Framing Formats for T1/E1 Systems
For E1 Systems
- Channel Associated (CAS) or Common Channel Signaling (CCS) Framing Formats
GL's E1 Cards permit either Channel Associated Signaling or Common Channel Signaling modes. In CAS mode, timeslot 16 contains
signaling bits for each channel. In CCS mode, timeslot 16 does not contain signaling information, it may contain user defined and
formatted data.
- Receive and Transmit Cyclic Redundancy Code (CRC)-4 Multiframe Formats
Two separate multiframe structures exist in CEPT environments: CRC-4 and Channel Associated Signaling (CAS). Both these
multiframes use the frame structure but when used together they may not be aligned. The CRC4 multiframe is used primarily to assist
in validating alignment at the FAS level but could also be used to monitor error performance.
For T1 Systems
Currently two framing formats are supported: 193S and 193E as shown in the figure below. The 193S format is also referred to
as the D4 framing format and the 193E format is also referred to as the ESF format.
- 193S (D4) Framing Format
There are 12 frames per multiframe in the 193S format. Signaling information associated with each individual voice channel, such
as on-hook/off-hook, call progress, and dial digits are transmitted within the voice channel in the LSB of each channel's codeword
during the 6th and 12th frames. This technique, known as "robbed-bit" signaling, replaces the LSB of the PCM codeword with the
signaling data. The signaling state in the 6th frame for each channel is known as the A bit. The signaling state in the 12th frame
of each channel is known as the B bit. For voice grade applications, robbed-bit signaling does not noticeably degrade the signal
quality.
- 193E (ESF) Framing Format
The 193E or Extended Superframe Format was developed to utilize the framing bit capacity for additional purposes. The number
of frames per multiframe is expanded to 24 frames. This multiframe is also called a superframe. The 24 framing bit positions of each
are divided into 3 channels. The FPS, or Framing Pattern Sequence, provides a synchronization signal for determining frame and
superframe alignment. A 4kHz Facility Data Link (FDL) provides a dedicated channel for system messages. The Cyclic Redundancy
Check (CRC) channel allows checksums to be transmitted with each superframe to monitor line quality.
Loopbacks
Three separate hardware loopbacks are provided on the T1/E1 Cards:
- Inward framer loopback (Inward Loopback (F)).
- Inward driver loopback (Inward Loopback (D)).
- Outward driver loopback (Outward Loopback (D)).
- An additional software loopback called "Rx-to-Tx Loopback" is also provided. In software loopback, the received data is looped
back to transmit data by the PC software
Connection Options - Terminate, Bridge, and Monitor Modes
The T1/E1 input signal may be terminated using impedance, monitored, or bridged depending on user requirements.
Terminate
The incoming signal is electrically terminated in 100 ohms for T1 signals, 120 ohms for E1 signals.
This form of connection is normally used when connecting directly to a T1/E1 signal rather than monitoring a signal on an existing T1/E1 connection.
Bridge
The incoming signal is in series with a pair of 420-ohm resistors prior to termination with either 100
ohm (T1) or 120-ohm (E1) resistance. The pair wise 420-ohm resistance appears as a high impedance connection to the signal, and thereby
renders the "tap" non-intrusive. This form of connection is normally used to non-intrusively monitor transmissions on an existing T1/E1
connection. A "T" type of tap is required to simultaneously pass the signal to its intended destination and to provide a connection to the
T1/E1 board.
Monitor
The incoming signal is assumed to originate from a monitor jack (which provides a tap and a pair
wise series resistance of 420 ohms). On the board the signal is terminated with either 100 ohm (T1) or 120ohm (E1) resistance. This form
of connection is normally used to non-intrusively monitor transmissions on an existing T1/E1 connection. A monitor signal is usually at a
level of-20dBsx that is 20 dB down from the nominal signal level measured at a DSX panel.
T1/E1 Clock Options
The T1/E1 analyzer has 3 clock options to choose. With internal clock option the transmit section is clocked using an internal oscillator.
For recovered clock option , the transmit section is clocked using the clock derived from the received signal. If no received signal is
present the internal clock is used. An external clock can be provided at the SMB input of the Cards or Dual USB Units. A TTL or CMOS
level is acceptable. Apply a 1.544 MHz clock for T1 or a 2.048 MHz clock for E1.
Internal Clk:
The transmit section is clocked using an internal oscillator. 1 ppm is provided, when this
option is selected, the internal clock is also provided at the Ext Clk SMB connector.
Recovered Clk:
The transmit section is clocked using the clock derived from the received signal. If no
received signal is present the internal clock is used. When this option is selected, the recovered clock is also provided at the Ext Clk
SMB connector.
External Clk:
An external clock can be provided at the SMB input of the Cards or portable USB Units.
A TTL or CMOS level is acceptable. Apply a 1.544 MHz clock for T1 or a 2.048 MHz clock for E1. The Ext Clk at the SMB connector may
be an input or an output depending on the clock settings. For Internal and Recovered clock options, the SMB connector provides an
output clock. For External Clock option, the SMB connector provides for an externally provided input clock.
The interfaces to the T1/E1 Cards or portable USB units are balanced 120 ohm (E1) or balanced 100 ohm (T1) electrically. Systems
employing 120-ohm interfaces can connect directly to the Cards or portable USB units. When connecting to systems employing 75-ohm
interfaces, as is sometimes encountered in E1 systems, a 120-to-75 ohm transformer may be used. Contact GL Communications for these
adapters. Connecting an unbalanced 75-ohm signal to a balanced 120-ohm interface (using a BNC to Bantam cable) will generally work for
short distances.
