57 terms

T-Carrier Systems

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T1 Digital Carrier
A carrier system that uses digital pulses rather than analog signals to encode information
Bell System
North American Telephone Standard recognized by CCITT (ITU-T)
T1 Carrier System
Uses PCM-TDM; Multiplexes 24 voice-band channels (1 Frame) each frame separated by a framing bit
DS-1
First level digital signal; processed by T1 line driver
System Clock [formula]
(n channels/frame) (8 bits/channel) (8000 frames/second)
# of Time Slots for USA
24
# of Time Slots for Europe
32
E1 Carrier
Gross data rate of 2.048 Mbps; 32 8-bit channels send in a 256 bit frame; two channels for signaling, other 30 for data; does not support bit robbing
Multiframe Alignment Bit
Added to the transmit signal of each frame as an overhead; occurs once per frame (8kbps rate) and is recovered in the receiver
Line Speed for T1 Digital Carrier System [formula]
=(193 bits/frame0 * (8000 frames/second) = 1.544 Mbps = 1.536 Mbps + 8000 bits = 1.544 Mbps
T1 Line Driver
Sits between TDM (DS1) and the External interface; Conditions T1 bit stream electrical characteristics to meet specifications, converts to AMI format
AMI Format
Alternate Mark Inversion Format (Unipolar - Bipolar); Alternates polarity of each 1 = Mark
BPV
Bipolar Violation; Occurs when two consecutive ones have the same polarity
T1 Physical Layout
Two balanced pairs of copper wire (twisted pair); full-duplex; no clock signal
CPE
Customer Premises Equipment; terminate T1 with a RJ-48C jack
S-Bit
Signaling bit; used for inband signaling (7 bits voice, 1 bit signaling); used for flash-hook, on hook, off hook, dial pulsing, ringing
Signaling Channel rate is ____, which is too small, resulting in ________.
8 kbps; Bandwidth waste
Voice Rate is _______, resulting in the suffering of _____________.
56 kbps; voice quality
Out of Band Signaling
Telephony signaling carried over separate network from telephone calls; allows for signaling between any switches, signaling during call, higher than voice data rate signaling; in band tone signaling helps phone phreaksl out of band signaling more secure
SS7
Signal System 7; telephone signaling network with packet-switching and out-of-band signaling; lots or redundancy
SF
Superframe; group of 12 frames
Payload
User data
Framing bits in odd-numbered frames
101010 alternating
Framing bits in even-numbered frames
001110 repeating
Framing bits are referred to an ________
Overhead
Channel bit rate (bps) [formula]
8000 * (n/m)
where n is the number of bits every m frames
D4 Framing
Frames 1-6 (A-highway) and 7-12 (B-highway), where 6 and 12 are the signaling frames
Robbed Bit Signaling
5 of 6 frames have an 8-bit resolution, while 1 of 6 frames has a 7-bit resolution; used in D-4 Framing
Clear Channel
No bit robbing data transfer
Bit Robbing
Fine method for voice data; humans cannot detect sound change in 125 usec; not appropriate for pure data (ASCII File Transfer)
D5 Channel Banks
Extended Super Frames; 24 frames; each frame has 24 DS-0 64 Kbps channels; frame = 193 bits; ESF = 4632 bits (193 * 24); 6 of 24 F-bits used for synchronization (4, 8, 12, 16, 20, 24)
ESF
Extended Super Frame
FPS
Framing Pattern Sequence
FPS for ESF
001011
CRC
Cyclic Check Redundancy
CRC-6
6 bits in an ESF used for error detection (1,5,9,13,17,21); short version of CRC-16
FDL
Facilities Data Link; 12 remaining F-bits in an ESF provide for this management channel
ESF Channels
Framing (6 bits)
Error checking channel (6 bits)
Management Channel (12 bits)
F-bit per framing channel (ESF) [formula]
(n Kbps/x fbits) * (24 Fbits/8 kbps)
F-bit per framing channel (SF) [formula]
(n Kbps/x fbits) * (12 Fbits/8 kbps)
Both SF and ESF used __-____ Signaling
In-band
In-Band Signaling
Signaling information is carried in the same bandwidth (channel) as the voice and data messages
For T1 to operate correctly, the transmitter and receiver is _____ ___________.
Frame synchronized
Synchronization is realized through the usage of _______ ____.
Framing bits
Transmitter and receiver must be clock synchronized, in a method called...
Sample Synchronization
Master/Slave Clock Relationship
Relationship where the master provides the clock, the slave derives the clock (clock recovery) from the master.
Master/Slave Timing
Loop Timing; slave takes its timing from the T1 loop
BSRF
Basic System Reference Frequency; used by the master clock; an atomic clock located in Hillsboro, Missouri; most accurate clocking device available for T1 circuits; STRATUM 1 CLOCK
Stratum Clock Accuracies
STRATUM 1 = .001 PPM
STRATUM 2 = .200 PPM
STRATUM 4 = 50 ppm (used by channel banks)
Regenerative Repeaters
T1 Repeaters; necessary since a digital signal is still electrical in nature
Maximum Transmission Distance without a Repeater
1 mile (600 ft)
T1 Repeaters
Maintain clocking of original signal; use incoming 1's to maintain the clock timing (clock recovery)
Zero Constraint Rule
Need to average at least one 1 every eight bits and cannot transmit more than 7 0's in a row; easy with voice transmission, difficult with data transmission
Bit Stuffing
For a sequence of eight zeros, the eight bit will be replaced with a 1
Bit Stuffing Limitation
No De-stuffing, when bit stuffing is applied, data is modified (56 kbps)
B8ZS
Bipolar 8 Zero Substitution; overcomes bit stuffing limitation; overrides 00000000 with 00011011, creating Bipolar violation when sending 11 and 11; receiver recognizes BPV and interprets 8 zero's; 64 kbps transmission rate remains
Transmission Rate for T1
1.544 Mbps
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