Issues of the synchronous digital hierarchy
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Issues of the Synchronous Digital Hierarchy. Twelfth Meeting. 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

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Network design elements
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)

    • Carries out a switching function with pointer mechanism

    • 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.

Vertical add drop multiplexer adm
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.

Ring add drop multiplexer adm
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.

Restoring traffic in a ring adms
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

Network architecture

Layer 1

Layer 2

Layer 3

Network Architecture


  • Two SDH networks, A and B

  • Each network is running a separate clock

  • An STM-1 line system connecting the two are shown below.

Synchronization clock drift
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.

Synchronization buffer
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.

Pointer operation controlled slips
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


  • 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

Bit error rate ber
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

Bit interleaved parity bip
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

Error performance
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.