t305: digital communications arab open university-lebanon tutorial 61 t305: digital communications...

T305: DIGITAL COMMUNICATIONS

Arab Open University-Lebanon Tutorial 6 1

T305: Digital Communications

Block II – Part I - Synchronous Digital Hierarchy


2Arab Open University-Lebanon

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Introduction

Topic 1: Network DesignTopic 2: Synchronization – Keeping in TimeTopic 3: PerformanceTopic 4: SonetTopic 5: Management of Networks



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Topic 1: Network Design The functionality of the SDH hierarchy can be realized with four types of functional units, called network elements, which can in turn be physically realized as equipment.



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Topic 1: Network DesignThe multiplexer translates STM-1 signals into STM-4, STM-16 or STM-64. Higher-order and lower-order VCs may be assembled directly into an STM-n, or STM-1 to STM-n conversion can be carried out. A multiplexer also carries out demultiplexing in the opposite direction of transmission. Regenerators are connected in the transmission path between multiplexers and operate at the same bit-rate as the multiplex.



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Topic 1: Network DesignAdd/drop multiplexers (ADMs) and digital cross-connects (DXCs) carry out a switching function. Both utilize the synchronous nature of SDH, together with the pointer mechanism, to switch virtual containers. Cross-connects are large network elements that have both SDH and PDH interfaces, and can make a large number of switching connections (cross-connects) between these interfaces. An add/drop will have fewer interfaces and will ‘drop out’ a VC which can then be replaced (add) with another VC.



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The interconnection of these four elements allows for a flexible network architecture as shown below.



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ADM: add drop multiplexer



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Topic 1: Network DesignIn (a), STM-1 line systems provide connectivity between two terminal multiplexers and two add/drop multiplexers. Path connections, at each add/drop, are shown at VC-12. VCs can be dropped out of the STM-1 frame at each of the intermediate (add/drop) nodes. Similarly, other VCs can be added in their place; however the total capacity of the path at any point cannot be greater than the STM-1 payload and a VC-12 can only be added into a vacant position.A ring configuration (b) can be constructed by joining the two terminal multiplexers together, and replacing them with ADMs. Access to and from the ring is via the add/drop capability of each ADM.



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Add/Drop ringsA common way of recording quality of service, for communication networks, is to measure the percentage of time that a service is available; this is called availability: time availableavailability (%) = 100% total time



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Add/Drop ringsThe availability of a path through an SDH network is dependent upon the combined reliability of all the components that make up that path. Design of equipment can reduce the frequency of failure through improvements in reliability. An add/drop ring is a configuration that can restore service, without manual intervention, in the event of a failure in any part of the ring. Full restoration of service can be achieved in a few milliseconds in most circumstances.



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Add/Drop ringsThe repair to the ring failure can be carried out using normal procedures with less urgency. For this reason, SDH ring configurations are often referred to as self-healing as shown below. In (a), connectivity is provided between node A and node C by fibre-optic cables. Under normal conditions the traffic is duplicated and passes around the entire ring, in opposite directions, on both the traffic and standby fibres. It is only the switching position at A and C that determines that the traffic is added and dropped from the ring.



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Add/Drop rings (b) shows what happens in the event of a break in the ring. Immediate switching actions at C allow the traffic, that was previously passing on the traffic fibre, to be picked up from the standby fibre. The unavailable time is determined by the actions necessary to detect the failure and activate the standby switching.



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Question 1A PDH network could have provided a similar capacity as the STM-1, using 140 Mbit/s. If we have to “add/drop” 2Mbits PDH at ADMs how many multiplexer we would need.?

2/8, 8/34 and 34/140 a full path is needed.



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Digital cross-connectsA digital cross-connect (DXC) provides the control and switching architecture to allow large numbers of paths to be interconnected at points of high traffic density. Unlike the add/drop it will normally be used as a node in the network rather than in a ring configuration.



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ArchitectureThe abstract view is shown in (a). At this level all users see a network that is characterized by the service they are using. A telephone user will see a phone network and an Internet user an Internet network, even if they are accessing the services via the same telephone line.



