulation schemes, Complementary Code Keying (CCK), used in 802.11b, and Orthogonal Frequency Division Multiplexing (OFDM), which runs at 54Mbps in the 2.4 GHz band. Other management scemes may be used. For example, ViXS adds QOS network management for managing video streams.

Dual Band Community Lan
A dual-band AP might drive three, dual-band, +15dB sector antennas (2/5 GHz) on a tower. A 5 GHZ client might use an Engenius 5 GHz bridge with a +19 dB Superpass panel for “wireless DSL”. Simultaneously, a 802.11g BuffaloTech AirStation WBR-G54 ($130) might use the 2.4 GHz band to link to multiple “repeater” hotspots.

Would it work? Beats me. What do you think?

Posted by Sam Churchill on Saturday, December 21st, 2002 at 12:31 pm.