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Beginner

Choosing Frame-relay, HDLC or PPP

Hi,

I have been trying to find information about advantages and disadvantages of the different data-link layer protocols on WAN links. I documents describing each of the protocols, but none comparing them.

I would be very helpful if someone could point me to any documents describing this or post comments into this forum.

Thanks in advance!

Regards,

Harald

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Participant

Hello,

Generally speaking they can be differentiated as follows - I am sure there's much more it:

Okay as you know the most common types of WAN communication links in use today are:

1) Leased Dedicated Point-to-Point permanent

circuits: A organization that leased the circuit &

is paying for the full dedicated bandwidth (such

as a T1) 24 hours a day, 7 days a week whether

they actually use the full bandwidth or not. PPP

and HDLC are commonly used over dedicated leased

lines. HDLC is commonly used between Cisco

routers, where's PPP is more suitable if

interoperability is desired.

2) Frame Relay Packet-Switched circuits:

Packet-switched networks enable orgs to

dynamically share the cloud, and the available

bandwidth and it is possible to only pay for the

bandwidth you need (CIR). Frame Relay is

considered a cost effective replacement for leased

line services such as PPP & HDLC

Regards

Pradeep

Highlighted

Thank you very much for your answer!

I am actually only interested in the encapsulation for dedicated leased lines. I have one customer using frame-relay also for a dedicated network and I am trying to understand the benifits of this compared to Cisco HDLC and PPP. I know that PPP can do authentication and as far as I know Frame-relay and PPP can do QoS, but not HDLC. Is that correct?

Highlighted

That's right, HDLC will not support authentication as each vendor's implementation of the protocol is different - Cisco's implementation adds an extra field in the frame that allows it to support different L3 thats encapsulated in it. Normal HDLC does not have it (also the reason for the interoperability issue).

As for QoS support I believe you could do some rtp header compression on hdlc links, but I don't believe you can do any LL queueing as you can on PPP & FR links. So if you are doing VOIP, you are better of using PPP or even FR.

If cost is not a factor, then the only reason to use HDLC is that it is easy to configure - you practically do nothing :-)

Hope this helps somewhat..

Regards

Pradeep

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