1. Issues of the Synchronous Digital Hierarchy
Twelfth Meeting

2. Network Design Elements
Multiplexer
Translates STM-1 signals into STM-4, STM-16 or STM-64.
Demultiplexes in the opposite direction of transmission.
Regenerators
Rregenerates a perfectly formed signal
Add/drop multiplexers (ADMs)
Carries out a switching function with pointer mechanism
Drop out’ a VC which can then be replaced (add) with another VC.
Cross-connects or Digital Cross Connecs (DXCs)
Has both SDH and PDH interfaces,
Has large number of switching connections (cross-connects) between these interfaces.
Allow large numbers of paths to be interconnected at points of high traffic density.

3. SDH Network Structure

4. Vertical Add Drop Multiplexer (ADM)
ADMs are installed between STM-1 terminal multiplexers
VCs can be dropped out of the STM-1 frame at each of the intermediate (add/drop) nodes. (ADMs)
Similarly, other VCs can be added in their place;
The total capacity of the path at any point cannot be greater than the STM-1 payload
VC-12 can only be added into a vacant position.

5. Ring Add Drop Multiplexer (ADM)
Joining the two terminal multiplexers together,
Then replace them with ADMs.
Access to and from the ring is via the add/drop capability of each ADM
Example:
A and C are connected by fibre-optic cables.
Traffic is duplicated and passes around the entire ring,
Traffic travels in opposite directions
Switching position at A and C only determines that the traffic is added and dropped from the ring.

6. Restoring traffic in a Ring ADMs
No manual intervention, in the event of a failure.
Full restoration in a few milliseconds
Referred to as self-healing.
Example:
The event of a break in the ring.
Immediate switching actions at C passes the traffic to the standby fibre.
Why C
The closest node the to the failure

7. Network Architecture
Layer 1
Layer 2
Layer 3

8. Synchronization Two SDH networks, A and B
Each network is running a separate clock
An STM-1 line system connecting the two are shown below.

9. Synchronization: Clock Drift
a) The original signal generated in using the clock of network A
b) The same signal, that is produced by network A, generated using the clock of network B
This clock drifting cause corruption of data
Buffers are used at the interface to control the differences.

10. Synchronization: Buffer
A buffer is a storage device used in time division multiplexing.
Data is ‘written’ into a buffer using clock A
Data is ‘read out’ using clock B
Buffer size is chosen based timing variations between reading and writing.
When will the buffer empty
When clock B is faster than A.

11. Primary Referencing Clock

12. Pointer Operation – Controlled Slips
The timing of the STM-4 frame is generated by byte-interleaving four VC-4s,
Each VC-4 is contained within an STM-1 frame
Example,
(a), (b) and (c) – originate from same station as the STM-4;
(d) comes into the station on an STM-1 line system.
STM-4 will drift apart from the STM-1 signal
The buffer approaches its lower limit
STM-4 makes a pointer adjustment to the location of VC-4(d). In frame n + 1
This is referred to as wander

13. Jitter
Jitter is a term used to describe the phase variation between signals
Reference signal is a bit-stream to be transmitted using a clock pulse
Jittered signal is the same bit-stream after it has been transported across an SDH network with the following difference:
It has pulses that do not line up with the clock.
If this clock is used, it will read a ‘0’ and not a ‘1’ as transmitted

14. Jitter: De-synchronizer

15. Bit Error Rate (BER) transmission A sequence of 100 bits
There are two bits in error at the end.
BER = 2 × 10-2.
Failure if BER >10-3
Acceptable if 10-6 < BER < 10-3
Normal if BER <10-6

16. Bit-Interleaved parity (BIP)
The frame is divided into blocks of bits
Organize them in a columns
Each column has an extra bit added (even parity of odd parity)
Perform a parity check

17. Memorize Error performance
Errored block (EB) – a block in which one or more bits are in error.
Errored second (ES) – a 1 second period with one or more errored blocks (includes severely errored seconds during available time).
Severely errored second (SES) – a 1 second period that contains 30% or more errored blocks .
Background block error (BBE) – an EB in available time not occurring as part of an SES.
ES ratio (ESR) – the ratio of ES to total seconds in available time.
SES ratio (SESR) – the ratio of SES to total seconds in available time.
BBE ratio (BBER) – the ratio of EB to total blocks, excluding SES and unavailable time.
Unavailable time – unavailable time commences at the start of a block of ten consecutive SESs, and finishes at the start of a block of ten consecutive seconds, each of which is not an SES.
Memorize