[e2e] Wireless Networks. An Example: GPRS.

[Detlef Bosau]

Am 11.03.2013 17:12, schrieb Martin Heusse:
> (Although I would tend to think you might need to read a bit about GPRS and cellular networks,

Be assured: I read quite some literature. However: Could you please be a 
bit more concrete?


>   I'll stick to a more general remark, replacing your reflections in an end to end perspective. And forgive me if I misunderstood you.)
>
> A sliding window (congestion window, anticipation window) allows a sender to use the links on a multi hop paths in parallel when the network device are store and forward.

Hang on. Sliding window is used to utilize a channel which is able to 
keep more than one packet in transit. Particularly on the media.

>   So to use a GPRS network properly you need a least 4 packets.
> UE -> BSC
> -> SGSN
> -> GGSN
> -> gateway to dest network
> Etc.

At a first glance, I tend to agree, however from what I read so far 
about mobile networks, these do not necessarily deal with IP packets 
internally.

Nevertheless, when the technology allows to keep more than one packet in 
transit and is underutilized otherweise, sliding window makes sense. On 
the air interface (in GPRS actually perhaps "behind" the GGSN) this (at 
least to my understanding) not always make sense.

Generally spoken: I've chosen GPRS because of it's huge latencies. And 
any example I would take, may turn out to be "bad" because the huge 
complexity of different technologies and implementation used in wireless 
networks.

> And that's one way, the return path also count for what's “inflight".
> (ok, high speed links don't hurt as much a low speed links beyond the bottleneck, but I hope you get the idea…)
>
> Also: RLC-ack (Is that what you call RLP?) does use ARQ… (Are you happy now? ...)

This is no matter of happiness. It is a matter of fact and it is part of 
the time necessary to convey a packet.

I mentioned the COMCAR project and I should provide a "QoS architecture" 
there - which included to describe the "rate" available for a stream.

Later on, in a conference, the first question was: "Mr. Bosau, how do 
you no the available rate?"

Period.

Latest in that moment, it became clear to me that TelCo guys and CS guys 
(packet switching guys) simply don't understand each other. They talk at 
cross purposes. Later on, it became clear to me that packet switching 
and line switching (used for voice) use different APIs. We talk about 
the years from 2000 to 2005 here.


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