For quality voice transmission the sample rate needs to be at least two times the highest frequency, typically 3500 Hz. For this reason a sample rate of 8000 times per second is used.
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Each sample of the signal has an amplitude component this is represented by an 8 bits binary code.
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Putting these numbers together we have an: •
8000 Hz (sample rate)
• x 8 bits (binary code) • = 64000 bits per second (representing one time slot channel) •
The coding technique used for E-1 transmission systems is called “A-law” companding. In T-1 transmission systems, “µ-law” companding is used which uses a different method of representing of the amplitude in its 8 bit binary code.
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The technique used to convert analog to digital is Pulse Code Modulation or PCM
E1 Transmission Format •
A Frame;
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An E1 system consists of 32 time slots which contain the 64 kbit/s consecutive samples. This is called a frame.
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The Transmission Rate;
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When combined together 32 time slots x 64 Kbits/s = 2048 Kbits/s which is the speed of the E-1 transmission system.
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ITU-T Specifications;
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ITU-T Rec. G.703 –
Bit rate: 2048 Kbit/s +/- 50 ppm
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Code: HDB3 (high density bipolar 3) Table 6/G.703
Pulse shape (normally rectangular)
All marks of a valid signal must conform with the mask (see figure 15/G.703) irrespective of the sign. The value V corresponds to the peak value.
Pair(s) in each direction
One coaxial pair
One symmetrical pair
Test load impedance
75 ohms resistive
120 ohms resistive
Nominal peak voltage of a mask (pulse)
2.37 V
3V
Peak voltage of a space (no pulse)
0 ± 0.237 V
0 ± 0.3 V
Nominal pulse width
244 ns
Ratio of the amplitude of positive and negative pulses at the centre of the pulse interval
0.95 to 1.05
Ratio of the widths of positive and negative pulses at the normal half amplitude
0.95 to 1.05
Maximum peak-to peak jitter at an output port
Refer to § 2 of Recommendation G.823
E1 Transmission Format
ITU-T Specifications; •
ITU-T Rec. fig.15/G.703 sample capture of an E1 signal with the pulse shape mask
Changes to 1 on loss of distant frame (remote alarm) Not-Frame alignment word
Time Slot 16 : Frames 2 through 15 are the same as frame 1 Time Slot 0: Even number frames 2 through 14 are the same as frame 0 Time Slot 0: Odd number frames 3 through 15 are the same as frame 1 1 = bit set to 1 1/0 = speech / signalling (varying data)
• PDH = Pleisiochronous Digital Hierarchy • One time slot channel = 64000 bits per second or 64 Kbps • 32 time slots = an E1 (2.048 Mbps)
(30 user channels + 1 synchronization and 1 framing channel) • 4 x E1 = E2 (8.448 Mbps), 120 channel PCM system • 4 x E2 = E3 (34.368 Mbps) 480 channel PCM system • 16 x E1 = E3 (34.368 Mbps) 480 channel PCM system
SDH Synchronous Transmission Format
• SDH = Synchronous Digital Hierarchy • 32 time slots = 1 x E1 (2.048 Mbps)
(30 user channels + sync and framing) • 63 x E1 = STM1 (155 Mbps) • 4 x STM1 = STM4 (622 Mbps) • 4 x STM4 = STM16 (2500 Mbps or 2.5 Gbps)
Theory of Drop & Insert • •
The conventional way of performing drop and insert is with the use of back-toback channel banks. At a Drop/Insert site all channels are converted to analog levels.
Site A Channel Bank
Site B Drop/Insert D/A A/D
E1
Site D Drop/Insert D/A A/D E1
E1
Drop/Insert analog channels
Site C Drop/Insert D/A A/D
Drop/Insert analog channels
Site E Channel Bank E1
Drop/Insert analog channels
While this technique works, there are several draw backs including: » Distortion caused by the multiple D/A and A/D conversion is cumulative across the system. » The overall high cost of the system » Increase in noise level
OMNIPlexer Drop & Insert
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The Omniplexer allows for drop and insert at every site but only converts to analog the channels that are required. The remaining are passed through digitally.
