If you are a telecom person, some of the causes of voice quality issues may be new to you. Most of those working with VoIP know that there are common impairment problems in the network, packet loss, latency, and jitter. There are other impairments as well which will be covered in this blog.
Expand Your Knowledge of VoIP and Its Operation
The Broadband Internet Advisory Group has issued a Technical Working Group report, "VoIP Impairment, Failure, and Restrictions" that can be a useful reference for those who need to expand their knowledge of VoIP. The paper begins with a primer on section 2, "Understanding VoIP and Real-Time Applications." It covers VoIP ports, Network Address translation (NAT) and dealing with IPv6. If you feel that that you have VoIP experience and knowledge, then skip this section.
The Broadband Internet Advisory Group (BITAG) is a non-profit organization focused on bringing engineers and technologists into a technical working group. The goal is to develop a consensus on broadband network management practices that can affect a user's Internet experience.
Some Causes of Technical Problems
Section 3 of the report provides extensive coverage of many impairments, failures, and restrictions that can influence the success of a VoIP call. Impairments reduce call quality. A failure means no calls can be established or the call fails during its operation. Restrictions are those conditions, not a failure, that prevent the use of VoIP. The problems outlined below are issues that providers have to contend with when they support VoIP, for example, SIP trunk providers. Many of these issues can also surface when the enterprise security staff enters the VoIP arena.
Port Blocking – This is the decision by enterprise security and ISPs to deny forwarding traffic, or delivery of traffic, to an intended recipient. This is a legitimate position for the enterprise but not the ISP to take. Sometimes ports can be indirectly blocked. This may result in one way audio, working in one but not both directions.
Application Level Gateways (ALG) – ALGs can automatically detect VoIP traffic and help traffic pass through NAT devices. One detection method is to observe that a consistent small packet rate, 50 packets per second, most likely means that the traffic is an audio transmission. ALGs can affect VoIP traffic operations, thereby impairing VoIP calls.
DNS Records – There can be filtering of relevant Domain Name Service (DNS) resource records that can prevent connectivity to VoIP services.
Network Flow Policing and Filtering - Network flow filtering and policing is designed to identify VoIP traffic patterns and apply specific treatment to VoIP traffic. This can be improperly configured.
Deep Packet Inspection – International VoIP communications may encounter deep packet inspection. In some countries, VoIP traffic is blocked. Deep packet inspection also adds latency to the call which can be excessive, beyond the 150 ms standard latency recommended.
Recommendations
The following recommendations in the report are directed to the ISPs and equipment vendors. Use this list of recommendations to ensure that your ISP and the equipment used is not causing VoIP problems. Have them validate that these are not part of VoIP problems:
• Network operators should avoid impairing or restricting VoIP applications unless no reasonable alternatives are available to resolve technical issues. Unfortunately, this means that if the ISP cannot resolve the issues, VoIP will not work.
• VoIP related ALGs in operator supplied home routers should minimize their impact on traffic other than the operator's VoIP service where possible. This is important for small offices and teleworkers.
• Check that the home router manufacturers disable VoIP-related ALGs by default.
• Port blocking rules in consumer equipment should be user configurable.
• If network operators intentionally use network policies or practices that impair or restrict VoIP, they should provide disclosures about those policies and practices and provide communications channels for feedback. Check your ISP agreements to see if you have agreed to these restrictions without knowing it.
Another recommendation directed at application developers, whether for products or internal development, is that developers should design VoIP applications to be port agile where possible.
Problems with SIP Trunks
Many enterprises have internal VoIP riding over their data networks. The problems that befall users are the responsibility of the IT staff. Enterprises are also pursuing the use of SIP trunks. The SIP trunk can have problems with the PBX, Session Border Controller, Network Address Translator and the SIP trunk provider.
The problems mentioned earlier in this blog will also surface when connecting to SIP trunks. Most of the SIP trunk problems are not with the technologies, but are caused by improper configuration and implementation. These problems are presented in the blogs, Provider Issues Still Dog SIP Trunks and SIP Trunk Equipment Problems Remain.
Useful Resources
Broadband Internet Technical Advisory Group,
"Port Blocking," August 2013
Broadband Internet Advisory Group, "Large--‐Scale Network Address Translation (NAT),," March 2012
Broadband Internet Technical Advisory Group, "Real--‐time Network Management of Internet Congestion," Oct. 2013
