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Chapter 4

4.1    SDH

SDH (Synchronous Digital Hierarchy) is the next generation of transmission technology after plesiochronous era. SDH gas a well-defined interface to existing plesiochronous networks, which is the basic message of the multiplexing and mapping structure. In addition to being a solution that integrates to and can also grow with the plesiochronous networks where needed new SDH networks provide very high flexibility and improved management possibilities to meet the needs of the future.

The basic building block of SDH network is the STM-1 frame with its extensive overhead capacity. The signal being synchronous, gives the possibility of direct access to its tributaries. For this reason, as an example it is possible to avoid “multiplexer mountains” at stations where only a few tributaries have to be dropped from the link passing by. In the MTT transmission network the STM-1 interface is used for SDH transmission.

4.2 SDH Frame Structures

Fig 4.1 SDH frame format

In SDH, signals are transported in the STM-1 (synchronous Transport Module) frame comprising 270 vertical columns and 9 horizontal rows. One element in the frame is one byte, or 8 bits. Duration of the frame is 125 micro seconds, or the same as the sampling frequency of a single 64 Kbit/s channel. 

With these data we can calculate the speed of the STM-1 signal as ;

                                  9 x 270  =  2430 bytes

                         2430 x 8 bits  =  19440 bits

               19440 bits / 125 ms  =  155.52 Mbit/s

The first 9 columns are the overhead field of the frame (SOH = Section Overhead). It contains the frame alignment word, error check sum, pointer for the payload start location, and the auxiliary channels.

STM-1 OVERHEAD AREAS

Fig 4.2 STM-1 Overhead areas

Both the multiplexer (MSOH) and the regenerator section (RSOH) overheads are located in the section overhead field (SOH) of the STM-1 frame. The path overhead (POH) belongs to a part in the virtual container (VC-4). POH reserves all 9 rows of the first column. RSOH reserves rows 1 to 3 and MSOH rows 5 to 9. Row 4 is used by the pointers.

4.3  MTT’s SDH PATH

Fig 4.3– MTT’s SDH Network

4.4 Radios Used In SDH

For the SDH transmission network MTT is using the NERA - NL290 Radio. This is a STM1 class  (155 Mbit/s) Radio produced by the NERA Telecommunications. They have different frequency bands according to the equipment code.

Fig 4.4 - NL290 NERA Radio             

The equipment codes that are used in MTT SDH transmission network are NL 291 and NL 294. They have following features.

• Compact slim rack construction.
• High reliability
• Advanced adaptive equalizers.
• Integrated supervision system.
• Flexible system design
• Space diversity receiver
• Automatic error free protection switching system.
• Co-channel operation with XPIC.
• Drop and insert facilities.

This STM-1 radio-relay equipment is designed to provide transmission performance in compliance with ITU-T recommendations for a high-grade system.

The versatile slim rack construction allows for an extensive range of equipment configurations to meet operational demands. High reliability combined with flexible system design has been achieved through applying advanced component technology.

• Surface mounted components.
• High-density integrated microwave circuits.
• High-density integrated signal processing circuits (VLSI)
• Microprocessor controlled operation.

This system can be equipped with 2.048 Mbit/s channels for wayside traffic (one 2.048 <bit/s for each RF-channel). Several 64 Kbit/s digital service channels are available in addition to the internal service channels for supervision and protection switching.

4.5 Operation Modes of the NERA Radio

There are several modes, which can be used in transmission networks. A transmission system can be set up using following modes.

·         Single terminal

·         Hot Stand-By (HSB)

·         Warm Stand-By (WSB)

·         Hot Stand-By + Space diversity

·         Space diversity with 2 Tx

·         Frequency diversity

In the MTT transmission network, they use following modes

·         Single terminal

·         Hot stand-By (HSB)

·         Hot Stand-By + Space diversity

Chapter 5 >>