ATM and IP tunneling Overheads calculation

[dandres]

We contract 30 Mbps of IP bandwidth to our customer, and I would like to know how much overhead I need to allow for a PVC on an ATM link set up as following:

PVC l3-eST 6/100

INARP 1

BROADCAST

UBR 35000

OAM-PVC MANAGE

ENCAPSULATION AAL5SNAP

Also, an IP Tunnel has been created and IP Tunnel overhead has to be considered as well. Are 35 Mbps enough to cope with ATM, AAL5SMP and IP tunnel encapsulation?

I would be interested in know how you calculate the overheads if someone knows.

Thanks,

David

[c.mcguinness]

Here’s the lowdown on the overhead. You didn’t state if it is a DS3 or SONET connection as there is a difference in DS3 (PLCP) overhead versus OC3 (SONET), so I’ll assume it’s a DS3.

DS3 44.736 Mbps

PLCP Payload 4.032

ATM Payload 3.84

AAL5 Payload .112 (using the default IP MTU size for ATM AAL5 PVC of 9180)

LLC/SNAP Payload .032

IP overhead .08

Total overhead 8.096

Total left for Transport 36.640

(TCP/UDP)

This why the line capacity of an ATM DS3 is 96,000 CPS or 36.864 Mbps

[44.736 – (PLCP 4.032 – ATM 3.84) = 36.864].

So if you give them a UBR contract for a PCR of 91,145 CPS (35 Mbps) at about 18 % overhead they are looking at about 28.7 Mbps left for TCP/UDP.

IMO - I would give them a PCR of 96,000 CPS

[dandres]

Thank you for your help on this. In fact, it is an STM-1 circuit, i.e. 155.520 Mbps circuit. Are you aware of the STM/SONET payload?

Also, what are CPS and PCR?

Thanks again

David

[c.mcguinness]

Here’s the overhead for a SONET OC3c

Line rate = 155.520

SONET payload = 5.76

ATM payload = 14.128

AAL5 Payload = .412

LLC/SNAP payload = .118

IP payload = .294

20.712 Mbps in overhead, which leaves 134.808 Mbps for transport (TCP/UDP)

CPS = Cells per Second

PCR = Peak Cell Rate

For example if the contract is for a rate of 35 Mbps:

8 * 48 = 384

35,000,000/384 = 91,145 CPS

When you create the UPC contract/connection on your ATM switch you’ll set the PCR for 91,145 CPS

Hope this helps …