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QuestionsList four SDH network elements.A VC-4 and VC-12 can be said to have a client-server relation ship. Which is which, and Why?

Multiplexer, add/drop, cross-connect and regenerator.

A VC-12 can be transported as part of a VC-4. In this case the VC-4 is the server and VC-12 is the client.



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ArchitectureSummarySDH networks can be built using four types of network element: multiplexers, regenerators, add/drops (ADMs) and digital cross-connects (DXCs)The connection of ADMs in a ring configuration provides a flexible method of providing service which can also offer improvement in availability.Network architecture is important when looking at complex networks. Different views of the network can be taken, each with different levels of complexity.



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Topic 2: Synchronization – Keeping in TimeTwo SDH networks, A and B, each running from a separate clock, with an STM-1 line system connecting the two are shown below

Fig. Two separately timed networks



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QuestionA 155.52 Mbit/s link connects two networks, as shown in the previous figure. The timing of the received STM-1 varies by 10-11 from the clock in the adjacent network. What is the time interval between byte slips?155.52*106 bits are received in 1 second.A variation of 10-11 will result in a 1 bit slip in every 10 11 .

Therfore , it will take 10 11 / 155.52*106 seconds for one bit slip. This will results in 643 seconds.A controlled byte slip accumulates 8 bits slip, so a byte slip would occur every 8*643= 5144 seconds (every 1.34 hours)



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Topic 2: Synchronization – Keeping in TimeAssume that the clock in network A is running more slowly than the clock in B by a very small amount. The consequence of this variation, at the interface (I), is shown below, where (a) shows the signal generated in network A together with pulses generated from the clock in A, and (b) shows the same signal with the clock pulse from network B superimposed.

Fig. Clock drift.



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QuestionTwo STM-1 signals are supposed to be synchronized. If one signal is exactly behind the other, what is the difference in time between each signal?STM-1 bit-rate is 155.52 Mbit/s =155520000 bits/sTime to transmit one bit =1s/ 155520000=6.43 nanosecond



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Topic 2: Synchronization – Keeping in TimeNotice that the two sets of pulses are drifting apart as the difference between the two clocks accumulates. If the signal were ‘read’ using the clock from network B then eventually the clock would drift across a 1/0 boundary and the data content would be corrupted. Buffers are used at the interface (I) to control the differences.A buffer is a common storage device used in time division multiplexing. Data is ‘written’ into a buffer at the incoming signal rate and ‘read out’ at a rate determined by a local clock. The storage capacity of a buffer is chosen to accommodate short-term timing variations between reading and writing.



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Topic 2: Synchronization – Keeping in TimeA hypothetical buffering arrangement is shown below. The data signal, from network A (STM-1), enters the diagram on the left-hand side where the timing content of the signal is extracted. This is necessary as the data can only be accurately read into the buffer using a clock having the same frequency as the one with which it was generated

Fig. Buffer operation



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Topic 2: Synchronization – Keeping in TimeThe derived clock is used to write the data into the buffer. Once the data is stored in the buffer it can be read out using a clock that is synchronized to network B. However, using a buffer does not eliminate the timing variation, and in this example the slower-running clock in network A will result in the buffer emptying. The size of the buffer and the timing difference determine how quickly the buffer will empty, or fill if the clock in A is faster than B.



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QuestionWhat would be the typical delay of a 155.52 Mbit/s signal that is passed through a buffer which delays the signal for 8 Bits?Aggregate delay is 8 bits.At 1/155.52*106=6.4*10-9 secondsSo 8 bit delay=0.05Micro second



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Synchronization – how timing is distributedAn SDH network is supported with a clock source with an accuracy of 10-11. Each network has a hierarchy of clocks which distribute timing signals to main nodes in that SDH network.The tree arrangement below shows how timing is distributed from the primary reference clock (PRC) to a series of slave clocks. The slave clocks operate to an accuracy less than that of the PRC.

