# T1/D4 Description

This feature provides support for T1 Channel Associated Signaling.

T1/D4 provides a simulation of analog trunk signaling over DS1 digital trunk facilities (or links). DS1 is the North American standard first order digital line multiplex system operating at 1.544 Mbps. The 1.544 Mbps link is divided into twenty four 64 Kbps channels. In conjunction with the supporting hardware, this feature eliminates the need for expensive channel banks when employing the T1/D4 signaling scheme.

NOTE: A link is defined as a 1.544 Mbps digital transmission medium which terminates on the system. A trunk is defined as a single usable 64 Kbps PCM channel within the link.

## Signaling Scheme

T1/D4 signaling is a channel-associated signaling (CAS) scheme. Channel-associated signaling is a method in which the signaling information is transmitted over the traffic channels themselves. This type of signaling is also known as bit-stealing because certain bits in specific frames are used to provide signaling. Because these bits are "stolen", the customer usable data rate on each of the 24 channels is limited to 56 Kbps.

## Network Directory Numbers

The numbering plan for a network is defined during the system programming. It must be a uniform numbering plan for two reasons:

1. To invoke certain features across an MSDN network, the destination system must know the calling system's identity. The callback feature is an example of where uniform numbering is essential because the destination system must know both the identity of the calling system and the station in order to recall the party who invoked the callback.

2. A Uniform Numbering Plan provides consistency between the digits dialed and the digits displayed, as the called party's identity, on calling display telephones and attendant consoles.

## Network Synchronization

When two or more systems are connected by digital transmission in a network, the sending and receiving equipment must operate at the same frequency to ensure that the data is interpreted accurately. Varying limits can be applied to synchronization depending on the type of traffic being carried by the links (i.e., voice or data). Voice traffic is more tolerant of fluctuations in synchronization than data traffic because the high sampling rate of the voice signal allows frames to be lost without noticeable degradation of the overall transmission. Transmission of data traffic, however, requires continuous synchronization throughout the transmission, since the loss of a data frame could noticeably corrupt the received information.

NOTE: Data transmission via modem is considered to be voice traffic.

The system allows operation of digital trunks in either the " Freerun" or "Network Synchronized" mode. " Freerun" mode synchronizes the digital trunks to the system clock at each end of the connection; "Network Synchronized" mode employs phase synchronization, a process by which clocks at each end of the connection are maintained at the same average frequency by synchronizing one clock to the other. A maximum of eight synchronization sources (i.e., incoming links) are specified in the Network Synchronization form. The order in which they are listed is the order in which they are selected for use by the system if a previous link in the list fails or goes out of tolerance.

Differences that occur in the frequency of the two clocks cause varying degrees of error. In digital trunk terminology, these errors are categorized as Slip, Jitter, and Wander.

- Slip: The incoming data is temporarily stored in a buffer to allow tolerable differences to exist between the incoming frame clock and the system frame clock. A slip occurs when the read clock of the buffer (provided by the system) "overtakes" the write clock (provided by the incoming link). In this event, the data frame is read twice. Conversely a slip may occur when the write clock "overtakes" the read clock. In this event, the data frame is lost.

- Jitter: A short-term frequency variation about the nominal frequency. Jitter is normally caused by successive regeneration of the clock by line repeaters.

- Wander: Long-term frequency variations about the nominal frequency. Wander is usually caused by environmental variations over the line segment.

## Features of T1/D4

Click the feature for information.

- Call Forwarding

- Call Hold and Three-party Working

- Conferencing

- Direct Trunk Select from Multiline Telephone Sets

- Distinctive Ringing

- External Line Select from Attendant Consoles

- Last Number Redial

- Station Message Detail Recording (SMDR)

- Timed Recall

- Transfer to a Busy Device