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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
synchronization
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.
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
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
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.

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