PBX Lines | Traditional PBX Systems

In telephony, a line (or station line) connects endpoint equipment (digital terminals, analog phones, fax machines, modems, or even an IP phone through an IP network) to the PBX (or central office) for switching. An analog line is the private equivalent of a local loop or loop transmission facility.

Note

A PBX is more likely than your phone company to support ground start phones and trunks on analog interfaces. Your phone at home seizes control of the line by using loop start, which involves shorting the two ends of the line together to activate the circuit. Ground start sends one of the leads to ground (typically ring) to seize the line, which is much less likely to cause glare (a condition that arises when both sides on a line or trunk simultaneously seize control of the line).

Typically, a PBX supports analog lines (and trunks) through a line card with 8, 12, 16, 24, or more lines per card, which are then wired to a patch panel for interconnection through a structured cabling system to the analog phone or device. Most of the security concerns around analog lines center on how well protected the equipment and cabling systems are from eavesdropping and tampering. Ground start loops will make theft of service less likely because a special phone is required, but otherwise the same basic rules for protecting a PSTN line from tampering apply.

Of course, line is also a generic term that may apply to power lines providing electricity to homes and businesses. But when we talk about an analog telephone line, we are talking specifically about the two wires involved: the tip (the first wire in a pair of phone wires, connected to the + side of the battery at the central office or PBX; it is named tip because it was the at the tip of an operator’s plug) and the ring (connected to the − side of the switch battery and named because it was connected to the slip ring around the jack). Any equipment that works with Plain Old Telephone Service (POTS) lines will work with a PBX analog line configured for loop start. From a PBX, an analog line will nearly always be 2-wire although 4-wire lines with Earth & Magnet (E&M, sometimes also called Ear and Mouth) interfaces are supported from the same card for analog trunks.

Tip

If you’ve ever taken a peek behind the phone jacks that litter the walls of your home, you are likely to see two (or three) pairs of wires, one Green/Red, the next Yellow /Black, then White/Blue, but for our purposes only the first pair is important. The Green wire, referred to as the Tip, is the positively charged terminal. The Red terminal, the Ring, is the neutral, which completes the circuit, enabling electrical signals to flow freely. Note that newer homes may use a more recent color scheme that is also used for Ethernet cabling. The first pair is White/Blue, then White/Orange, then White/Green and finally White/Brown. This scheme is what you’re most likely to see in structured cabling systems within buildings.

Analog PBX systems supported only analog lines, but with the introduction of digital switching, a new class of line was developed: the digital line. In most PBX systems, a proprietary format for digital line signaling (and media) was created that requires the use of digital phones manufactured by that vendor. Some vendors, however, also support Integrated Services Digital Network (ISDN) standard phones directly (or through the PSTN) via the ITU-standardized ISDN BRI. Most proprietary digital formats use a 2-wire system with 8wire plugs and jacks, although some are 4-wire systems. ISDN uses a 2-wire system from the CO switch, but is 8-wire to the interface used by a phone terminal, so the actual number of wires used will depend on several factors (such as whether the phone has a built-in NT-1 interface). Also, many proprietary switch features will not be supported on ISDN phones, particularly when the phone is manufactured by a different vendor. And even within a vendor product line, you may discover that newer features are supported only on newer phones or phone firmware. In any case, digital lines for proprietary digital terminals typically are supported by digital line cards with 8, 12, 16, 24, or more lines per card, and ISDN lines for ISDN phones are supported by either ISDN trunk cards or special ISDN BRI line cards, which may come in several flavors depending on the ISDN BRI type.

In the case of the modern hybrid PBX or IP-PBX, there is an equivalent concept for IP lines to IP phones, but unlike analog or digital lines the IP line isn’t necessarily tied down to a single electrical interface on the PBX. In fact, the PBX can use multiple Ethernet ports to support an IP line, and IP phones can fail over to multiple IP-enabled PBX systems. The first IP line support built into most PBX systems leveraged the H.323 suite of protocols or proprietary protocols like Cisco “skinny,” but almost all new development on PBX systems today uses Session Initiation Protocol (SIP). The bottom line is that the concept of an IP line exists in virtually every VoIP system out there, and understanding how the line concept is expressed in a specific VoIP system will give you an important handle with which to analyze its architecture and security.

This flexibility and versatility are a huge advantage to VoIP, but it does come at a price. Because the phones are now sharing infrastructure and bandwidth with other devices (and perhaps the entire data network), quality-of-service (QoS) guarantees for packet loss, latency (how long each packet takes to arrive from the phone to the PBX), and jitter (variability of latency across packets in a stream) now become the responsibility of the party providing the network infrastructure. Additional vectors for Denial-of-Service attacks on IP lines (either to the phone or the PBX) and Man-In-The-Middle (MITM) attacks must be considered. In my experience, the resulting loss of accountability from a single organization or vendor to multiple entities rarely is included in planning (or ROI calculations) for VoIP deployments.