Pseudo Ternary Line Coding and the ST Interface
ISDN and the first layer of Frame Relay use a coding scheme called pseudo-ternary signaling, which is used by the S/T interface. This technique provides DC balance and non-voltage drifting procedures for signaling by using positive and negative 0s. In pseudo-signaling, 0 is represented as a line signal of approximately 750 millivolts that alternates between positive and negative polarity, and the 1 represents the absence of voltage. As you can see from Figure 2-6, synchronization is important because there must be a way to recognize the two consecutive 1s from 0s, where two or more consecutive 0s change the polarity. The synchronization is based on the bipolar violation (BPV), which is two consecutive 0s. In pseudo-ternary signaling, the binary values show a 0 as negative or positive, and 1 as the absence of signal.
Figure 2-6 Pseudo-Ternary Signaling
|
1 0 |
Time |
||
56 Chapter 2: Telecommunication Basics
The frame structure is organized into blocks of bits. Figure 2-7 shows the short physical layer frame format. From the CPE terminal equipment (TE), the outgoing frames have 2 bits offset from incoming frames. The network timing drives the process. The TE cannot activate the line (activation bit A), as only the NT confirms the activation. Figure 2-7 shows a significant difference between incoming and outgoing frame formats. Each frame contains 16 bits from every B channel and 4 bits from the D channel.
Figure 2-7 Short and Detailed Frame Formats
|
Channel |
B1 |
D |
B2 |
D |
B1 |
D |
B2 |
D |
|
Number of Bits |
8 |
1 |
8 |
1 |
8 |
1 |
8 |
1 |
Detailed Frame Format
NT-to-TE — Incoming Frames
FLB1B1B1B1B1B1B1B1EDAFaNB2B2B2B2B2B2B2B2EDMB1B1B1B1B1B1B1B1EDSEbB>B2B2B2B2B2B2EDL
OfOfFLB1B1B1B1B1B1B1B1 LDLFa LB2B2B2B2B2B2B2B2L DLB1B1B1B1B1B1B1B1LDLB2B2B2B2B2B2B2B2LDL TE-to-NT — Outgoing Frames 48 Bits in 250 Microseconds
The full description of the bits and rules follows:
• F —Framing bit. Always positive 0 and is based on the BPV, which is used for timing.
• L—DC balancing bit. Independent DC balanced; can be +0,-0, or 1.
• D—D channel bit. +0, -0, or 1 in the first format; 1,-0 in the second.
• E—Echo D channel bit. Can be +0, -0, or 1; exists only in the first format.
• Fa—Auxiliary framing bit. Only in the first format.
• A—Activation bit. Can be +0, -0, or 1; only in the first format.
• N—Complement to Fa. Can be +0, -0, or 1; only in the first format.
• B1, B2—Data bit within B1 and B2 channels respectively; can be +0, -0, or 1.
• M—Multiframing bit. Can be +0, -0, or 1; only in the first format. I.430 specifies multiframe, which is a group of 20 I.430 frames. M-bit and Fa are used in the multiframing procedures. The M-bit is set to 1 in the first 20 frames, which are 48 bits each, and set to 0 in every other frame. The I.430 also specifies that the first 0-bit transmitted F and L bits are another BPV.
For every 250 ms, 48 bits are transmitted through the channel, yielding a total of 4000 frames per second. 4000 frames x 48 bits each results in a total rate of 192 kb. For every 16 B-bits, the channel transmits 4 D-bits. Therefore in a Basic Rate Interface (BRI), if every B channel is 64 kbps, the D channel is 16 kbps. A BRI is also known as 2B+D. This signaling is not contention-free, so a simple contention-resolution mechanism was designed to prevent contentions. It is based on the following traffic considerations:
• B channel traffic—No additional functionality is necessary to control access to the two B channels because each channel is dedicated to a given TE at any particular time.
• Incoming D channel traffic—The D channel is available for use on all member TEs. The Link Access Procedure on the D channel (LAPD) addressing scheme (described later) is sufficient to resolve any unit and its destination, based on the fact that each LAPD frame includes an explicit address of every destination TE. All TEs can read this address and determine whether the frame was sent to them.
• Outgoing D channel traffic—Access must be regulated, so that only one device transmits frames at a time.
The following is how the contention resolutions works:
• When the TE is ready to transmit an LAPD frame, it listens to the stream of incoming D channel echo bits. If it detects a string of 1s of equal length to a threshold value Xi, where i = priority class for this LAPD frame, it can transmit. Otherwise, it waits because another TE is transmitting.
• If several TEs transmit 0 at the same time, all are using the same polarity. Sending 1 means no signal. The TEs are attached to the bus in parallel. Based on Ohm's law, the total voltages are not a sum of all voltages. Therefore, 1 is detected if all TEs apply 1 (no voltage) and 0 is detected if one or more TEs apply a voltage. This process uses the logical AND function.
• A NT-to-TE frame carries an E-bit, which is an echoed D-bit in the opposite direction. The E-bit performs an important contention mechanism, especially in the p2mp designs. The mechanism ensures that only one TE is transmitting frames in the TE-to-NT direction. If more than one TE tries to transmit, a collision can result. To avoid a collision, a transmitting TE monitors the echo bit with the transmitted bit. If the E-bit is different from the last transmitted D-bit by this TE, the TE knows that it does not control the D channel and ceases the transmission. This procedure is called perfect scheduling.
Continue reading here: Two Binary One Quaternary Coding the Frame Format and the UInterface
Was this article helpful?