Understanding CODECS ??

[Phone_User]

Can some one shed some light on codecs.

From my readings alaw and ulaw (g711) are pretty much the standards. If you have lots of bandwidth you should use them. If you have lower bandwidth you should try to use g729. However the g729 uses more processing power so you should have a decent server.

My questions are, does g729 offer comparable quality or because of the compression will it be of lesser quaility than standard phone lines?

Are all codecs available over all channels (ie. SIP, IAX).

Now I know that VoIP is not recomended over a wireless connection, however if the only type of connection I can get to a location is wireless I would think the g729 would give the best connection with the least amount of jitter or breakup of the sound, right? Or are there other options to obtain a fair quality connection over wireless.

Thanks

Rob

 

[wardmundy]

G.729 may be the worst choice over wireless. Remember that G.729 involves compressed data so, when you lose a packet with compression, you lose a lot more actual bits of conversation than you would with uncompressed data. If we're talking about wireless as in cellphone, keep in mind that they also compress data so you'd really be compounding your problem with anything other than a perfect wireless signal.

 

[Phone_User]

No, it's not a cell phone. It is a remote office that has internet over a wireless connection.

Rob

 

[GregKeys]

VOIP over wireless is not bad by default. A GOOD wireless connection, with minimal traffic, can support VOIP without ant issues. The questions are;
What kind of wireless connection is it?
What kind of latency is there?
What kind of customer load is there (other clients sharing the node).
The real answer will come from testing, and the results may not be the same each time as conditions fluctuate.
I run a wireless bridge in my house, upstairs to downstairs, 802.11g, and use uLaw with no problems. If one of the kids upstairs starts really pounding the bandwidth on the bridge, it does affect voice quality a bit. 85-90% of the time, it is just fine.
The biggest issue with wireless is that you are dealing with the equivalent of a hub environment rather than a switched one. Because each wireless client sees all the data on the wireless network, you do not get the point to point isolation that a switched environment gives you and collisions become rampant with moderate usage. VOIP suffers under such conditions.
In the case of a wireless provider, it gets worse because it is not just your users utilizing the freq, it is ALL the clients in the area. If the provider uses multiple frequencies to split up the customer load, it may work just fine. If they run just one antenna and overload on the customer side you could get lots of loss. Due to equipment cost, the second layout is far more common.

Anyway, the answer to all of this is to test, test, test, and test some more...... different times, with and without other traffic from PC's at the location, etc. Then, after you determine that you have a connection that this will work on, tell the end users that sometimes it will sound like crap. Maybe they will never see it or notice, but cover your tail because sooner or later somebody will be uploading their entire photo collection or streaming porn, or discover youtube, or, or, or.... and voice quality will go to the bad place in a heartbeat. And on a wireless system, it might not even be someone at your location, but rather the kid who lives half a mile away using the same node.

Greg Keys

 

[krzykat]

I've seen where bandwidth on wireless becomes an issue and the 729 performs better than the 711

 

[jroper]

Hi

Some Codecs can reconstruct dropped packets from the packets around them, so G729 could be a choice, but in general, the uncompressed codecs give more consistant results.

VoIP on Wireless can work absolutely fine, but depends on a lot of other factors, including the processing power of the wireless access point. a little soho router may not do the job when the traffic increases, or the number of customers increases.

The key is zero packet loss and plenty of bandwidth, and a well constructed wifi network, and don't even ask about wifi phones, most are truly awful.

Joe

 

[someguysname]

The biggest issue with wireless is that you are dealing with the equivalent of a hub environment rather than a switched one. Because each wireless client sees all the data on the wireless network, you do not get the point to point isolation that a switched environment gives you and collisions become rampant with moderate usage. VOIP suffers under such conditions.

This isnt the way wireless works. Wireless uses csma/ca "carrier sense multiple access with collision avoidance". In other words, there are no collisions because only one device is sending on the channel at any given time. Wireless can not use carrier sense multiple access with collision detection because it cant send and listen at the same time. It's more like token-ring than a hub...

Our IT dept is currently running on a point to point wireless connection using 2 off-the-shelf linksys access points while we wait on our fiber connection to get dropped through the roof. We are shooting about 1/4 mile to another building. All 14 of us can be on the phone (cisco 7940's) while using our PC's and quality is never an issue because of the way our network is setup. No high compression codecs required...

Maybe we just arent heavy users but it can be done without much issue when done right.

 

[toddhutch]

I think this might seem like a dumb question. How do you change the codecs?

Is there someplace in pbx in a flash to do this? We have lots of bandwidth, 10/MB by 10/MB and would rather have quality than anything else.

Our voice is via PRI in and out to the PBX. So we'd like the best quality we can have in the office, and from home everyone is either on cable, or fios.

Suggestions on the right codec, and how to change it to be the default in the PBX?

Thank you

Todd

 

[jroper]

By default the codecs are set to Alaw or Ulaw in Piaf, so you should not have to do anything.

For reference, in extensions and trunks, you would set disallow=all and allow=ulaw (or alaw), making sure that the disallow comes before the allow.

Also in the phone, you need to make sure that ulaw/alaw is set as the first choice of codec, and it is available to the phone.

Joe

 

[Phone_User]

someguysname, Your point to point has no other traffic on it than yours correct?

My wireless connection is a shared wireless, we get it from a provider. When doing speed test it shows a 2.5 up and 1.5 down. However when making calls they are always broken and spotty.

At times the dundi lookups won't even find the number you are calling. Yes at times the calls are crystal clear. Most of the time they are just plain bad.

I'm curious to know what codecs you are using on phones, trunks, etc.

Rob

 

[wardmundy]

I'd suspect a crappy firewall/wireless router. In the bargain basement department, many of the newer dLinks work well as do the Tomato-based Linksys routers (from other reports).

 

[someguysname]

someguysname, Your point to point has no other traffic on it than yours correct?

My wireless connection is a shared wireless, we get it from a provider. When doing speed test it shows a 2.5 up and 1.5 down. However when making calls they are always broken and spotty.

At times the dundi lookups won't even find the number you are calling. Yes at times the calls are crystal clear. Most of the time they are just plain bad.

I'm curious to know what codecs you are using on phones, trunks, etc.

Rob

...mine and 13 other people's pc's, their phones, various servers, shared printers on other servers across the link, 3 other people in 2 other buildings (the buildings have fiber between them back to the wireless link) with pc's, phones, etc and 2 of those have home folders on a server across the link along with the 14 of us so all of our file access goes across it as well. Our programmer keeps an internet stream up all day as well

Our traffic does get slow, way slower than your tests even, but we dont have any packet loss and voice gets priority over data both ways(you probably wont have control of that though, so maybe it's a moot point all together).

We use cisco call manager so I am using the default CCM codec which I believe is G711 (The phones are registered to the CCM across the wireless). I do use other codecs outside of that to trunk our asterisk stuff to CCM and to run over 64k ISDN , but it doesnt play into the wireless scenario...

I wasnt trying to say you were doing something wrong, if I somehow implied it I'm a network guy, so I am sometimes dry in my delivery of internet conversation