A Channel Bank
B Drop/Insert
Drop/Insert channels
Drop
Insert
C Drop/Insert
Drop/Insert channels
D Drop/Insert
Drop/Insert channels
Pass through
E Channel Bank
Synchronization Point - Point
E1 Tx with Clock
E1 Rx
Bi-Directional E1
E1 Rx
E1 Tx with Clock
Master Clock Source - Internal Stratum III clock Or - External Clock
• Only one E1 clock in the network • Far end is loop timed to keep both directions of E1 synchronized
Loop Time Mode Redirects incoming clock to outbound E1
Synchronization Drop/Insert
West E1
Master Clock Source - Internal Stratum III Clock Or - External Clock
East E1
• Mux clock derived from West and East E1 clocks +/- 128 bit slip buffer Recovered Mode - Clock extracted from incoming E1
• Only one E1 clock in the network • Drop/Insert terminal recovers clock from incoming E1 • Far end is loop timed to keep both directions of E1 synchronized
Loop Time Mode - Redirects incoming clock to outbound E1
Point-to-Point E1 Single Hop
E1; 1 – 30 channels
• Voice • Full-Rate Data • Sub-Rate Data
• Voice • Full-Rate Data • Sub-Rate Data
Point-to-Point E1 3 Hops E1; 12 channels
E1; 8 channels
E1; 10 channels
• Poor utilization of E1’s • Excessive multiplex equipment • Does not support multipoint interface
Drop/Insert E1 3 Hops E1; 30 channels (12+8+10)
• 12 channels
E1; 18 channels (8+10)
• 30 channels
• 8 channels
• Good utilization of E1’s • Minimal multiplex equipment • Supports multipoint interfaces
E1; 10 channels
• 10 channels
Drop/Insert Advantages •
Supports time-slot channel re-use
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Supports sub-rate data channel re-use
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Supports multipoint (miltidrop) voice and data interfaces
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Low throughput delay (less than 100 microseconds)
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Low jitter accumulation
Drop/Insert Channel Re-Use Site 1
Site 2
V F
V F
Site 3
V F
V F
V F
TS #1 D/I to Site 1
TS #1 D/I to Site 2
TS #1 D/I to Site 3 (Re-Use) TS #2 To Site 3 Pass Through Site 2
• Time-slot #1 is re-used at Site 2 • Individual channels may be dropped and re-used • Time-slots “pass through” site where channel is not accessed
TS #1 D/I to Site 2 TS #2 To Site 1
V F
Drop/Insert Sub-Channel Re-Use
Site 1
Site 2
L S D C M
Site 3
L S D C M
L S D C M
L S D C M
Drop TS-1 Sub-Ch A,B from Site 1
Insert into TS-1 Sub-Channels A,B to Site 2 C,D to Site 3
• Time slot divided into 8 data sub-channels • Individual sub-channels may be dropped and inserted
Time slot digitally bridged and dropped at each site
E1 Testing E1 Bit Error Rate Testing
Site 1
Site 2
Site 3
E1 Test Set
Laptop or PC connected to Local Port
• From one location, technician can selectively test each link • E1 remote loopback enabled sequentially to test link-by-link • Testing performed in both “West” and “East” directions (using drop/insert E1 test set) • Can test entire E1 bandwidth or time slot increments • Duration of test is ½ to 1 hour • Error rate of 10E-6 is acceptable, error rate of 10E-8 to 10E-9 is expected • OMNIPlexer’s built-in E1 error rate testing will raise alarm if error rate rises above pre-programmed threshold (10E-6 to 10E-9)
E1 Testing Data Channel Testing
Site 1
Site 2
D C M
Site 3
D C M
Data Test Set
• From one location, technician can perform end-to-end bit error rate test for each data circuit • Data channel remote loopback enabled • Can test entire one time slot, multiple time slots or sub-rate time slot data channels • Duration of test is ½ to 1 hour • Error rate of 10E-6 is acceptable, error rate of 10E-8 to 10E-9 is expected
E1 Testing Voice Channel Testing
Site 1
Site 2
Site 3
E & M
E & M
Analog Level Meter
Analog Signal Generator
• From one location, technician can perform end-to-end bit test for each analog circuit • Analog channel remote loopback enabled • Measure analog level, signal-to-noise ratio, noise floor, distortion
