Please Help!! ATM Type 3

[icw27]

Can anybody please explain the difference between ATM type 3 and ATM type 1.

I understood that if the service provider provided ATM type 3 then on Cisco IOS we set the PCR=SCR because you can permanently burst (i.e. always send at PCR).

However on type 1 ATM the SCR is half the PCR.

Does anybody know for sure?

Regards, Ian

 

[VHDLguy]

I am not certain on this information, most of my ATM knowledge is low level implementation. So I am not intimate with how it is used and more importantly what is is marketed as. And my explanation is opposite to what you stated, so Im guessing that either you got it around the wrong way, or I am talking about something else.

But I think maybe you are talking about AAL1 vs AAL3? ATM Adaption Layer 1 or 3.

There are many differences, but it sounds like you are interested in the bit rate.

Class A or AAL1 is a CBR or Constant Bit Rate. ie fixed interval's between cells, in which case Sustained Cell Rate = Peak Cell Rate - or SCR = PCR.

Class C and D or ALL3/4 (they have kind of merged together, one is connection oriented, the other connection-less) uses VBR or Variable Bit Rate. I don't recall there being any rules that the PCR must be twice the SCR, it could be any number that the service provider would like. I am not sure if AAL3 is rt-VBR or nrt-VBR or what (real-time or non-real-time).

The difference between nrt-VBR and rt-VBR is the Cell Delay Variation (or CDV) and Cell Transfer Delay (CTD). real-time have specific values for CTD and CDV (ie the timing variation between cells is more tighly controlled), nrt uses a mean CTD only.

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http://www.lateralsands.com/

 

[yewenyi]

VBR atm services have three types. Only Type 1 and 3 are commonly used. They are called VBR.1 and VBR.3.

VBR.1 allows you to burst to PCR for a number of cells equal to the MBS. However, over a period of twice your MBS or higher you can only transmitt data at your SCR. So if you have a circuti that is 1000 cells per second SCR and 500 Cells per second PCR and MBS 100, and send a continuous 1000 cell per second, the first 100 cells will get through, the second 100 cells will be discarded and then the pattern repeats.

VBR.3 allows you to send the full PCR. In the example above the first 100 cells will be allowed through, the second 100 will be tagged with CLP=1 (Presuming you send all your traffic with CLP=0) and then the pattern repeats. CLP=1 is only a best efforts service and if there is congestion in the network it is the first to be discarded.

My understanding is most CISCO Routers are all VBR.1 settings under the nrtVBR category. So if you have a VBR.3 service you need to set your SCR = PCR to get the full capacity of your circuit. Some CISCO BPX devices allow you to set the VBR type.