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The basic entry level in bandwidth or speed for leased lines that provide synchronous connections between sites is known as the T1. It serves up 24 Digital Signal 0 (DS0) 64-Kbps channels in the United States, Japan, and South Korea. There is a slightly bigger/faster version with 32 DS0 channels that’s available in Europe and called the E1 or E carrier line.
T-series connections are digital connections that you can lease from the telephone company. They can use copper pairs like regular phone lines, or they can be brought in as part of a backbone, which is also called a trunk line. T-series connections use time-division multiplexing (TDM) to divide the bandwidth into channels of equal bit rate.
EXAMPLE
The table below lists some of the T-series connections and their maximum data rates. The most commonly used T-series lines are T1 and T3.| Connection | Maximum Speed |
|---|---|
| T1 | 1.544 Mbps |
| T1C | 3.152 Mbps |
| T2 | 6.312 Mbps |
| T3 | 44.736 Gbps |
| T4 | 274.176 Gbps |
A T1 is a 1.544 Mbps digital connection that is typically carried over two pairs of copper wires. This 1.544 Mbps connection uses DS1 and aggregates 24 discrete 64-Kbps channels that use DS0, which refers to the time slots within a channel. Each channel can carry either voice or data. In the POTS world, T1 lines are used to convert and bundle analog phone conversations over great distances because of the better quality of digital signals and the use of a great deal less wiring than would be needed if each pair carried only one call. This splitting into independent channels also allows a company to combine voice and data over one T1 connection or to use the T1 as if it were an unchannelized 1.544 Mbps pipe.
You can also order a fractional T1 (FT1) circuit that’s delivered on a T1 but doesn’t allow the use of all 24 channels. While this may seem slow given today’s standards, the T1 line is still relevant for many applications.
A T3 works similarly to a T1 connection but carries 44.736 Mbps. This is equivalent to 28 T1 circuits or 672 DS0 channels. A T3 connection uses a signal known as Digital Signal 3 (DS3) that is definitely not the same as DS1, which is generally delivered over fiber-optic cables. Many local ISPs have T3 connections to their next-tier ISPs. Other entities that often opt for T3 are large multinational companies because they need the capacity to send voice and data between their major regional offices.
Similar to the T1, the T3’s European counterpart is the E3, which operates at 34.368 Mbps. The Japanese Digital Hierarchy specifies the J3 circuit, which operates at 32.064 Mbps.
The transmission media is another aspect of WAN technologies that can have a big effect on the speed, bandwidth, and volume of data that can be transferred. Wireless transmissions use air as transmission media. This not only creates a challenge concerning security but also creates signal degradation. That’s because the further the signal has to travel from the original source, the weaker it gets. This is called attenuation and refers to the loss of signal strength in networking cables or connections. A solution to this issue is microwave repeaters, which retransmit signals through the air and bolster them. This approach is a lot more efficient because it can preserve signal strength over much greater distances.
Wired connections use either copper wire or glass fiber to carry bits as voltages or light pulses, respectively. That attenuation issue, where the signal gradually weakens over distances, also relates to copper wire transmissions and limits the length of wire you can use.
The international equivalent of SONET is Synchronous Digital Hierarchy (SDH). SONET defines a base data rate, or throughput, of 51.84 Mbps, and multiples of this rate are known as optical carrier (OC) levels, such as OC-3 and OC-12. Table 16.2 depicts common OC levels and their associated data rates.
| Level | Data Rate |
|---|---|
| OC-1 | 51.84 Mbps |
| OC-3 | 155.52 Mbps |
| OC-12 | 622.08 Mbps |
| OC-48 | 2.488 Gbps |
| OC-192 | 9.953 Gbps |
| OC-768 | 39.813 Gbps |
Regardless of the media used to carry WAN traffic, the growing volume of voice and video traversing data networks has led to new traffic problems. Regular data traffic can arrive out of order and be reassembled back into its original order on the receiving end, but voice and video data require real-time delivery to be intelligible. Clearly, this makes regular data a lot less vulnerable to congestion or busy traffic conditions that can delay voice and video communications and totally mess them up.
As one option to address this demand, wavelength-division multiplexing (WDM) is a technology that multiplexes several optical carriers on a single optical fiber by using different wavelengths.
Another option is found in dense wavelength-division multiplexing (DWDM), which multiplexes within a specific band (1550 nm), allowing for the use of erbium-doped fiber amplifiers (EDFAs) that boost the signal. This allows for upgrading the bit rate of a single-strand line by simply replacing the equipment at either end of the line.
Yet another option is found in coarse wavelength-division multiplexing (CWDM), which uses larger chunks of the light spectrum and is defined by wavelengths, whereas DWDM is defined by frequencies and fits 40+ channels into the same frequency range used by just 2 CWDM channels. CWDM allows ISPs to help customers in a metropolitan area network (MAN) physical location where fiber is still too pricey to implement.
Finally, the passive optical network (PON), also called fiber to the premises, is an option for connecting homes and businesses to the internet. It is point-to-multipoint technology with a single fiber strand used for multiple premises (typically, 16–28). Unpowered optical splitters are used in the process and are the reason for the use of the term “passive.”
The system consists of an optical line termination (OLT) at the telecommunicator’s office and a number of optical network units near end users. These systems typically have downlink speeds of 155–655 Mbps and uplink bursts of 155Mbps.
As has been discussed previously, different technologies offer different distance ranges and optical fiber carries signals much farther than copper cabling can. We also discussed that fiber comes with a much higher price tag to match its higher capacity and that it’s much harder to install.
As you may be aware, Bluetooth is a type of wireless technology that’s only used for short-distance wireless transmissions. Bluetooth is actually a wireless protocol that creates personal area networks (PANs). It utilizes short-range communications technology, enabling data transmission between fixed and/or mobile devices.
Microwave radio relay is a technology for transmitting digital, and sometimes even analog, signals between two locations on a line-of-sight radio path through the atmosphere. During microwave radio relay, radio waves are transmitted between the two locations with directional antennas that form a fixed radio connection between them. A really long connected series of links can form line-of-sight transcontinental communication systems.
A communications satellite (comsat) is an artificial satellite stationed in space for telecommunications purposes. Modern communications satellites use a variety of orbits.
It’s important to understand point-to-multipoint services and how communications satellites provide microwave radio relay technology, as shown in the diagram below.
This technology is also used for mobile applications like GPS communications to ships, vehicles, planes, and handheld terminals as well as for watching satellite HDTV or listening to satellite radio broadcasting. These all require capabilities that are impractical or impossible to use with other technologies like cable.
Source: This content and supplemental material has been adapted from CompTIA Network+ Study Guide: Exam N10-007, 4th Edition. Source Lammle: CompTIA Network+ Study Guide: Exam N10-007, 4th Edition - Instructor Companion Site (wiley.com)
