Can we improve client wireless transmissions?

Q: We are a wireless ISP providing full data connectivity through wireless LANs We have had problems with clients sending e-mail via WLANs, as it becomes very difficult and very slow. Web browsing and receiving e-mail is OK, but sending mail from a client becomes slow, especially when attachments are used. We seem to have a problem between the client’s PC and the mail server (once it reaches the mail server it sends the mail quickly). Is there a way to improve wireless transmission efficiency? Karthi, Madagascar.

The Wizards ponder carefully and respond:

Seth Goldhammer, Roving Planet:

An RF analysis here might help. It could be that while the access points have enough power to reach the clients, the client’s power settings are low and are having difficulty sending data back to the access point. 

Albert Lew, Legra Systems:

If you are using Wi-Fi Alliance certified access points, the certification process ensures that the access points will have good performance downstream to clients and upstream to the wired network. In fact, the requirements to pass certification usually stipulate that the performance exceeds T-1 rates. Therefore, there should not be significant differences between upstream and downstream performance, and the problem you are experiencing is probably not a wireless problem, but rather a terrestrial one.

Some wired broadband access technologies, such as HFC/cable and ADSL, do not have symmetric performance and are anywhere from two to 8 times slower in the upstream direction versus the downstream direction, which is probably causing the problem you are experiencing. One way to test this hypothesis is to test whether the same behavior occurs when you replace the access point with a wired connection to the network. If the same behavior occurs, it is definitely the wired broadband access.

Mike Montemurro, Chantry Networks:

There are several probable causes for your problems: network congestion, lower rate transmissions, or client configuration issues. SMTP uses the TCP/IP protocol for transport, so it adjusts the data packet size depending on the network latency. In congested networks, TCP/IP connections back off to maintain a connection and allow other communications to occur.

Typically, a WLAN uses a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism for transmission of packets. As the amount of traffic on the wireless network increases, multiple stations try to transmit at the same time. When a collision occurs, each station uses a back-off algorithm to gain access to the medium. The back-off time increases as the station encounters more collisions. In the presence of congestion, it is more difficult for a station to access the channel and transmit a packet.

In the presence of interference, or as a station gets farther from the access point, the station will drop its transmission rates to allow for a more reliable link. At lower transmission rates, it takes longer to transmit a packet, which takes up more of the available channel bandwidth and introduces more latency.

The problem could also be related to people having their SMTP server configured for authentication. Typically hotspots intercept SMTP traffic either in-band or via DNS proxy returning a different IP address for the mail server than official.  That results in the mail client hitting a different SMTP server, which can’t handle the authentication request and therefore denies access until the client tries without authentication. If you do not run a transparent SMTP proxy, I would recommend you check your SMTP server logs for possible hints (authentication failures, DNS resolve issues).

If the problem is congestion, you may need to add another access point to increase the capacity of the WLAN in the coverage area. It the problem is with stations transmitting at lower rates, you may want to try fixing the access point operational rate at 11M bit/sec., rather than 1,2, 5.5 and 11M bit/sec. You will decrease your effective coverage area, but you will reduce latency.  One way to prevent problems like performance issues from arising is to look for a solution that provides RF management services that can automatically adjust the configuration of the access point to compensate for performance issues.

Dan Simone, Trapeze Networks:

Traditionally, wireless networks are synchronous in the amount of bandwidth that both access points and clients can send. Of course, unlike switched networks, sending and receiving devices are contending for the same bandwidth since wireless is a shared medium.

One area to look at is whether the problem occurs all the time or just under load. If it appears to be load-dependent, it may be that the access points are taking priority over the clients when they’re busy serving numerous clients. Consider investigating whether you can define the QoS on your WLAN to enable weighted fair queuing and more evenly spread the load.

As with any WLAN anomaly, it’s also important to make sure your WLAN clients are all using the latest drivers – driver software has an incredible impact on the client’s WLAN performance.

Randy Chou, Aruba Wireless Networks:

This may be a case where the transmit power at the access point and the clients are not symmetrical. There are vendor implementations of access points that have a higher transmit power. While this does increase the coverage area of the access point from the perspective of how far a client needs to be to receive packets transmitted from the access point, it does have some undesirable side effects. One is where a client can receive packets transmitted by the access point because of higher transmit power, but the access point may not hear the packets transmitted because the maximum transmit power of the client isn’t strong enough. It’s possible the client is at a location where its signals are strong enough to send smaller packets through, but larger packets are dropped due to CRC errors (note that for a given bit-error-rate, the probability of a packet error increases with the size of the packet). Most Web browsing results in larger packets downstream, with smaller TCP ACK packets in the upstream. But while sending e-mail or one with large attachments, the number of larger packets upstream is higher, and results in fewer packets getting through.