Fig. Inter-station Synchronization.



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A star configuration is used to distribute the timing source within a telecommunications station (single building). NE stands for network element.

Fig. Intra-station Synchronization

Synchronization – how timing is distributed



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JitterJitter is a term used to describe the phase variation (in the time domain) of a digital signal from its normal, or ideal, time position. Figure below shows the normal (reference) position of a pulse train and a continually changing, jittered, train; the vertical dashed lines represent a clock signal.

Fig. Jitter.



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JitterThe reference signal, which could be a bit-stream transmitted over an SDH network, is timed using a clock pulse that corresponds to the centre of every pulse. The jittered signal, which could be the same bit-stream after it has been transported across an SDH network, has pulses that do not line up with the clock. If this clock is used to read the digital signal it will miss the fifth pulse completely, and an error will occur as a ‘0’ will be decoded and not a ‘1’ as transmitted.



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Topic 3: PerformanceConsider a signal entering a transmission system to be a known sequence of 100 bits, with two bits in error at the distant end. It is now possible to record the error rate of this path as 2 errors in 100 bits, or express it as a ratio of 2 10-2. An error ratio is commonly referred to as bit error ratio: number of bits in error bit error ratio (BER) = -------------------------- total number of bits



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Topic 3: PerformanceIn practice, an error rate would be calculated over a much longer sequence of bits. For example, if error measurements were made on a 155 Mbit/s signal then there would be a sequence of 155 million bits in one second. Typically, a signal at 155 Mbit/s may be measured over a period of one day or even one month. Such long periods would be necessary because very few errors occur with the very high performance of modern transmission systems.For a modern optical transmission system, a BER of 10-3 is regarded as failure, 10-6 is acceptable, and better than 10-8 is what would normally be expected. SDH overcomes the ‘known’ signal problem by performing bit-interleaved parity (BIP) checks on each frame.



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Bit-Interleaved parity (BIP)

The frame is divided into blocks of bits which can be organized in a column as shown below. This allows for a certain type of check known as a parity check to be carried out. Each column has an extra bit added, the value of which (0 or 1) is chosen so that the total number of 1s in that column, including the extra bit, is an even number. This is known as even parity.



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Bit-Interleaved parity (BIP)

Fig. Bit-interleaved parity.



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Bit-Interleaved parity (BIP)An even parity check is carried out on each column, and the result (11000111 in this case) is inserted in the overhead of the frame immediately following the one upon which the calculation was carried out.The parity check is repeated at the receiving end of the transmission path, where the result is compared with the result sent in the overhead. If the results of the two calculations are the same the block is recorded as error-free; if not, a block error event is



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Error performanceThe following error performance events are based upon ITU-T Recommendation G.826: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.



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Error performanceES 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.unavailable time = total time - available time



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Topic 3: PerformanceSummaryPerformance measures can be divided between loss of signal and degradation measures.SDH uses bit-interleaved parity (BIP ) to measure block errors.From the block error result it is possible to derive a range of parameters that include errored blocks (EBs), severely errored seconds (SESs) and unavailable time.



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Topic 4: SonetSynchronous optical network, known as SONET, is the standard for synchronous operation used in North America, and is defined by the ANSI T1 (American National Standards Institute – Telecommunication 1). As SDH is the internationally defined standard, SONET is technically a subset of SDH. In practice SONET and SDH share many common principles of operation and architectural concepts; in fact the SDH standard developed out of the early work on SONET, and the development of both standards was undertaken with due consideration for the other. A comparison of the transmission rates is given in Table below.



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Topic 4: Sonet



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Topic 5: Management of NetworksSDH has three features which make it suitable for management:It has dedicated overhead which can provide information to the manager.The add/drop and cross-connect functions enable network changes to be carried out at the command of the manager.The data communication channel (DCC) provides the necessary communication between the manager and SDH.Because these features are built into its architecture, SDH is well placed to operate within a managed environment.

t305: digital communications arab open university-lebanon tutorial 61 t305: digital communications...

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