Frequently Asked Questions | Hardware Infrastructure

Q: How is a PBX different from a switch in a telephone central office?

A: In many ways, the two switches serve the same basic function, but with differnt target customers. PBX systems are usually smaller-scale systems with more enterprise-specific feature functionality, and tend to interconnect a larger percentage of digital and IP phones than a PSTN switch would.

Q: Do I need an analog PBX to use an analog phone or trunk? Or a digital PBX to use a digital phone or trunk?

A: No, a digital PBX or VoIP gateway can handle analog lines and trunks just fine. These signals are converted to digital signals before bing switched on a digital PBX’s Time Division Multiplexing (TDM) bus or Gateway VoIP media stream. A digital phone does require a digital PBX, but digital trunks can be split out on a channel bank for an analog switch if the signaling also is converted to an analog format.

Q: Where do the names “ring” and “tip” come from? What do they mean?

A: In the old days of telephones, operators connected calls using quarter-inch phone plugs (the same plugs that later were used with stereo headphones before the mini-phone plug became commonplace). The tip of the plug was the positive side of the circuit. The ring (or slip-ring) was a conductive circle around the plug above the tip and was the negative side of the ciruit. Sometimes another conductor was present on the plug after the ring—this was called the sleeve.

Q: What does “codec” mean, and what common codecs should I consider using? Is any kind of codec more secure than another?

A: Codec is short for COder/DECoder (and in more modern usage, COmpressor-DECompressor—though the first PCM codec was not compressed). In audio, a codec like the name implies, compresses audio before transmitting it, and decompresses the received audio. This helps pack more traffic in the same bandwidth. G.711 is standard PCM encoding, G.721 uses Adaptive Differential PCM (ADPCM) to cut the bandwidth required in half, and G.729 can compress a 64 kbps speech channeld down to 8 kbps, but with significant loss of quality (and it won’t work for fax or data connections). In general, your choice of codec will not affect the security of your VoIP system one way or the other.

Q: Why do regular firewalls have so many problems with VoIP traffic?

A: There are several reasons for this. First, VoIP packets have three characteristics that make traversal more difficult: separate signaling streams from media streams, broad ranges of port numbers for media, and embedded IP addresses. Second, VoIP standards are always changing and firewall vendors have a hard time keeping up. Finally, VoIP packets are real-time by nature and firewalls aren’t friendly to real-time packets under load.

Q: What is a WEP initialization vector and how is it used? Why is it not enough to protect me?

A: WEP is a stream cipher, which uses a value known as an initialization vector to ensure every signal is a unique signal, despite being encrypted by the same key.

Q: WEP’s fatal flaw is that its IVs are too short, and duplication occursCan I use WPA2 with any access point?

A: Most access points, but not all, now support WPA2 encryption. To be sure, consult the manual that came with your router (or they can usually be downloaded from the manufacturer’s site) and look up the encryption they support. Some routers can be upgraded by uploading a special firmware to the device. Check the manufacturer’s Web site, just to be sure.

Q: Can I run my own RADIUS server?

A: RADIUS, which stands for Remote Authentication Dial in User Service, has many free implementations for Linux and other operating systems. For a typical list of commercial and open source options, visit the VoIP-Info wiki at www.voip-info.org/wiki-Radius+Servers.

Q: What are some of the security concerns involved with using the popular instant messaging clients?

A: The same vulnerabilities that exist on the desktop are found in IM clients. This includes man-in-the-middle attacks, keylogging, and even audio capture and reconstruction with freely available tools on the Internet. And just as we’ve seen in the operating system world, the more widespread an IM client becomes, the more attractive a target it is to the hacking community.