The Need for a Larger Structure: The Multiframe

The basic E1 frame, with a duration of 125µs, provides two dedicated overhead timeslots (TS0 and TS16). While TS0 is sufficient for basic frame alignment, the 8 bits available in TS16 within a single frame are insufficient to carry all the necessary signaling information for 30 different voice channels. How can a network signal events like "ringing", "busy", or "call answered" for all 30 channels using just 8 bits at a time?

The solution is to group multiple basic frames into a larger, repeating structure. This structure is called a multiframe. For the E1 system, this is specifically the MF16 Multiframe.

Anatomy of the MF16 Multiframe

The MF16 multiframe is a sequence of 16 consecutive E1 frames, numbered from Frame 0 to Frame 15. This structure expands the capacity for signaling and management information by allowing the content of the overhead timeslots (TS0 and TS16) to vary from frame to frame within the multiframe.

• Frame Count: 16 frames (Frame 0 to Frame 15).
• Total Duration: $16 \text{ frames} \times 125 \frac{\mu s}{\text{frame}} = 2000 \mu s = 2 \text{ milliseconds (ms)}$.

Role of TS16 within the Multiframe: CAS Signaling

The multiframe's primary purpose is to expand the capacity for Channel Associated Signaling (CAS). Inside the MF16 structure, TS16's function is meticulously organized:

• Frame 0 - MFAW: TS16 of the very first frame (Frame 0) carries the Multiframe Alignment Word (MFAW). This allows the receiver to synchronize not just to the frame, but to the entire multiframe sequence.
• Frames 1-15 - Signaling Data: The TS16 of each subsequent frame (Frame 1 through Frame 15) is used to carry signaling information for two of the thirty user channels.
• Channel Allocation: For instance, TS16 in Frame 1 carries signaling for voice channels 1 and 16. TS16 in Frame 2 carries signaling for channels 2 and 17, and so on. Over the course of the 15 signaling frames, all 30 user channels receive their dedicated signaling bits. Each channel gets 4 bits of signaling information every 2 milliseconds.

Enhanced Role of TS0: Frame Alignment and Error Checking (CRC-4)

The multiframe structure also enhances the role of TS0. Instead of transmitting the same Frame Alignment Word (FAW) in every frame, its function is alternated to carry additional management information.

• Frames with FAW: The FAW (e.g., pattern "0011011") is transmitted in TS0 of every even-numbered frame (Frame 0, 2, 4, ...).
• Frames with CRC-4: In every odd-numbered frame (Frame 1, 3, 5, ...), the TS0 is repurposed. In these frames, the bits that would have held the FAW are now used to carry a Cyclic Redundancy Check (CRC-4) code. This CRC-4 value is a checksum calculated across the entire previous multiframe. It allows the receiver to check for bit errors that may have occurred during transmission, significantly improving the link's monitoring capabilities.

This clever interleaving of frame alignment, multiframe alignment, signaling, and error checking within the two overhead timeslots is what made the E1 standard so robust and versatile.