T1 Tutorial

(Information compiled from internal and external sources. Wikipedia.org, etc.)

What is T1?

Digital signal 1 (DS1, also known as T1, sometimes "DS-1") is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside of North America and Japan. Technically, DS1 is the transmission protocol used over a physical T1 line; however, the terms "DS1" and "T1" are often used interchangeably. T1 meant "Transmission - Level 1", and had to do with the media that the signal was passed over. DS-1 meant "Digital Service - Level 1", and had to do with the service to be sent (originally 24 digitized voice channels over the T1). The terms T1 and DS1 have become synonymous and include a plethora of different services from voice to data to clear-channel pipes. The line speed is always consistent at 1.544 Mbit/s, but the payload can vary greatly.

A DS1 circuit is made up of twenty-four 8-bit channels (also known as timeslots and DS0's), each channel being a 64 kbit/s DS0 multiplexed pseudo-circuit. A DS1 is also a full-duplex circuit, meaning, in theory, the circuit can send 1.544 Mbit/s and receive 1.544 Mbit/s concurrently. A total of 1.536 Mbit/s of bandwidth is achieved by sampling each of the twenty-four 8-bit DS0's 8000 times per second. This sampling is referred to as 8-kHz sampling.

T1 Uses

Before the jump in Internet traffic in the mid 1990s, DS1s were found almost exclusively in telephone company central offices as a means to transport voice traffic between locations. DS1s have been and still are the primary way cellular phone carriers connect their central office switches (MSCs) to the cell sites deployed throughout a city.

Today, companies often use an entire DS1 for Internet traffic, providing 1.544 Mbps of connectivity (allowing for 1.536 Mbit/s of usable traffic, and 8 Kbit/s of framing overhead). However, DS1 can be ordered as a channeled circuit, and any number of channels can be reserved for non-data (for example, voice) traffic.

Additionally, for voice T1s there are two types: so-called "plain" or Inband T1s and PRI (Primary Rate Interface). While both carry voice telephone calls in similar fashion, PRIs are commonly used in call centers and provide not only the 23 actual usable telephone lines (the 24th line carries signaling information) but also Caller ID (CID) and Automatic Number Identification (ANI) data, commonly referred to in industry parlance as 'signalling data'.

Inband T1s are also capable of carrying CID and ANI information if they are configured by the carrier to do so but PRI's handle this as a standard and thus the PRI's CID and ANI information has a much better chance of getting through to the destination. While an Inband T1 seemingly has a slight advantage due to 24 lines being available to make calls (as opposed to a PRI that has 23), each channel in an Inband T1 must perform its own set up and teardown of each call. A PRI uses the 24th channel as a data channel to perform all the overhead operations of the other 23 channels (including CID and ANI). So even though an Inband T1 has 24 channels, the PRI can actually dial more calls faster because of the dedicated data (also called "D" channel).

T1 Cables

It is very important T1 cables use T1 Shielded Wire and not Category 5 Wire. In addition, the T1 Cables should also use Shielded RJ45 connectors. These quality cables can be purchased from PacificCable.com using the following order buttons.
T1 Cables
Part Number Description Click Picture for Specs Price / Qty
Custom Length T1 Cables
T110C SHIELDED R45 - R45 T1 CABLE (Custom) T110C Shielded R45 - R45 T1 Cable

Custom Length T1 Crossover Cables
T110XC RJ45 - RJ45 T1/DS1 Crossover Individual Shielded Pairs (Custom) T110XC RJ45 - RJ45 T1/DS1 Crossover Individual Shielded Pairs


DS1 frame synchronization

Frame synchronization is necessary to identify the timeslots within each 24-channel frame. Synchronization takes place by allocating a framing, or 193rd, bit. This results in 8 kbit/s of framing data, for each DS1. Because this 8-kbit/s channel is used by the transmitting equipment as overhead, only 1.536 Mbit/s is actually passed on to the user. Two types of framing schemes are Super Frame (SF) and Extended Super Frame (ESF). A Super Frame consists of twelve consecutive 193-bit frames, whereas an Extended Super Frame consists of twenty-four consecutive 193-bit frames of data. Due to the unique bit sequences exchanged, the framing schemes are not compatible with each other. These two types of framing (SF and ESF) use their 8 kbit/s framing channel in different ways.

Connectivity and Alarms

Connectivity refers to the ability of the digital carrier to carry customer data from either end to the other. In some cases, the connectivity may be lost in one direction and maintained in the other. In all cases, the terminal equipment, i.e., the equipment that marks the endpoints of the DS1, defines the connection by the quality of the received framing pattern.

Alarms are normally produced by the receiving terminal equipment when the framing is compromised. There are three defined alarm states, identified by a legacy color scheme: red, yellow and blue.

Red alarm indicates the alarming equipment is unable to recover the framing reliably. Corruption or loss of the signal will produce "red alarm". Connectivity has been lost toward the alarming equipment. There is no knowledge of connectivity toward the far end.

Yellow alarm indicates reception from the far end of a data or framing pattern that reports the far end is in "red alarm." Red alarm and yellow alarm states cannot exist simultaneously on a single piece of equipment because the "yellow alarm" pattern must be received within a framed signal. For ESF framed signals, all bits of the Data Link channel within the framing are set to data "0"; the customer data is undisturbed. For D4 framed signals, the pattern sent to indicate to the far end that inbound framing has been lost is a coercion of the framed data so that bit 2 of each timeslot is set to data "0" for three consecutive frames. Although this works well for voice circuits, the data pattern can occur frequently when carrying digital data and will produce transient "yellow alarm" states.

Blue alarm indicates a disruption in the communication path between the terminal equipment. Communication devices, such as repeaters and multiplexers must see and produce line activity at the DS1 rate. If no signal is received that fills those requirements, the communications device produces a series of pulses on its output side to maintain the required activity. Those pulses represent data "1" in all data and all framing time slots. This signal maintains communication integrity while providing no framing to the terminal equipment. The receiving equipment displays a "red alarm" and sends the signal for "yellow alarm" to the far end because it has no framing, but at maintenance interfaces the equipment will report "AIS" or Alarm Indication Signal. AIS is also called "all ones" because of the data and framing pattern.

These alarm states are also lumped under the term Carrier Group Alarm (CGA). The meaning of CGA is that connectivity on the digital carrier has failed. The result of the CGA condition varies depending on the equipment function. Voice equipment typically coerces the robbed bits for signaling to a state that will result in the far end properly handling the condition, while applying an often different state to the customer equipment connected to the alarmed equipment. Simultaneously, the customer data is often coerced to a 0x7F pattern, signifying a zero-voltage condition on voice equipment. Data equipment usually passes whatever data may be present, if any, leaving it to the customer equipment to deal with the condition.

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