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  1. Member tomset's Avatar
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    #1

    Default Wireless IP Phones - duplex question

    What up wireless gurus!

    I was thinking about this the other day. Wireless LANs operate in half-duplex mode. So how can people have conversations using wireless IP phones? Is the conversation choppy since you can't talk and listen at the same time?

    It doesn't seem like wireless IP phones would be designed that way so what am I missing?
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    #2
    Quote Originally Posted by tomset View Post
    What up wireless gurus!

    I was thinking about this the other day. Wireless LANs operate in half-duplex mode. So how can people have conversations using wireless IP phones? Is the conversation choppy since you can't talk and listen at the same time?

    It doesn't seem like wireless IP phones would be designed that way so what am I missing?
    tomset,

    Are you talking (pun intended) about 802.11a, 802.11b, 802.11g, 802.11n, or don't know?
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    #3
    Quote Originally Posted by tomset View Post
    I was thinking about this the other day. Wireless LANs operate in half-duplex mode. So how can people have conversations using wireless IP phones? Is the conversation choppy since you can't talk and listen at the same time?

    It doesn't seem like wireless IP phones would be designed that way so what am I missing?
    The voice data is compressed and split up into packets. The bandwidth of the wireless connection is greater than the bandwidth of a single call. If your link is at capacity then yes, you'd get a choppy sounding call.
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  5. Member tomset's Avatar
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    #4
    Quote Originally Posted by tech-airman View Post
    tomset,

    Are you talking (pun intended) about 802.11a, 802.11b, 802.11g, 802.11n, or don't know?
    I'm talking about 802.11g.

    Tiersten -

    Let me see if I can clarify what I'm asking. If you and I are having a phone conversation, there may be times when I talk and you talk at the same time. When that happens, I talk and can hear you talk at the same time. That requires a full-duplex connection (which the POTS network is). If you're now using a half-duplex connection, we wouldn't be able to talk (and hear each other at the same time). Doing so would cause a collision on the half-duplex connection. Am I wrong?
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    #5
    Quote Originally Posted by tomset View Post
    Let me see if I can clarify what I'm asking. If you and I are having a phone conversation, there may be times when I talk and you talk at the same time. When that happens, I talk and can hear you talk at the same time. That requires a full-duplex connection (which the POTS network is). If you're now using a half-duplex connection, we wouldn't be able to talk (and hear each other at the same time). Doing so would cause a collision on the half-duplex connection. Am I wrong?
    Why are you assuming that a VoIP call will monopolise the entire connection? It is a packet switched network and not circuit switched.

    VoIP splits up the data into packets. The packets are sent to the other side. If the link isn't at capacity then there are points in time where the network is free. Packets from the other side can come in at this point. From the point of view of the users, they're having a full duplex voice call.
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  7. Member tomset's Avatar
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    #6
    Quote Originally Posted by tiersten View Post
    Why are you assuming that a VoIP call will monopolise the entire connection? It is a packet switched network and not circuit switched.

    VoIP splits up the data into packets. The packets are sent to the other side. If the link isn't at capacity then there are points in time where the network is free. Packets from the other side can come in at this point. From the point of view of the users, they're having a full duplex voice call.
    You are familiar with how a half-duplex connection works, right? Bandwidth can't overcome the restrictions of a half-duplex connection no matter how much bandwidth you might have. The duplex settings of the link will still determine how the link will operate. The wireless AP and the wireless IP phone use the same frequency to transmit and receive. If they try to talk to each other at the same time, collisions will occur and both devices will have to wait the random back-off period before re-transmitting.

    It just seems like you wouldn't be able to have as smooth a conversation on a wireless IP phone as you would with a traditional POTS or landline VoIP call.
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    #7
    Quote Originally Posted by tomset View Post
    You are familiar with how a half-duplex connection works, right?
    Yes. I know what a half duplex connection is.

    Quote Originally Posted by tomset View Post
    Bandwidth can't overcome the restrictions of a half-duplex connection no matter how much bandwidth you might have. The duplex settings of the link will still determine how the link will operate. The wireless AP and the wireless IP phone use the same frequency to transmit and receive. If they try to talk to each other at the same time, collisions will occur and both devices will have to wait the random back-off period before re-transmitting.
    If my VoIP call is using g729 then it is split up into 10ms packets and consumes 8kbps assuming no extensions etc... Every second, the phone needs to send 1KB and receive 1KB. Is the fact that WiFi is half duplex going to adversely affect this requirement?

    Quote Originally Posted by tomset View Post
    It just seems like you wouldn't be able to have as smooth a conversation on a wireless IP phone as you would with a traditional POTS or landline VoIP call.
    Yes. You're just switching sides really quickly but from the point of view of the callers, they've got a full duplex connection.

    There is a inherent delay in using packet switching due to the need to actually packetize the data. The restrictions caused by having a half duplex network link aren't going to be significant enough for users to actually notice because the packet sizes are so small. Your POTS call is going to be going over a packet switched network at some point anyway.
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    Tomset, I don't think you're taking into account how many milliseconds that all occurs in.
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  10. Went to the dark side.... Moderator networker050184's Avatar
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    #9
    Quote Originally Posted by dynamik View Post
    Tomset, I don't think you're taking into account how many milliseconds that all occurs in.
    +1

    The delay is so small that a person wouldn't notice it.
    An expert is a man who has made all the mistakes which can be made.
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  11. Member tomset's Avatar
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    #10
    Great guys, thanks for the info!
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    #11
    Quote Originally Posted by networker050184 View Post
    +1

    The delay is so small that a person wouldn't notice it.
    Exactly. Packets are moving at nearly the speed of light.. even if they can only go one way at once it's still so fast you can't hear the milisecond of delay
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