Two Binary One Quaternary Coding the Frame Format and the UInterface
ANSI standard T1.601 is used in the U.S. to provide the necessary specifications for the U-reference point because the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) does not define the local loop specifications between NT and the LE across U-reference points, or the otherwise called U-interface. The interface and
58 Chapter 2: Telecommunication Basics line coding is specific for U.S. ISDN, IDSL, HDSL and SDSL local loop specifications. The physical connection over twisted pair provides distances up to 5.5 km (18,000 ft). The U-interface supports serial, synchronous, full-duplex, and point-to-point designs. The signaling technique associated with the U-interface is called two binary one quaternary (2B1Q), as shown in Figure 2-8.
Figure 2-8 Two Binary One Quaternary (2B1Q) Signaling
Binary Values Quats
0.833V
Binary Values Quats
0.833V
2B1Q is a four-level, single-symbol code. Every combination of two bits (first two columns), where the first bit represents the polarity and the second one the magnitude, has a voltage representation and Q symbol representation called a quat. The frame format in the U-access point (often called the U-interface) is different, as shown in Figure 2-9.
Figure 2-9 2B1Q Frame Format
|
Synchronization Word |
12 Groups of (2B+D) Each |
Overhead (M Channel) |
|
9 Quats, 18 Bits |
108 Quats, 216 Bits |
3 Quats, 6 Bits |
The Synchronization Word (SW) is a self-explanatory term, which is used for physical layer synchronization and frame alignment. B and D channel information is formed into 12 groups that contain data and control information as follows: B1 (8 bits), B2 (8 bits), and D (2 bits). Therefore, 18 (8 + 8 + 2) multiplied by 12 yields 216 bits, representing 108 quats. The overhead field is used for channel maintenance, limited bit error detection, and power status indication. Using two different interfaces, S/T and U, requires signal transformation that is performed by the device known as the network termination type 1 or NT1. The timing for the NT1 in a U-interface (reference point) is still provided by the LE.
Unlike a S/T reference point, the U-access point operates at 160 kbps and sends 666.666 frames per second, or one frame every 1.5 ms. Every eight groups of frames is a SF. To indicate the beginning of the SF, the technique uses an inverted SW. The 6 overhead bits of all frames represent a 48-bit (8 frames x 6 bits) block, called a M-channel. This block is capable of increased error-detection, signaling maintenance, and power error detection.
Continue reading here: T1 ESF Signal Formats
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