The structure of the Plesiochronous Digital Hierarchy (PDH), and how this carried on Synchronous Digital Hierarchy
What does SDH mean?
Synchronous digital hierarchy: It is a transmission protocol based on sending and receiving information in the form of digital bits on fiber cables. (That is why we call it the optical transmission protocol)
What is the difference between it and the PDH?
Plesiochronous digital hierarchy, it works with more than one standards, for example
E, T, J Europe, America and Japan.
The E1 is standard:
A copper cable with digital information, a data transfer rate of 2 megabits per second
Because the frame has a 32-time slot, it takes 8000 frame / sec, and 8 bit / byte
Date rate = 32 *8000*8 = 2.048 mbps
And there are:
• E2 = 4 E1
• E3 = 4 E2
• E4 = 4 E3
We take the E2, E3, and E4 from the E1 by multiplexer
The highest data rate in the system is 140 mbps
Fig. 1 PDH Plesiochronous digital hierarchy
Each group works on its own standard, so we do not achieve compatibility
Note:
We mean in compatibility: the compatibility between the equipment and the standard
i.e. between the countries that manufacture the cabins and the countries that download the standard for any new technology.
The benefit of achieving compatibility
It is very important to unify the standards because all the cabins understand each other and can install any types of equipment and you will not have a problem in that they understand each other's signals and are able to deal with each other easily
Other difficulties in the E system?
• The data transfer rate is relatively low
• The cost of the network is high, so many multiplexers are needed
• To solve this problem, we use SDH
Note:
We know that the lowest data rate in the SDH is higher than the highest data rate in the PDH
In the PDH
It is available to use the following:
E1, E2, E3 and E4
The highest data transfer rate is 140 megabits per second
In the SDH
It is available to use the following:
STM-1, STM-4, STM-16 …. UP TO STM-N
The minimum data transfer rate in the STM-1 is 155 Mbps
SDH Rates:
• STM-1 = 155.520 Mbit/s
• STM-4 = 622.080 Mbit/s
• STM-16 = 2,488.320 Mbit/s (~2.5 Gbit/s)
• STM-64 = 9,953.280 Mbit/s (~10 Gbit/s)
Thus, it is as if you were taking information from more than E1 and collecting them in stm-1
Put the bits into the Matrix
9 rows * 270 columns
Each square in this matrix is a byte, which is why they call it byte interleaved multiplexing
STM-1 (Synchronous Transport Module level-1)
The STM-1 base frame is structured with the following characteristics:
• Length: 270 column x 9 row = 2430 bytes
• Byte: 1- byte = 64kbit/s speech channel
• Duration (Frame repetition time): 125 μs i.e. 8000 frame/s
• Rate (Frame capacity): 2430 x 8 x 8000 = 155.520 Mbit/s
• Payload = 2349bytes x 8bits x 8000frames/sec = 150,336 Mbit/s
Thus, the data return will be:
• STM-1 data rate = 9 rows * 270 colums * 8 bit / byte * 8000 frame / sec = 155.5 mbps
Fig. 2 STM-1 frame
To transfer from STM, E1 and vice versa, we use ADM (Add drop multiplexer), and so from STM to e2, e3, e4.
The STM frame as shown in Fig.3 consists of:
A regenerator section over head
Bits we use them for error detection and correction, coding, synchronization
Multiplexer section over head
Bits we use them in the process of multiplexing, DE multiplexing in the frame
Administrative unit pointer
Bits we use them as an address to consult the beginning and end of the data pay load
Path over head
Information about the path of the data and this is when I put it in the payload, as far as we can say that it is a VC virtual circuit
Fig. 3 STM frame
Thank you.
