TaffTel

By Linda Orlando

If you go POP fishing in a scalable upstream LAN and your ADSL payload is getting lots of bits per second, landline packet switching may be possible with a GSM digital microwave! Isn’t that terrific? That’s what my new cable company told me! (Well, they said something like that, anyway…)

The first step in learning how to speak basic telecom as well as the neighborhood geek is to understand the various types of products provided by your local exchange carrier (LEC), the company that owns and operates lines to consumer locations in your area. In the olden days, telephone service was provided through analogue signals (sound waves or frequencies) on landlines (copper or fiber optic wiring). But most phone companies now offer advanced digital and wireless services, so analogue signals and even landlines are quickly becoming telecom dinosaurs.

Digital signals travel longer distances more quickly, with clearer voice quality and less static, and they require less complex equipment. Regardless of the type of communications channel used—copper wire, fiber optic, or wireless—the capacity of the channel is referred to as the bandwidth. Digital signals are transmitted in binary bits, and their bandwidth is measured in bits per second (Bps) or kilobits per second (Kbps). Faster speeds are measured in megabits per second (Mbps—millions of bits), while telecom backbone networks transmit phone calls and data at Gigabits per second (Gbps—billions of bits). Newer technologies are even boosting transmission speeds to terabits per second (Tbps—trillions of bits)! But perhaps this is a bit too much information to bite off this early in the game; let’s take a breather and talk about the basics of mobile phone calls.

When analogue mobile phones started increasing in popularity over twenty years ago, it quickly became clear that most telecom networks would have a hard time handling all those calls. Other issues such as security, mobile phones being illegally “cloned,” and people wanting to use their mobile phones in other countries made it clear that the analogue telephone system couldn’t keep up with evolving mobile phone technology. So in 1982 a committee called the Groupe Spécial Mobile (GSM) was formed to develop a mobile phone standard that would use the radio spectrum efficiently, provide for using a mobile phone while travelling, give satisfactory voice quality, have low equipment costs, be compatible with other systems such as Integrated Services Digital Network(ISDN) and be ready to support new services as they were developed. The committee developed specifications for a system, and they courteously recycled their GSM acronym by naming the standard Global System for Mobiles. The first commercial GSM mobile telephone system was restricted to large cities and airport areas, but as the networks expanded more countries signed on to the system, and GSM is now the dominant mobile phone service.

Here’s how it works. A GSM mobile phone connects you to a telephone network. Your phone number is associated with a subscriber information module (SIM) card, a small “smartcard” that also stores your personal phone book. While your phone is turned on, it keeps tabs on which network base stations it can reach, and the network keeps track of where your phone is. Base stations provide coverage in a series of cells, arranged like patches in a quilt, with each cell being anywhere from 50 meters to many kilometers across. When you make a call, the phone searches for the best base station to log onto, and then the spoken words are converted to a stream of data passed to and from the nearest base, while your phone and the base monitor what’s going on to decide whether there’s a better cell to transfer to. If you’re moving while talking (hopefully not driving and talking!), then your call may be transferred from base to base, with the network switching the call so that the changes are seamless. The network has to keep track of where all its mobiles are and be able to send an incoming call to wherever the mobile is. This is accomplished using a gateway mobile switching center (GMSC), which performs switching functions to route mobile calls to their destinations efficiently.

The most important networking technologies for long-distance and international telecommunications links are fiber optic cable, hybrid fiber-coaxial cable, geostationary satellites (satellites that are permanently located above a particular spot on the Equator), and microwave links. Microwave links provide a relatively quick and cheap method of connecting points, but because they travel in a straight line, they require a clear line of sight between transmitter and receiver. Because of this, microwave frequencies are often used for linking to satellites in orbit.

Although many mobile phone conversations are priceless gems, they aren’t the only important streams of data sent across an LEC’s network. The most vigorous use of telecom networks comes from people and businesses sending and receiving packets of data—logical groupings of information—over the Internet. To keep things amusing for people learning telecom jargon, packets are also often called datagrams, frames, messages, or protocol data units (PDUs)—and these are just a few of their common aliases. The portion of a packet that contains upper layer information (raw data) is called the payload. One popular method of sending data over networks, packet switching, uses network capacity efficiently by breaking up large communications (voice, video, graphic files, e-mail, etc.) into individually addressed data packets, sending them over different routes, and then reassembling them on the other end.

Any digital transmission speed of about 1.5 Mbps (megabits) or higher is called a broadband connection. The most common consumer forms of broadband Internet connections are cable TV lines and digital subscriber line (DSL) technology. DSL enables high-speed voice, video, and data connections over the standard copper phone lines that connect most people to larger telecommunications networks. Most companies use DSL technology to offer bundled high-speed services in order to compete with alternate carriers such as cable and satellite companies.

Asymmetric digital signal line (ADSL) was one of the first DSL technologies made available to consumers; the A in the acronym refers to the fact that upstream data transmission speeds are slower than downstream transmission speeds. Okay, before visions of speckled trout steal your attention, let’s back up and explain the analogy. Picture the communications between you and your LEC’s network as a flowing stream. Data from a user to the network flows upstream, and downstream data flows the opposite way, from the network to the user. Upstream speeds are usually slower than downstream speeds; hence, the two speeds are asymmetric (hence the A in ADSL).

Finally, any discussion of telecom terms would not be complete without mentioning scalable equipment—one of the most venerable telecom terms, meaning equipment that can handle huge increases in usage without quality being affected. But the most fascinating telecom term is probably Points of Presence (POPs), because it has a different meaning depending on what part of the telecom industry you’re discussing. (Who knew telecom companies were so playful?) For telecom carriers, a POP refers to the connection between the facilities of an interexchange carrier and a local phone provider, where the two companies transfer voice and data transmissions to each other’s networks. For Internet providers, a POP is an access point to the Internet, with a distinct Internet Protocol (IP) address. And for wireless carriers, a POP is equivalent to one potential customer their network has the capacity to serve.

Obviously the subject of telecommunications, with its endless litany of acronyms and tongue-tangling terminology, can quickly make you feel as though you’ve been sucked into an alternate universe. Don’t despair—there are plenty of resources available for helping you figure it all out. The Mobile Phone User Guide at http://www.mobileshop.org/ provides lots of easy to understand information about mobile phone systems and telecom networks. For a more exhaustive reference of all things telecom, you can download the Mpirical Companion Telecom Terms Encyclopedia from http://www.mpirical.com/companion.html and keep it handy while talking to your LEC. (And once you’ve gotten fluent in telecom, you can use it for those nights when you just need something mind-boggling to numb your brain to sleep.)