Voice Over Internet Protocol

by Kong

I was recently hired as a field-network technician at a major cable company.

I don't want to name names, but I will drop a hint and let you know that they own AOL, (((CNN))), and several other big names.

The title of my job really means nothing.  I just go to customers' homes or businesses and set up wireless and wired networks.

Interesting stuff but nothing too interesting.  I did this for a month or so until I was given an opportunity to switch over to the Voice over IP (VoIP) department.

Being an avid phone phreak I decided to take this opportunity.  After an intense training session, I was left with a little more knowledge then I had before and a training manual.  Since selling the manual on eBay seemed out of the question, I decided the best place to share my new information would be in an article.

The first misconception many people have with VoIP is that your phone calls go over the Internet.  While this is true with Vonage and other Internet phone companies, it is far from the truth with the phone system I work on.

The VoIP system consists of the following:

• MTA:  Media Terminal Adapter - cable modem.
• Coaxial Network:  Coaxial cable is television cable, enough said.
• CMTS:  Cable Modem Termination System - more on this later.
• MGC:  Media Gateway Controller - see above notes.
• PSTN:  Public Switched Telephone Network - telco's existing network.

The MTA works on the same basic principals as a standard Data Over Cable Services Interface Specification (DOCSIS) cable modem.

It even uses the same channel in the RF spectrum.  It can even look the same as a standard cable modem except in addition to an RJ45 jack and USB port, it will also have an RJ11 jack for a phone.

This means in almost all cases Internet and phone are run from the same device and the same coaxial cable.  Both functions have their own MAC address and also their own IP address.

Most cable modems have a buffer of 1500 bytes which will last about 10 seconds and will cause some noticeable delays on streaming video or music as packets are loss.  Since delays for voice are unacceptable, the phone part of the modem only has a buffer of 160 bytes or about 20 milliseconds.  This means that if a packet is lost for voice, there is no chance of it being resent.

As mentioned earlier data and voice share the same channels for upstream and downstream.

To cut down on lost voice packets, they are given priority over data packets.  This could cause some performance drops while surfing but they are hardly noticeable.

The RJ11 jack on the MTA acts the same as a jack that is hooked up to telco wiring, meaning it supplies -48 volts DC for on-hook and 90 volts AC for ringing and all that good stuff.  It also supports Dual-Tone Multi-Frequency (DTMF).

The MTA also has the job of changing the analog voice signal into digital packets.  Once the MTA has transferred the packets, it sends them through the coaxial cable in your neighborhood to the CMTS.

The CMTS is also the same as with a standard cable modem.  It is located at a cable company office and terminates the packets from the coaxial cable to either fiber optics or Ethernet.

or Internet, it routes the packets from their office to the Internet.  In the case of phone, it keeps the packets on a managed network controlled by the cable company and used for VoIP only.

Packets are routed to different parts of the network depending on who is calling whom.  Eventually they are dropped off at the MGC.

Once the packets arrive at the MGC they are further analyzed to decide where they are going one last time.  The job of the MGC is to send and receive packets to and from the PSTN.  So basically all the cable company has to do is get the packets from your house to their office and then drop them off at the telco and let them deal with it from there.

This article is a condensed version of a 500 page manual but I have included the most important parts.

There are a few minor details I have left out such as various servers that do nothing more than make sure your phone is on the hook or off the hook, let people know your number is disconnected, etc.

A good section of the training manual also deals with how to hook the MTA up to the customer's exiting phone wiring so they can use a phone in every room instead of just plugging a phone into the MIA.  That section is not that interesting and most people with any phone experience professional or not shouldn't have to worry too hard about that.

The main idea of this article was to outline how and why the system works.  Keep in mind that once the packets leave the MTA they are standard IP data packets and can be sniffed like any other packet regardless of medium (coax, Ethernet or fiber).