Answer Key
- What is the primary scope and purpose of ITU-T Recommendation G.704? This recommendation provides the functional characteristics of interfaces used with network nodes, particularly synchronous digital multiplex equipment. It defines the frame structures, including frame length, frame alignment signals, and Cyclic Redundancy Check (CRC) procedures, for specific digital transmission hierarchies. It is intended for use with equipment like digital exchanges in IDNs and ISDNs, as well as PCM multiplexing equipment.
- List the five hierarchical levels (in kbit/s) for which this recommendation defines synchronous frame structures. The recommendation defines synchronous frame structures used at the 1544, 6312, 2048, 8448, and 44 736 kbit/s hierarchical levels. These standards ensure interoperability in digital transmission systems and networks.
- In the 1544 kbit/s frame structure, what is the F-bit, and what are its primary functions? The F-bit is the first bit of a frame in the 1544 kbit/s structure. Its primary functions are to provide frame alignment, performance monitoring, and a 4 kbit/s data link. In a 24-frame multiframe, the F-bit is allocated to the multiframe alignment signal (FAS), CRC check bits, and the data link messages.
- Describe the role of the Cyclic Redundancy Check (CRC) as a performance monitoring method within the 1544 kbit/s 24-frame multiframe. The CRC-6 is a method of performance monitoring where six check bits (e₁ to e₆) are contained within the F-bit positions of specific frames (2, 6, 10, 14, 18, and 22) of the multiframe. A receiving terminal calculates the CRC on the incoming multiframe and compares it to the received CRC bits. A mismatch indicates the presence of transmission errors.
- What is the basic frame length and frame repetition rate for the 2048 kbit/s interface? The basic frame for a 2048 kbit/s interface has a length of 256 bits, which are numbered 1 to 256. The frame repetition rate for this interface is 8000 Hz.
- Explain the purpose of the Alarm Indication Signal (AIS) as defined for the 44 736 kbit/s signals. The Alarm Indication Signal (AIS) is a special pattern used for the 44 736 kbit/s signal to report an alarm condition. It is a signal with valid multiframe and M-subframe alignment signals, and valid P-bits. The information bits are set to a 1010… sequence, and specific C-bits and X-bits are set to binary zero and one, respectively, to indicate the alarm state.
- In the 44 736 kbit/s multiframe structure, what is the general purpose of the C-bits (C11 through C73)? The C-bits in the 44 736 kbit/s signals can be used for either unchannelized bulk data transport or for channelized multiplex applications. For specific applications like C-bit Parity, these bits are used for functions such as Application Identification Channel (AIC), Network Requirements (Nₓ), Far-End Alarm Control (FEAC), alarm signals, and control signals for loopbacks.
- How are Synchronization Status Messages (SSM) used in the 2048 kbit/s system, and which bits carry this information? Synchronization Status Messages are used to convey information about the quality of the synchronization source. In the 2048 kbit/s system, one of the Sₐn bits (where n=4, 5, 6, 7, 8) is organized as a 4-bit nibble (Sₐn1 to Sₐn4) to carry the SSM, indicating the Synchronization Quality Level (QL).
- What are the two main types of signalling discussed for interfaces like the 1544 kbit/s carrying 64 kbit/s channels? The two primary methods of signalling discussed are common channel signalling and channel-associated signalling. In common channel signalling, one 64 kbit/s channel time slot is dedicated to carrying signalling information for other channels. In channel-associated signalling, signalling information is carried within the frames associated with each individual channel, such as by using an S-bit in a multiframe structure.
- Describe the composition of a multiframe at the 44 736 kbit/s level in terms of M-subframes and bits. A multiframe at the 44 736 kbit/s level contains 4760 bits. This multiframe is divided into seven M-subframes, with each M-subframe containing 680 bits. Each M-subframe is further divided into eight blocks of 85 bits, with each block consisting of one overhead bit and 84 bits for payload.