What method does router use to forward packets....

[just1moretime]

I am curious to how a router is able to forward
packets from 1 subnet to another. My guess is, it acts
like a repeater from 1 interface to another. I am not
asking about its function but how it accomplishes it.
Maybe a follow up question....how does a non ip
telephone system connect its calls. To me, the uninformed
it would seem that linking up voice is more difficult than
data because of the inherent xmit/rec ongoing exchange. I
am not looking for an explanation of time division multiplex
concept.
Thank you.

 

[NetworksMD]

Routers forward packets based on the routing table and the best match found or default route. Depending on the protocol you are using, a router will look for a parent route match or a child route match for an ultimate route and then forward the packet out of the interface that is assigned to the ultimate route. If the ultimate route has an next hop IP instead of a exit interface, the router then has to do a recursive lookup to find what interface the next hop IP address is out of.

Non ip based telephone systems (TDM) are circuit switched instead of packet switched. Most of the public telephone system uses SS7 signaling and sets up a nailed down connection from the originator CO to the end user CO based on area code and exchange. This means that a circuit switched network has to find a single consistent path from one end point to another and keeps this path up for the duration of the call. Because the circuit is connected from start to finish, this has the advantage guaranteeing the bandwidth and path for the entire duration of the call. However this also means consuming the entire circuit even if there is no actual voice transmission (i.e. both parties put each other on a silent hold where IP can use silence compression). In addition if any portion of the path or circuit fails the entire calls is lost and must be reestablished as a new call. In a IP call there may be certain lost packets (due to voice packets being UDP) if there is an issue or congestion in one the path, but can recover if other packets take alternate routes. Many times this results in degradation in the quality of the call such as clipping or chop.

I don't and haven't worked on the carrier side, but my guess would be that most calls these days are a combination of circuit swiched to IP and back to circuit in the carrier network for many calls.