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New Member

IP telephony

Pl Help me in understanding, if IP telephony can be implemented over WAN on slow links ( Analog lines, PSTN Lines - this are dedicated as Lease lines with bandwidht of 28KBPS speed ).

Is it possible to trigger IP telephony call on PSTN line with DDR enabled if the given lease network is Analog lines with 9.6 KBPS bandwidth


Re: IP telephony

Can you do it, yes. Is it a good idea, probably not. Depending on what other traffic you are sending down that pipe, you might have a hard time keeping the voice quality at an acceptable level, much less a good level.

Also, you do have some demand routing that you can do, but how you would implement that depends on how you implement VoIP. You can simply use routers or a full-blown CallManager system. Hope this helps.


New Member

Re: IP telephony

Thanks Mr. Rick

I have only an plain text traffic other than routing updates, as we are in progress of rolling out the other project in next one year time. For the first case that I have posted pl send an possible configuration on my E-mail ID :

With Regards,


New Member

Re: IP telephony

It really depend on how you implement it. A typical Voice channel using Cisco router with codec g729r8 would cost you 18kbps without compression. With compression, it probably give you about 13kbps.

Moreover, you have not state the response time between each end.

So far, the slower speed I have in my locations are 64kbps with 2 voice channels coupled with Data use with help of QoS and managed to keep the quality in place.

Think you might have to think about your Leased line speed again.

New Member

Re: IP telephony

Well, In our location we have all mixed types of lease lines ranging from Digital 9.6, 19.2, 64, 128 upto 2 MB and Analog lines 4.8 and 9.6.

In places were the bandwidth is in the range of 4.8 KBPS and 9.6 KBPS , we are looking at an possiblity of triggering its backup PSTN line ( PSTN gives an bandwidth of approximately 28 KBPS ) to route the IP telephony call.

Hence we are seeking help of Network Professionals to transhout this challenging issue.

Pl note our Hardware : Call Manager 3.3 is at central site, which has 7507 core router, 3745 as distribution router and at remote locations we have placed 2621 XM router - The design is Cisco Standard hirerchical, IP telephone is 7509.


Re: IP telephony

9.6 Kbps is not enough to route a VOIP call, even with a G729 codec and header compression.

A G729 call with header compression and 40 msec packetisation period (40 bytes samples rather than the default 20 bytes samples) still needs 12Kbps. It just isn't going to work, and any data being sent while a call is up would just get discarded.

The minimum you should consider is 64K - even if you could use a very low bit rate codec , the serialisation dleay (time taken to clock out the individual bits of the packet) is just too long to make it acceptable. 64K has reasonable delay, and it has enough B/W for a couple of calls plus space for data apps.

New Member

Re: IP telephony

As I have mentioed can we combine Analog lease line with PSTN line which will give us approximately (9.6 KBPS + 28 KBPS = ) 37 KBPS and trigger IP Telephony call, If yes request you to help us in its configuration.

We have standard cisco hardware at Branches we have deployed 2621 XM router and Cisco 2950 Switch, at district office we have 3745 and at Head office we have Cisco 7507, Call Maanger 3.3 is at Central site

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