OC-1

OC-1 is a SONET line with transmission speeds of up to 51.84 Mbit/s

This base rate is multiplied for use by other OC-n standards. For example, an OC-3 connection is 3 times the rate of OC-1.

OC-3 / STM-1x

OC-3 is a network line with transmission speeds of up to 155.52 Mbit/s Depending on the system OC-3 is also known as STS-3 (electrical level) .

When OC-3 is not multiplexed by carrying the data from a single source, the letter c (standing for concatenated) is appended: OC-3c.

OC-3c

OC-3c concatenates three STS-1(OC-1) frames. Concatenated STS(OC) frames carry only one column of path overhead because they cannot be divided into finer granularity signals. Hence, OC-3c can transmit more payload to accommodate a CEPT-4 139.264 Mbit/s signal. The payload rate is 149.76 Mbit/s and overhead is 5.76 Mbit/s.

OC-12 / STM-4x

OC-12 is a network line with transmission speeds of up to 622.08 Mbit/s (payload: 601.344 Mbit/s; overhead: 20.736 Mbit/s).

OC-12 lines are commonly used by ISPs as WAN connections. While a large ISP would not use an OC-12 as a backbone (main link), it would for smaller, regional or local connections. This connection speed is also often used by mid-sized (below Tier 2) internet customers, such as web hosting companies or smaller ISPs buying service from larger ones.

OC-24

OC-24 is a network line with transmission speeds of up to 1244.16 Mbit/s (payload: 1202.208 Mbit/s; overhead: 41.472 Mbit/s). Implementations of OC-24 in commercial deployments are rare.

OC-48 / STM-16x / 2.5G Sonet

OC-48 is a network line with transmission speeds of up to 2488.32 Mbit/s (payload: 2405.376 Mbit/s; overhead: 82.944 Mbit/s).

With usually cheap interface prices and being faster than OC-3, OC-12 connections, and even surpassing gigabit Ethernet, OC-48 connections are used as the backbones of many regional ISPs. Interconnections between large ISPs for purposes of peering or transit are quite common.

TDR stands for Time Domain Reflectometer . In short, it’s a test instrument that works by shooting a pulse down
the cable and then measuring any reflections that return. These reflections are caused by changes in impedance of the cable, which can be caused by water, split pairs, bridge taps, load coils, shorts, and opens. One main benefit of a TDR is its accuracy and ability to pinpoint the exact location of a fault. Whereas a Coil Detection feature can detect if a load coil is present, only a TDR can determine its location.

A TDR is an excellent tool for prequalifying the copper plant for DSL. DSL is affected by the traditional faults found on a cable pair like opens, shorts, and wet cable. However, it is also affected by some common elements of the local loop- namely load coils and bridge taps. Whereas load coils were beneficial for analog voice over long loops, they have the opposite effect on high-frequency digital services like DSL. A single load coil prevents DSL service. Therefore, it is critical to understand how to properly upgrade your copper plant for DSL deployment.

There are two key factors for determining the effect a bridge tap has on DSL performance. First, the length of the lateral: shorter bridge taps are more harmful than longer ones. The reflected signal encounters little attenuation over a short lateral and thus is more powerful. With long laterals, the reflection may be so attenuated that it has little effect on performance. The second factor is the distance of the bridge tap to either modem (xTU-C or xTU-R). Again, attenuation is the key here. When the noise source is closer to the receiver, there is more damage than from a distant source that has been attenuated. Based on field testing, the worst scenarios for bridge taps (for ADSL) seem to be when
the bridge tap is within 1,000 feet from either modem and between 200 to 500 feet long.

Bridge Taps

Bridge taps, unused and unterminated lines to customers, have proven to be one of the most Central Office performance-affecting faults on ADSL circuits. The length of the bridge tap is commonly referred to as a
lateral and is any length of cable that is not in the direct path between the central office and customer.
Bridge taps cause problems with high-frequency digital signals like ISDN and DSL. A lateral creates a second path for the digital signal. The signal travels down the lateral and is reflected by the open at the end. Bridge taps are harmful because the reflected signal that bounces off the end of the bridge tap creates noise back onto the real cable pair.

Load Coils

A single load coil prevents DSL service. Therefore, it is a good idea to first check for load coils on the pair before installation. Or, if you’re trying to turn-up the link and cannot connect with the other end, check to see if there
are any load coils which might be preventing service. Load coils are used to extend a voice signal’s range over
long loops (greater than 18,000 feet). A load coil is an inductor, typically 88 mH (and some places,66mH). It works by boosting the transmit power level for voice frequencies (between 300 Hz and 3 kHz). However, after 3.1 kHz, the power drops below that of unloaded cable. This is ideal for voice transmission, since it is limited to the 300 Hz to 3.1 kHz bandwidth. But, what happens with ADSL or other DSL services that use the higher frequencies? They simply
cannot pass through load coils. Therefore, it is critical to remove all load coils before deploying ADSL or other high frequency signals.

Load coils are placed at regular intervals- a key factor for locating and removing them. The first coil appears 3,000 feet from the central office or exchange and subsequent load coils are placed every 6,000 feet after that.

On a TDR, a load coil appears as a smooth upwards bump. It will look very similar to an open on a TDR. Placing the cursor at the beginning of the upwards slope gives you the distance to the load coil. Knowing the spacing interval is a key to identifying a load coil. For example, if this signature appears approximately 9,000 feet from the central
office, chances are high it is a load coil.

A TDR can detect only the first load coil on the cable. You will need to run the TDR, remove the first load coil, and then run the TDR again to check for other load coils. This process should be repeated until there are no more load coils on the line.

Split Pairs

A split pair means that one wire of a pair is spliced onto a wire of an adjacent pair. It is generally caused by improper splicing or wire labeling. Split pairs lead to crosstalk which can impair DSL performance.

Opens

An open is a break in the cable pair; it does not allow electrical energy to flow through. One major cause of an open is careless, or unauthorized, digging in an area. As expected, an open prevents DSL service. If the two modems at either end cannot communicate at all during installation, there could be an open in the cable between them. On a TDR, an open appears as an upwards bump.

Shorts

A short occurs when the tip and ring wires come in contact. A short prevents the DSL signal from passing through. Shorts can be caused by improper splicing or worn sheaths. On a TDR, a short appears as a downspike.