One-way audio is an annoying and common occurence with VoIP. The primary cause for one way audio is the NAT enabled device hiding the topology of the customers network. Many legacy devices do not have a built in ALG (Application Layer Gateway), which changes the headers of the VoIP packets (either SIP or MGCP) to allow the customer to preserve their private network topology and allow them to use VoIP service.
One-way audio is caused when one side of the RTP stream is not setup or terminated correctly. RTP is the UDP media stream that carries the audio of a phone call on VoIP. As discussed earlier, the primary cause for one leg of the RTP stream to terminate incorrectly is the customer premise equipment. Lets use the example below to illustrate the problem. This is a very common small network design.
|
|
--------- ---------- ----------
| Phone | ------ | Switch | ------- | Router | -------- INTERNET
--------- ---------- ----------
|
Private Network | Public Network
(192.168.0.0/24) (ISP Provided Public IP's)
In this illustration the router does not have an ALG and is performing NAT. When the phone places an outbound call the phone sends out its private IP to the VoIP carrier in the header. The call sets up, but when the carrier tries to send the RTP media back to the private IP the RTP fails because the private IP sent by the phone is not routable on the public internet. See the below example for an RTP setup with this scenario.
09:04:01.245261 IP 4.58.212.235.50015 > 4.79.212.217.2727: UDP, length: 195
E`..uk..6....,.+.O..._
.....200 11509 OK
I: 0
v=0
o=- 7960 7960 IN IP4 192.168.0.3 <= Private IP
s=MGCP Call
c=IN IP4 192.168.0.3 <= Private IP
t=0 0
m=audio 17698 RTP/AVP 0 101
a=rtpmap:0 PCMU/8000
a=rtpmap:101 telephone-event/8000
a
=fmtp:101 0-11
In this example the phone sent the IP 192.168.0.3 to the media gateway, the media gateway attempts to set up the RTP stream with this IP, which fails, resulting in one way audio to the calling party. An ALG solves this problem by rewriting the header with it's own public IP allowing the carrier media gateway to terminate the RTP media to the ALG. Since the ALG sits in both the public and private networks, it then relays that media to the corresponding endpoint in the private network.
There are two deployment scenario's for an ALG, one is on the customer premise, the other is in the carrier cloud. There are advantages and disadvantages to both. Here are some of the pros and cons.
Customer Premise Deployed ALG
Pros:
- Media takes the shortest path to the terminating media gateway, reducing latency.
- Customer is able to failover to pots lines in the event of an outage.
Cons:
- Cost, ALG devices range from $100 - $800 depending on scalability and feature set.
Carrier Deployed ALG (Session Border Contoller)
Pros:
- No Cost to the customer
- No hardware to maintain on site
Cons:
- Media must "hair pin" through the carrier ALG, this introduces latency into the RTP stream which can affect call quality.
- Introduces artificial bottleneck by routing through carrier's network.
