The telephone system isn’t what it used to be. Mostly that’s good. Technology has given us wireless mobile smartphones, enterprise VoIP, cloud hosted services and sophisticated call center operations. Many companies, though, can’t afford to shuck their considerable infrastructure investment in order to gain the advantages of the latest advancements. Is there any way to mash up the old and the new and have it work?
Indeed, there is. The magic is in something called a “handoff”. This is the interface between your telecom lines and your equipment. Let’s take a look at what type of handoffs are available to keep your phone system up to date.In the beginning, “Ma Bell” offered one type of handoff. It’s the venerable analog POTS (Plain Old Telephone Service) line that is still popular today. Every phone was an analog phone and every line was an analog line. The handoff was in the form of a single twisted pair of copper wires. All the switching was handled by the public telephone system.
This type of connection still works well for businesses that need only a single telephone line. The phone plugs into a jack on the wall that connects to the telco POTS line. It’s the ultimate in simplicity. This arrangement supports both single wired handsets and the newer cordless phones. You can have multiple cordless handsets connected to a single POTS line.
A slightly more sophisticated business phone system allows multiple desk phones along with multiple outside lines. Each line is still an analog POTS line with its own phone number. It’s called a key telephone system because each phone has a button or “key” for each line. You can see the status of each line, typically 4 or 8, on the light for that line. You select the line you want by pushing the button for that line. You have to provide the switching intelligence for the system by answering the line that is ringing or selecting an unused line for calling out.
You can have as many analog phone lines as your key system will support. Since the lines are independent, there is no economy of scale with this arrangement. Two lines cost twice as much as one line, and so on. To gain a cost advantage, you need to move up to a trunk ing arrangement.
A trunk is a telecom line that supports multiple independent phone conversations. Originally, these were also analog using frequency division multiplexing. In the last 50 years, the old analog trunks have been replaced by digital trunks. T1 phone lines are a popular trunking system. Each T1 line supports up to 24 simultaneous phone calls carried on separate time multiplexed channels. The physical connection is two copper twisted pair. One is for transmit and the other is for receive.
Lets say you have a good size key telephone system with a dozen outside lines. You can provide those lines as individual POTS lines or with a single T1 line that carries all 12 phone lines with room to spare. The T1 line will likely cost less for a dozen lines. If you used all 24 channels, the cost savings would be even more dramatic. The issue you have, though, is how to connect a digital T1 line to your analog telephone system.
The answer is in a piece of interface equipment called a “channel bank.” This device converts between analog and digital signals so that both the phone system and the trunk line have the signal format and electrical characteristics they expect. As far as your phone system is concerned, the connection is exactly the same as if you had separate analog lines all the way to the phone company. This is called an analog handoff.
Another application for analog handoff is older in-house PBX (Private Branch Exchange) phone systems. A PBX is a key system with a brain. The PBX takes care of routing incoming calls and the bank of outside lines. When you dial “9” for an outside line, the PBX gives you the next available one. You don’t know or care which line it has selected. They all work the same.
Many PBX systems will accept one or more T1 lines directly. There is no need to go through the intermediate analog conversion using a channel bank. This is called a digital handoff. It may also be called a T1 CAS (Channel Associated Signaling) digital handoff or an ISDN PRI digital handoff.
ISDN PRI is a special type of T1 line that supports 23 simultaneous calls. What happened to the other channel? It’s used for signaling and switching to make the system faster and capable of slightly higher voice quality. Many PBX systems now have a connector for one or more PRI (Primary Rate Interface) trunks, as this is the most popular service for medium and larger phone systems.
Enterprise VoIP is the current technology being adopted en-masse by medium and larger companies. This replaces the analog or proprietary digital telephone wiring with network connections to the company LAN. The PBX is updated to an IP PBX that supports IP telephones. It also supports a packet switched phone line technology called SIP Trunking.
A SIP Trunk is digital, like T1 or PRI, but not separated into individual channels. Instead, the transmission protocol is Ethernet packets and a VoIP-centric switching format called SIP or Session Initiation Protocol.
An all-VoIP business telephone system would have IP phones and an IP PBX sharing the company LAN. A SIP Trunk connects the PBX to the telephone service provider. This is called a SIP handoff.
Another use of SIP Trunks is for cloud hosted VoIP services that provide the switching as well as connectivity to the public telephone network. With a cloud solution, you no longer have a PBX or IP PBX in-house.
Now, here’s where it gets interesting. You can often select your preferred phone trunk technology and specify the handoff you require. Just like T1 and PRI can be set up to provide analog handoff, SIP trunks can provide SIP, T1 digital, PRI digital, or analog handoffs. This means that your old phone system, with whatever interface it supports, can connect to modern networks and telephone trunking options. It often makes economic sense to continue using your current phone system until you can migrate to the latest technology.
Do you want to connect your current phones or phone system to the telephone service of your choice but aren’t sure how to make that connection? Find out what business telephone trunking options and pricing are available for your business location.
