Solved: Re: Some help on ATM basics

ankurbhasin · ‎01-18-2006

Hi Champs,

I am new to this old technology and have some basic questions.

1) What are the scenarios where VPC will be used and what are the scenarios where VCC will be used?

2) Also these are 2 different kinds of connections which customer will ask for or the ISP will configure as per his ease?

3) I understand that bundle of VCC can be used over a single VPC but what will be the advantage of doing the so? Will there be more throughput?

I know these are very basic concepts but waht to get good hands on ATM technology so pleae take sometime to answer my doubts and incase I am on a wrong forum please redirect me to the right one.

Regards,

Ankur

mheusinger · ‎01-19-2006

Hello,

the first question is, whether there is a technical need to separate the 23 channels in the ATM network.

What usually is done when you want T1 point-to-point (for PBX interconnection NOT using voip) is called CES (circuit emulation service), which takes T1 or E1 frames (OSI layer 1) or even just the Bit stream, places this into ATM cell payload (AAL1) and transports it to the other end. The customer will have a normal T1 interface to the SP, just the transportation inside the SP network is through ATM and not TDM.

Assuming you want to demultiplex the T1 into 23 channels with different destination ports, then ATM VCC connections will be setup. But VPC is not an option in this case anyhow because of different destinations.

Again, with Cisco WAN switches you could create "channel-groups" - they are actually called ports in the WAN switching terminology - and do CES for those single ports. Again a VCC would be used. Each VCC for a port can have a different destination - up to 23, if you want.

So a VPC is only needed if you have an ATM interface to the SP and you order ATM VPC service to be able to setup VCCs yourself. Typical scenario, as mentioned above: interconnection of customer ATM switches.

Hope this helps! Please rate all posts.

Regards, Martin

View solution in original post

mheusinger · ‎01-18-2006

Hello,

so ATM is still alive! OK on to the questions then.

A1) A VPC transports ATM cells through a SP network solely based on the VPI value. The idea is to leave all VCI settings untouched. This enables the customer to configure his own PVCs without intervention of the SP needed. A typical scenario would be a so called "virtual trunk", i.e. a VPC connection between two ATM switches operated by the customer. This could be part of his ATM backbone replacing - for cost reasons - a dedicated line usually used to connect aTM switches.

A VCC is typically used to connect two ATM end devices (mostly routers).

A2) Partially answered in A1, but the SP will usually not assign more "VPI/VCI address space" to a customer than needed. With one VPI across his trunks he can service up to 65000 PVCs (really depends on the hardware used). There are however a max of 4096 VPI available - infact usually less because of hardware restrictions. So SP will configure a VCC unless the customer asks for VPC (and pays for it ;-)

A3) Throughput will not change between VPC and VCC as the size of ATM cell is not modified and the hardware will still have to switch cell by cell.

The use of a VPC versus VCC should be answered by A1 - I hope. The main question is who sets the VCI values, i.e. sets up the VCC. The advantage is that this can be done by a customer independantly from the SP, so less hassle with change management of the SP, time to setup the VCC across the VPC etc. - and it can be done dynamically. A customer could run PNNI across a VPC link transparently through a SP ATM network not supporting it.

Hope this helps! Please rate all posts.

Regards, Martin

ankurbhasin · ‎01-19-2006

Hi Martin,

Firstly thanks for your reply.

For suppose a customer has taken T1 conneciton from any ISP and using all 23 channels so he need to specify 23 VCI to pass through the ISP to his remote office or he will be grouping all the 23 channels as one bundle and having one VCI value represting the same and letting it pass through the ISP on his VPC?

Please explain me bit more in detail taking an example of customer with T1 line trying to connect via the ISP (ATM backbone) to 2 different remote locations. Please your help me be highly appreciated.

Any links also in which a practical scenario is given with configuration will be helpfull.

Also will like to know in what scenario ISP will configure VCC?

Your answers will be highly appreciated.

Regards,

Ankur

mheusinger · ‎01-19-2006

Hello,

the first question is, whether there is a technical need to separate the 23 channels in the ATM network.

What usually is done when you want T1 point-to-point (for PBX interconnection NOT using voip) is called CES (circuit emulation service), which takes T1 or E1 frames (OSI layer 1) or even just the Bit stream, places this into ATM cell payload (AAL1) and transports it to the other end. The customer will have a normal T1 interface to the SP, just the transportation inside the SP network is through ATM and not TDM.

Assuming you want to demultiplex the T1 into 23 channels with different destination ports, then ATM VCC connections will be setup. But VPC is not an option in this case anyhow because of different destinations.

Again, with Cisco WAN switches you could create "channel-groups" - they are actually called ports in the WAN switching terminology - and do CES for those single ports. Again a VCC would be used. Each VCC for a port can have a different destination - up to 23, if you want.

So a VPC is only needed if you have an ATM interface to the SP and you order ATM VPC service to be able to setup VCCs yourself. Typical scenario, as mentioned above: interconnection of customer ATM switches.

Hope this helps! Please rate all posts.

Regards, Martin

ankurbhasin · ‎01-19-2006

Hi Martin,

Thanks for your answer!! It helped a lot!!

Regards,

Ankur