La planeación de 1988 a 1999

9 X N

Regenerator Section Overhead (R SO H )

Administrative Unit Pointer(s)

Multiplex Section Overhead (M S O H ) 270 X N columns (bytes) 261 X N STM -N payload 9 rows

Figure 2,2 The format of an STM-N frame

rate PDH signal. When VCs are mapped into the payload area of an STM-1 frame control information referred as the Section Overhead (SOH) is added to fill the remaining area of the STM-1 frame. The SDH multiplexing hierarchy is well discussed in the international literature and is shown in figure 2.1 [6-20].

In North America, a different synchronous transmission hierarchy has been developed, leading to the Synchronous Optical Network (SONET) standards [2,7,9]. In SONET the plesiochronous signals are mapped to Optical Carrier (OC) modules and the first level of the hierarchy is the Synchronous Transfer Signal (STS-1). SONET can be considered as a subset of the worldwide SDH standards. Interworking between SDH and SONET required the addressing of a number of issues that have been discussed in the literature [21,22].

The information type in an STM-1 frame repeats every 270 bytes, the frame can therefore be considered as a structure with 270 columns and 9 rows. The format of an

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STM-N frame is shown in figure 2.2. The first 9 columns correspond to the "Section Overhead" and the remaining 261 to the payload.

An STM-1 frame generally consists of three main sections - the Payload, the AU Pointer and the Section Overhead. The payload contains the actual data which needs to be transmitted, such as PDH signals or ATM cells. The AU Pointer is used to indicate the start of the payload data in the payload area. The Section Overhead is used for communications between different pieces of synchronous equipment, frame synchronisation and a wide variety of management and administrative duties.

2.4 SDH Equipment

SDH equipment has been defined by standards committees and is already manufactured and deployed [23-29]. Three types of SDH equipment have been developed. Multiplexers, cross-connects and line systems. SDH equipment can accept a wide range of tributaries and offer a number of possible output data rates. On the tributary side, all current plesiochronous bit rates can be accommodated, from 2Mbit/s (1.5 Mbit/s for North America) to 140Mbit/s, together with STM-1 or STM-4 tributaries, both electrical and optical. On the aggregate side, the standards define four transmission rates from STM-1 to STM-64, although STM-16 is the highest rate deployed in networks today.

ITU-T Recommendation G.782 identifies examples of SDH equipment providing combinations of SDH functions, classifying them into multiplexers (type I to IV, with sub-typing to give a total of 7 variants) and cross connects (type I to III) [23]. From a network architecture perspective, it is perhaps simpler to consider SDH equipment as belonging to one of three classes: line systems (type I.l, II. 1 and IV multiplexers), add-drop multiplexers (type 1.2, II.2, III.l and III.2 multiplexers), and digital cross connects.

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2.4.1 Add-Drop Multiplexers

An Add-Drop Multiplexer (ADM) is a type of equipment that can implement the line

termination, multiplexing and cross connect functions. ADMs can be deployed in point-to-point configurations (or as the end points of ADM chains), where their cross connect capability makes them somewhat more flexible than the simpler line systems. (In G.782, type 1.2 and II.2 multiplexers are examples of ADMs deployed in such a manner.) However, the real strength of ADMs is exploited in ring configurations. Three different types of multiplexers have been designed as network elements in SDH networks. The first type is the terminal multiplexer, which is used to multiplex sixty three primary rate PDH tributaries of 2Mbit/s into an STM-1 (155Mbit/s) aggregate signal. Terminal multiplexers are widely used at the edges of the SDH network and at the boundaries between the SDH and PDH networks, to allow primary rate signals to access the SDH network. The line terminal function is integrated in the equipment and therefore the STM-1 signal is available at a standard optical interface.

The second type of multiplexer is the higher order multiplexer. It terminates the section overhead of N STM-1 signals and multiplexes them in an STM-N aggregate signal which contains a newly formed section overhead. Many variations of this multiplexer are possible, and the tributary signals can be STM-N signals (STM-1, STM-4 or STM-16) multiplexed in an STM-M signal, where M > N.

The third type is the ADM which has 2 STM-N aggregate ports and a number of tributary ports in the add-drop side, that can be of different VC level. Variants could employ low (STM-1) or high (STM-16 or recently STM-64 [30]) aggregate ports and low (VC-12) or high (VC-4) tributary ports.

The add-drop multiplexers are the elements that form SDH resilient ring sub­ networks. The Add Drop Multiplexer equipment used in the nodes of a self healing ring architecture supports both bi-directional and unidirectional transmission. It is worth noting that one of the main causes of misunderstanding in SDH network

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planning is that the use of the term “multiplexer” obscures the fact that an ADM is functionally a cross connect.

2.4.2 Digital Cross-Connects

Cross-connect equipment in SDH networks can provide a number of key functions [30]. It is the most complex and therefore the most expensive kind of SDH equipment. The primary function of a cross-connect machine is to provide routing and path management processes for signals at the higher and lower-order path layers such as the VC-4 and VC-12 path layers. It also provides a traffic grooming function between two different layers of the multiplexing hierarchy and traffic consolidation functions within a layer. Grooming is performed by allocating low order paths in the lower tier to higher order paths in the higher tier according to their destination, type of service or protection required. Consolidation is performed by more efficiently filling (improved fill factor) high order paths with lower order path connections from other partially filled high order paths. Cross-connects can provide path level protection both to low and high order paths and test access facilities. In addition, cross-connects can be used as direct interfaces to a PDH network. PDH signals can terminate in a cross-connect and be demultiplexed and mapped into the appropriate VCs. The whole range of PDH signals can terminate at a cross-connect.

However, it is not the inclusion of the cross connect functional block(s) defined in G.782 which distinguishes the cross connects from the multiplexers, but the presence of the Higher Order and Lower Order Connection Supervision (HCS and LCS) functional blocks. Thus, we can conclude that the distinguishing feature of a DXC is its ability to provide supervision of the connections. There are two types of dedicated SDH cross connects, generally referred to as 4/1 DXCs and 4/4 DXCs.

4/1 DXCs can usually accept combinations of 2, 155 and 622 Mbit/s inputs and can cross connect VC-12s, although many 4/1 DXCs will also be able to cross connect