Carrier System : Digital Signal Level (DSx)

Digital signal level (DSx) transmission is a hierarchy of digital communication channels and lines that range from 64 kbps to 565 Mbps. Lower level DS structures are combined to produce higher-speed communication lines. There are different structures of DS levels used throughout the world with significant variations between North American and European systems. DSx has been used to represent the digital transmission standards where the “x” denotes which service is under discussion.

Trunk carrier (T Carrier) is often used to describe the DSx level. Trunk carrier uses (Tx) to represent the digital transmission standards where the “x” denotes the multiplexed level of trunk service. Tx is the actual transmission structure where DSx is the digital signal levels. Outside the United States, E Carrier (Ex) is used to represent the transmission carrier.

The following represent the North American DS structure:

Digital Signal 0 (DS0) - is the smallest digital channel operating at 64 kbps. It represents a single digitized analog voice channel;

Digital Signal 1 (DS1) – normally referred as a T-1, it is composed of twenty four voice channels packed into a 193 bit frame and transmitted at 1.544 Mbps. The unframed version, or payload, is 192 bits at a rate of 1.536 Mbps;

Digital Signal 2 (DS2) – normally referred to as a T-2, it is composed of four T-1 frames packed into a higher level frame transmitted at 6.312 Mbps;

Digital Signal 3 (DS3) – normally referred to as a T-3, it is composed of twenty-eight T-1 frames packed into a level frame transmitted at 44.736 Mbps.

Fractional T1 (FracT) - A data transmission rate that is a portion of the total capacity of a T-1 communications line (1.544 Mbps) but greater than a DS0 (64Kbps).

European standards are different from the North American noted above. The European hierarchy is as follows:

Digital Signal 0 (DS0) - is the smallest digital channel operating at 64 kbps. It represents a single digitized analog voice channel;

Digital Signal 1 (DS1) – normally referred as an E-1, it is composed of thirty voice channels and two controls channels. It uses a 256-bit frame and operates at 2.048 Mbps.

Digital Signal 2 (DS2) – normally referred to as a E-2, it is composed of four E-1 frames packed into a higher level frame transmitted at 8.448 Mbps;

Digital Signal 3 (DS3) – normally referred to as an E-3, it is composed of sixteen E-1 frames packed into a level frame transmitted at 34.368 Mbps.

Digital Signal 4 (DS4) – normally referred to as a E-4, it is composed of sixty-four E-1 frames packed into a higher level frame transmitted at 139.268 Mbps;

Digital Signal 5 (DS5) – normally referred to as an E-5, it is composed of two hundred fifty-six E-1 frames packed into a level frame transmitted at 565.148 Mbps.

Digital Signal Level (DSx)

Carrier Systems : Plain Old Telephone Service (POTS)

Carrier systems are a combination of a transmission medium, specification of signal types and levels along with specific protocol controls (communication rules). The types of carrier systems range from simple audio telephone POTS carriers to high-speed optical carrier (OC-x) transmission systems.

Plain Old Telephone Service (POTS)
Plain old telephone service (POTS) is a transmission system that is used to provide basic telephone service. It is the common term used for residential telephone service.

Between the late 1800’s through the 1990’s, telephone transmission had remained basically the same. Acoustic energy from the customer was converted to electrical signal by a microphone in a handset. This electrical energy was applied through a hybrid electrical device to the telephone line through the speaker in the handset. A telephone hybrid device (often called a “magic” device by telephone personnel) transferred energy from the microphone to into the telephone 2-wire line while extracting most the remote microphone energy and applying it to the speaker. At the same time at the other end of the connection, the same process was occurring.

Figure 1 shows a hybrid telephone transmission system. This diagram shows that microphone energy from user #1 is added to the 2-wire transmission circuit via the hybrid assembly and the microphone energy from user #2 is subtracted from the circuit by the speaker #1. At the same time, the microphone energy from user #2 is added to the 2-wire transmission circuit and speaker #2 subtracts the microphone energy from user #1. This hybrid process allows 2 wires to contain a composite of both microphone signals.


Figure 1: Hybrid Telephone Transmission

Local Exchange Carriers (LEC)

Local exchange carriers (LECs) or post and telegraph and telecommunications (PTT) companies provide telephone services directly to residential and business customers located within a localized geographic area. Typically, these telephone companies provide services via copper lines that extend from a local carrier’s switching facilities to the end customer’s premises equipment (CPE). This is referred to local loop.

Until the early 1990’s, most countries had a single company that provided local telephone services. This company was either owned or highly regulated by the government. To increase competition and reduce telephone service prices to consumers, some governments have begun to allow other companies to provide basic (local) telephone service. These competitive local exchange company (CLEC) or competitive access providers (CAPs) provide alternative connections to the public switched telephone networks (PSTN). The established telephone companies are now called the incumbent local exchange carriers (ILECs),

Traditional incumbent local exchange company (ILEC) may be connected with one or more competitive local exchange companies (CLECs) who provide local telephone service in a defined geographic area. End customers (the houses) in the geographic area are connected to the End Office (EO) switching center by copper wire (local loop). The local loop length is approximately 1k to 10k feet from the EO. Rarely will the distance of the local loop exceed 20k feet.

The end office interconnects calls between local customers. Each end office switch can usually supply service up to 10,000 customers. In larger areas (such as a city), established LECs may have several EO switches. The EO switches are interconnected using a higher level tandem switch. If is a significant amount of calls regularly processed between end offices, they may be directly connected via high-speed communication lines (trunks).


Figure 1.7 shows that CLECs must be connected to the ILEC to allow calls to be routed between the different networks. There may be several CLECs in a specific local telephone service area.

Local Telephone Service Networks

Local telephone service areas are connected to each other by inter-exchange switches. These higher level (meaning above tandem) are often operated by long-distance service providers called inter-exchange carriers (IXCs). The IXCs interconnect their long distance switches to the local network through a point of presence (POP). The POP is the toll center that allows separation of billing for local and long distance service.

There are several other types of competitive access companies that are starting to provide local telephone service. These include cable television companies, Internet telephony service providers (ITSPs) and wireless local loop (WLL) providers.