• Here's the issue to which I can't seem to be able to find a solution. I'm running an 802.11g wireless network with one 802.11b device (HP JetDirect 380x print server). My advertised broadband bandwidth is 4000 Mbps down. When I connect a workstation to the Cisco 831 router and measure the bandwidth on dslreports, the bandwidth is something about 3800 Mbps. When I connect a workstation (either desktop or laptop) to the Internet via my wireless network (Cisco 1121g AP), which is connected to Cisco 831, the downstream bandwidth drops to about 2000 Mbps or less. There're no collisions between the cable modem and the Cisco 831 or between the Cisco 831 and Cisco 1121. The ping times from a client to the router (on the wireless network) are between 1 & 2 msec.

    What's causing this?

    P.S. The wireless cards are also Cisco 802.11a/b/g

  • Hi Sirozha:

    I assume you mean 4.0 Mbps line speed and 3.8 Mbps throughput with Ethernet client and 2.0 Mbps throughput with Wi-Fi client.

    I suggest you focus on why your wireless network throughput is so slow. An 802.11b HR/DSSS based network might achieve 6 Mbps and an 802.11g ERP based network might achieve 30 Mbps. You may be contending for the medium with your neighbors' Wi-Fi or interference from non Wi-Fi RF technology.

    Take the DSL out of the picture and measure throughput between your several Ethernet and Wi-Fi stations.

    I hope this helps. Can you add your location to your forum profile? Thanks. /criss

  • By (Deleted User)

    Wireless LAN vs. Wired LAN
    An IEEE 802.11 wireless LAN does not exactly work like an Ethernet LAN but shares some similarities. The WLAN access method is CSMA/CA where CA stands for Collision Avoidance. Notice that's not the same as Collision Detection, where all clients are guaranteed of hearing each other's transmission. With a WLAN, it is quite possible for two clients, located on two sides of an AP, to both communicate with the AP, yet not hear each other. Plus the radio medium has a vastly poorer bit error rate, around 0.1%, compared to a wired LAN rate of 1.0E-10. Hence more frames will require retransmission. All 802 WLANs employ a handshaked transmission to compensate, with the client NIC and the AP responsible for positive acknowledgment with retransmission. But this adds overhead and reduces throughput by 50-60%!. A WLAN is like push-to-talk radio - it is a half-duplex broadcast access method. In this regard it is analogous to a hub (repeater) of a wired LAN, where all stations (should) hear each others' transmissions and all compete for the shared bandwidth.

    Most organizations have deployed layer two switches (bridges) across campuses to provide dedicated bandwidth. In some case, the second generation of layer two switches has been deployed, with higher speeds and QoS functionality. WLANs will be a step backward. Slower speeds, half duplex, shared media. (Is that three steps backwards?) When you become unfettered from the wire, you gain freedom, but you give up something in return.

    A wireless access point (AP) usually is a layer two bridge, performing store and forward of frames between a wired LAN and a wireless LAN. When we combine the hub-like nature of the WLAN, an AP is really more like the combination of a two-port bridge with a wireless hub on one side.

    Since the AP acts as a layer two bridge, it should also be capable of performing Spanning Tree Protocol (STP) on both the wired and wireless sides. Note that if two APs share the same frequency channel, they will indeed create a STP loop via their combined WLAN. (The airwaves are no different than a crossover cable on the wired side!) This is one reason why APs should only be installed as part of an integrated campus design.

  • Criss_Hyde Escribi?3:

    Hi Sirozha:

    I assume you mean 4.0 Mbps line speed and 3.8 Mbps throughput with Ethernet client and 2.0 Mbps throughput with Wi-Fi client.

    Certainly that's what I meant. I'm lying in bed recuperating from a surgery, so I guess the anesthesia hasn't worn off yet. :)

    The second response indicates that the throughput on a wireless network is reduced by 50-60%. That's exactly the pattern I'm seeing. So, is this a well known phenomenon?

  • Hi Sirozha of Kennesaw:

    Maximum throughput of a wireless network is roughly half of its data rate. But you are observing a still lower throughput than what an 802.11b HR/DSSS network can bear.

    I hope this helps. Thanks. /criss

  • half duplex, can't send and recieve at the same time, that's why it's halfed!
    A 802.11g network supports "b" but will have to be in protect mode, meaning, it talks the same as 802.11b (no multi plexing), like 100mb slowing to talk to 10mb. And the AP and client will see how fast they can talk. Move the printer to the wire. Or move the AP closer to the printer. I bet they are to far away (or way to close) and they auto'ed to 2mb.


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