Chapter 6 Broadband
VMG1312-B Series User’s Guide
102
Multiplexing
Ther e are t wo conventions to identify what prot ocols t he virt ual circuit (VC) is carrying. Be sure to
use t he m ult iplexing m ethod required by your I SP.
VC- based Mult iplexing
I n t his case, by prior m utual agreem ent, each protocol is assigned t o a specific virtual circuit; for
exam ple, VC1 carries I P, et c. VC- based m ultiplexing m ay be dom inant in environm ents where
dynam ic creat ion of large num bers of ATM VCs is fast and econom ical.
LLC- based Multiplexing
I n t his case one VC carries m ultiple protocols wit h protocol ident ifying inform ation being contained
in each packet header. Despit e the extra bandwidt h and processing overhead, this m ethod m ay be
advant ageous if it is not practical t o have a separat e VC for each carried protocol, for exam ple, if
charging heavily depends on t he num ber of sim ultaneous VCs.
Traffic Shaping
Traffic Shaping is an agreem ent between t he carrier and the subscr iber t o regulate t he average rat e
and fluct uat ions of dat a t ransm ission over an ATM network. This agreem ent helps elim inate
congestion, which is im port ant for transm ission of real tim e data such as audio and video
connections.
Peak Cell Rat e (PCR) is the m axim um rat e at which t he sender can send cells. This param eter m ay
be lower ( but not higher) than the m axim um line speed. 1 ATM cell is 53 bytes ( 424 bit s), so a
m axim um speed of 832Kbps gives a m axim um PCR of 1962 cells/ sec. This rate is not guaranteed
because it is dependent on the line speed.
Sust ained Cell Rate (SCR) is the m ean cell rate of each burst y t raffic source. I t specifies the
m axim um average rate at which cells can be sent over the virtual connection. SCR m ay not be
great er than the PCR.
Maxim um Burst Size (MBS) is the m axim um num ber of cells t hat can be sent at the PCR. After MBS
is reached, cell rates fall below SCR unt il cell rate averages t o the SCR again. At t his t im e, m ore
cells ( up to the MBS) can be sent at the PCR again.
I f t he PCR, SCR or MBS is set to the default of "0", t he system will assign a m axim um value t hat
correlat es to your upst ream line rat e.
To Next Page
Loading...
