Jaycee's Networking

May 8, 2009


Filed under: Information, IOS — Tags: — Jaycee @ 6:10 am

A. T1 is a means for digitally trunking multiple voice channels together between locations.

1. T1 are full-duplex links.

2. All T1s are digital. T1 uses digital signaling within the data channel even with an “analog” T1. Each channel’s audio must be converted to digital to be sent over the T1.

3. Basic types of T1s:

a. Channelized T1:

(1) It’s a voice circuit that has 24 voice channels.

(2) It’s in-band signaling:

i) each channel contains its own signaling information, which is inserted into the data stream of the digitized voice.

ii) The signals within the channel are not audible. These signals, called ABCD bits, are embedded in the voice data and they are used to report on the status of phones.

b. PRI:

(1) It’s a voice circuit that has 24 voice channels and one of which is dedicated to signaling.

(2) Bearer channels: the number of available voice channels is 23.

(3) Data channel: the signaling channel.

(4) It’s out-ot-band signaling: One entire channel is reserved for signaling which reduces the number of usable channels from 24 to 23.

c. Clear-channel T1:

(1) It’s not framed, no channels and no organization of the bits flowing through the link.

(2) It’s a rarity, as most data links are provisioned with ESF framing.

4. Two types of encoding:

In T1 signaling, there are two possible states: mark (1) and space (0). A space is 0V, and a mark is either +5V or -5V.

a. AMI (Alternate Mark Inversion): When using AMI, a long progression of spaces will result in a loss of synchronization. The risk of an all zeros signal exists, so AMI sets every eighth bit to a 1. 16 zeros in a row can cause the remote end to lose synchronization. Voice signals can easily absorb having every eighth bit set to 1, but data signals can’t tolerate having any bits changed. If one bit is different in a TCP packet, the CRC (Cyclic Rdundancy Check) will fail and packet will be resent. Thus, AMI is not an acceptable encoding technique on data T1.

b. B8ZS: If 8 zeros in a row are detected in a signal, they will converted to a pattern including international BPV (Bipolar Violation)s. Remote side will converts it back to all zeros. This technique allows data streams to contain as many conseccutive zeros as necessary while maintaining ones density.

5. Framing:

T1 use time-division multiplexinthat each channel is actually a group of serial binary values. The receiving equipment needs to know when the first channel starts, and when the last channel ends. The way this is done is called framing.

a. D4/Superframe: a standard voice framing that each 8-bit sample is relayed from each channel in order.

i) Frame: 24 8-bit channels + 1 framing bit = 192bits + 1 bit = 193 bits.

ii) Superframe: 12 x 193-bit frames = 2,316bits

iii) 8,000 frames per second are sent: 8,000 x 1932bits = 1,544,000 bps

iv) If removing the framing bit (1 bit in each frame), we get: 8,000 x 192 = 1,536,000 bps

v) Thus, Some texts will show a T1 to be 1.544 Mbps, while others may show 1.536M Mbps. 1.536 Mbps is the usable speed of a T1 when framing bits are taken into consideeration.

vi) D4 is lack of error detection, thus it’s not suitable for data.

b. ESF (Extnded Superframe):

i) ESF is composed of 24 frames instead of 12.

ii) Frames 4,8,12,16,20 and 24 (every 4th frame): The framing bits are filled with the pattern 001011.

iii) Frames 1,3,5,7,9,11,13,15,17,19,21 and 23 (every odd-numbered frame): The framing bits are used for a new 4,000 bps virtual data channel. This channel is used for out-of-band communications b/w networking devices on the link.

iv) Frames 2,6,10,14,18 and 22 (the remaining even-numbered frames): The framing bits are used to store a 6-bit CRC value for each superframe.

6. T1 should be provisioned with B8ZS encoding and ESF framing.

7. T1 CSU/DSU WIC Troubleshooting:

Router# sh service-module s0/1 performance-statistics

a. It shows which events occurred during each of the last 96 15-minute intervals.

b. In the result of the command, if you see a number of line code violations, or any other error that shows up in all or most of the intervals, you’ve got a problem.

B. Configure an internal CSU/DSU for a WAN connection:

interface serial0/0
 ip add
 service-module t1 timeslots 1-12 speed 56
 service-module t1 clock source internal
 service-module t1 linecode ami
 service-module t1 data-coding inverted
 service-module t1 framing esf
 service-module t1 fdl ansi
 service-module t1 remote-alarm-enable

1. “service-module t1 timeslots 1-12“: tells the internal CSU/DSU to use the first 12 time slots of the T1 circuit.

2. “service-module t1 timeslots 1-12 speed 56“: by default, the CSU will assume that all of these time slots are 64Kbps DS0 channels. If the circuit actually uses 56Kbps channels, then add “speed“.

3. “service-module t1 clock source internal“: WAN carrier usually provide the clock signal for a T1 circuit. But for lab networks, you’ll need the internal CSU/DSU to act as the DCE device and supply the clock signal.

4. “service-module t1 linecode ami“: by default, the router will use B8ZS (Binay 8 Zeros Substitution) line coding which tends to be the most common way that T1 circuits are delivered. Some use AMI (Alternate Mark Inversion) instead. “service-module t1 data-coding inverted“: When use AMI line coding, you have to set the speed of each channel to 56Kbps or use inverted data coding.

5. “service-module t1 framing esf“: by default, the router uses ESF (Extended Super Frame). Here uses SF (Super Frame) framing.

6. “service-module t1 fdl ansi“: Some network vendors require a special FDL (Facilities Data Link) configuration. When using a WIC-1DSU-T1 module, FDL is disabled by default. On Cisco 2524 and 2525 routers, the built-in T1 CSU/DSU uses both ANSI and AT&T simultaneously by default.

7. “service-module t1 remote-alarm-enable“: allows the CSU to send remote alarms, called yellow alarms, to the CSU on the other end of the circuit. It does this to let the other device know that it has encountered an alarm condition such as a framing error or loss of signal on the circuit. Only use this with ESF framing because it conflicts with SF framing.


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