[Cialug] Commerical Interruption

[Zachary Kotlarek]

On Nov 12, 2010, at 5:23 PM, Josh More wrote:

> So the headers are not fixed length?
> 
> I would have thought that, transmission-wise, something like:  0000000000 would take just as much as 0110100110.  (Unless there's built-in compression, I suppose.) 


It's less about "headers" and more about, let's say for the sake of common naming across technologies, "transmission slots". The specifics vary among MAC schemes, but it's more or less like this:

In multi-access radio systems the available bandwidth is divided into slots by time or frequency or code or any combination thereof. In order to avoid collisions each device engaged in an active call or data transmission is assigned a particular subset of the available slots for its exclusive use.

But in order to get these dedicated slots you first need to tell the tower that you have a call to place or data to transmit, or that your handset is now available, etc. So there are also some slots available for use by all stations during which they content for media access, potentially colliding, and upon gaining control of the media they ask the tower for reserve services for them.

So SMS is "free" in that it uses the contention-based slots for transmission, rather than using the dedicated slots used for normal voice and data traffic. Thus sending an SMS message doesn't reduce the number of voice calls or amount data transmitted to/from a tower. But obviously there's a limit to the total number of stations that can transmit during any particular contention slot, and as with WiFi the more stations that are contending during any given slot the more likely a collision. So at some point there won't be enough space in the "free" slots to handle all the SMS traffic and messages will start to fail.

	Zach

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 2746 bytes
Desc: not available
Url : http://cialug.org/pipermail/cialug/attachments/20101112/f08d9344/attachment.bin