[BC] UDP so what!!

[Edwin Bukont]

Yadda Yadda Yadda

 

UDP does not necessarily mean you have an RTP or RTSP session.  It is the RTSP,  not the UDP itself, that enables streaming. UDP merely removes the ongoing TCP connection process and its overhead so that there is less bandwidth necessary and less chatter that delays data.  What happens for RTSP (and to some degree even RTP which does have RTCP) instead of TCP is the need for a server to monitor and identify (but not necessarily control) the traffic.  Much of the control in streaming is at the intermediate path and receiver.  The uplink is very much at the mercy of the earlier stages.

 

So SIP in the IP sets up the path for UDP to enable the VoIP using RTP/RTCP or streaming RTSP.  Yeah, we know all that.

 

BUT RTSP does not matter if the bandwidth is just not there to handle the traffic.  That bandwidth is in part a function of the server policing the gateway.  Satellite and Cable are both RF gateways that do not handle IP directly.  They are essentially a fixed address and a limited number of processes.  You don't add RF ports as users are added.  What you open are added port-shared processes in the server, if it has the guts to do so.  None of the uplink carriers avaialble to end users are really full time.  While it may be argued that the system is FDM, there are still a limited number of channels.  The RF system is essentially time shared, not TDM...data asks for time to payload to the modem and excite the carrier for a fixed period.  There is a wait for permission aspect.  Permission can be denied.  The UDP of the data has nothing to do with the control of the RF carrier in the uplink. (Whereas the downlink side may be a full time service that spits an aggregated raw stream to be deciphered at the modem).  Demand of data to be uplinked does not cause a like availability of bandwidth.  Just because you have permission to drive a car does not mean you get to go from local roads to the highway as you please because the on-ramp may  have a 'one car per green' signal. The server then controls what gets loaded as payload (how many cars on the highway so that traffic moves, not the max number possible on the highway).  Its a mux, a bridge, not a switch. Its all aggregated and upconverted to RF. Buffering to a sync signal is the name of the game.   
At some point, if there are more packets of a like class claiming priority than there are avaiable frames to stuff bits into in a timely manner, then latency increases and eventually timeouts increase and packets are dropped even if other classes maintain throughput.  

 

Edwin Bukont CSRE, DRB, CBNT 
V- 240.417.2475; F- 240.368.1265