ing error. When a T1 device reaches a certain threshold of framing errors, an alarm is triggered. You may have seen a discrepancy in the reported speed of T1 links in your reading. Some texts will show a T1 to be 1.544 Mbps, while others may show 1.536 Mbps. This discrepancy is a result of the framing bits. Because the framing bits are used by the hardware and are not available as data, they are considered overhead. Thus, 1.536 Mbps is the usable speed of a T1 when framing bits are taken into consideration. Extended Super Frame The D4/superframe standard was developed for voice and is not practical for data transmissions. One of the reasons D4 is unsuitable for data is the lack of error detection. To provide error-detection capabilities and to better use the framing bits, a newer framing standard called Extended Super Frame (ESF) was developed. Chapter 21: T1 11111111 One channel sample = 8 bits Phone audio is 8khz = 8,000 samples/second Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 23 Channel 24 Framing bit 24 channels= 192 bits Frame (24 8-bit channels + 1 framing bit)= 193 bits Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7 Frame 8 Frame 9 Frame 10 Frame 11 Frame 12 Superframe (12 193-bit frames)= 2,316 bits 8,000 frames per second are sent. 8,000*193= 1,544,000 bps Removing the framing bit overhead (1 bit in each frame), we get: 8,000*192= 1,536,000 bps Figure 21-5. DS1 framing ESF works under the same general principles as D4/superframe, except that an ESF is composed of 24 frames instead of 12. The framing bit of each frame is used to much greater effect in ESF than it is in D4. Instead of simply filling the framing bits with an expected pattern throughout the course of the superframe, ESF uses these bits as follows: Frames 4, 8, 12, 16, 20, and 24 (every fourth frame) These frames' framing bits are filled with the pattern 001011. Frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23 (every odd-numbered frame) These frames' framing bits are used for a new, 4,000 bps virtual data channel.