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KNOWLEDGE: Latency or how fast is your packet sent
Posted by Arne Hess - on Tuesday, 21.05.02 - 18:32:00 CET under 09 - Thoughts - Viewed 10917x
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While the whole wireless industry and the consumers are talking about bandwidth in wireless networks (GSM is able to provide up to 14.4 KBps, GPRS in theory 171.000 KBps and 3G in theory 2 MBps) one not less important fact is forgotten - the round trip delay or better known as latency. defines latency as follow:

In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. In some usages (for example, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency.
The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.

But what does latency for you as the user means and why is latency so important for the wireless Internet?

The explains this very good:

GSM's wide frequencies give it scalability advantages, but the short time slots cause problems in keeping phones synchronized with each other. Radio signals take just over 0.003 microsecond to travel a kilometer, which adds up to a round-trip delay of around 0.4 microsecond for a phone only 60 km (40 miles) from the base station. The time slot only lasts 0.577 microsecond, so this delay is enough to make the phone miss its slot entirely, even though it would be unnoticeable to a human listener.

Okay so here we go: As closer you are to a GSM Base Station as shorter is the round trip delay. And this is for the pure air interface only, not included is the IP backend a packet will go afterwards.

However, on the other hand you have to watch the hardware you are using, depending on this, the latency time also varies. It also depends on the wireless technology you are using, GSM CSD and GPRS means a longer round trip delay while other wireless technologies like IEEE 802.11b means a shorter round trip delay.

A 32 bytes ping to with different technologies:

  • Fixed Line DSL: 123 ms
  • Wi-Fi on the same DSL line: 126 - 219 ms
  • GPRS [Compaq Wireless Pack]: 1103 - 1339 ms
  • GPRS [Bluetooth with Ericsson T68]: 931 - 1004 ms
  • GSM CSD [Bluetooth with Ericsson T68]: 1058 - 1085 ms

Final Conclusion

But what does it means for you today and in the future? With a latency like in today's GSM/GPRS networks, real-time multi-player games are out of the scope! We've tried it a year ago with a multiplayer game (a Bomberman clone) and it wasn't playable through GSM/GPRS. The other players just jumped through the screen - crap; while it was perfect playable through IEEE 802.11b. This fact means that you can not expect multi-player real-time games today on any wireless devices in Wide Area Networks like GPRS. And the future? Well, the feedback I got from different network vendors doesn't looks even better. There is the possibility that even with 3G networks we will not reach the minimum of latency to play multi-player games (which is at least something around 250 ms according to different sources). If this happens, 3G will becomes into real trouble.

And what does it means for today's services? Let us take a service like WAP. The pages has a real small footprint but even with GPRS (which serves up to 35 KBps) WAP doesn't seems to work faster than using CSD and it's true. it's - again - because of the round trip delays. WAP pages are small enough not to require the GPRS bandwidth but due to the latency time the WAP page doesn't appears faster on a GPRS device than on a CSD device.

You see, it's not about bandwidth only but also about latency.

Cheers ~ Arne


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Posted by Anonymous on 25.05.02 - 00:00:00

Hopefully the Nagle Algorithm is turned off in these mobile TCP/IP stacks.

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