Telecom Revolution
Telecom industries have gone through an exponential transformation since the last century. In the early 19 th century, telephone comes as a surprise addon to the mankind. At that point of time it would have been beyond imagination that the future will be connected more closely by the telecom more than the family relations. The late 19th century saw the boom in the usage of telephone in every day life of common man, then the dot com boom added to the communication revolution.
We are going through the fastest phase of life where anyting and everything is available virtually infront of you. The Internet of Things(IOT) is happening now and will be a legacy soon to be replaced by more advancements in the field. Right from ordinary conversation via wired telephone connection to the latest online shopping via smart phone, this all didn't happen overnight. The journey was piggy backed on top of power full protocols. We call them telecom protocols.
Telecom protocols are the rules and regulations followed by the telecom players/vendors to make the life easier and better for implementation, interoperabilty and usability. There are no of protocols right from the beginning of the telecom journey. Here I will focus on some important protocols which revolutionized the telecom industry. They are Plesiochronous Digital Hierarchy(PDH), Synchronous Digital Hierarchy(SDH)/Synchronous Optical Network(SONET) and Optical Transport Network(OTN).
Introduction To Digital Transmission Hierarchies
In many countries, digital transmission networks were developed based upon the standards collectively known today as Plesiochronous Digital hierarchy. Also, various parts of the world use different hierarchies which lead to problems of international interworking; for example, between those countries using 1.544 Mbits/s systems (U.S.A. and Japan) and those using the 2.048 Mbits/s systems (Europe).
In order to overcome these difficulties a new standard was formulated. SDH (Synchronous Digital Hierarchy) is formulated by the International Telecommunication Union (ITU) as a standard for telecommunications transport in 1988. It’s deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It’s based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fiber-optic cable.SDH satisfies the increasing bandwidth demand and operation, administration, maintenance and protection facilities for the network. Also lower level PDH signals can be carried in the SDH envelope, which makes them useful for supporting already existing PDH rates and demands.
Plesiochronous Digital Hierarchy (PDH).
Digital transmission systems and hierarchies was based on multiplexing signals which are Plesiochronous (running at almost the same speed) in nature. Also, there were different hierarchies in various parts of the world, this lead to problems of international interworking; for example, specialized interfacing equipments were required to interface those countries(U.S.A. and Japan) using 1.544 Mbit/s systems and those using the 2.048 Mbit/s system(Europe). Also inorder to recover a low speed 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH do step wise multiplexing and demultiplexing (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel. This is caused due to the bit-stuffing at each level.
Limitations of PDH Network
Although it has numerous advantages over analog, PDH has some shortcomings, The main limitations of PDH are:
- Inability to identify individual channels in a higher-order bit stream
- Insufficient capacity for network management;
- Most PDH network management is proprietary.
- There’s no standardized definition of PDH bit rates greater than 140 Mbit/s.
- There are different hierarchies in use around the world, specialized interface equipment is required to interwork the two hierarchies.
To overcome these limitations, they started thinking of a new multiplexing technique based on byte interleaving and with optical interface compatible with multiple vendors. The new transmission hierarchy was named Synchronous Optical Network (SONET). The International Telecommunication Union (ITU) established an international standard based on the SONET specifications, known as the Synchronous Digital Hierarchy (SDH), in 1988.
Synchronous Digital Hierarchy (SDH)
SDH (Synchronous Digital Hierarchy) is a standard for telecommunications transport formulated by the International Telecommunication Union (ITU).SDH was first introduced into the telecommunications network in 1992. It’s deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It’s based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fiber-optic cable. SDH is also defined for use on radio relay links, satellite links, and at electrical interfaces between equipment.The increased configuration flexibility and bandwidth availability of SDH provides significant advantages over the older telecommunications system.
These advantages include:
- A reduction in the amount of equipment and an increase in network reliability.
- SDH is synchronous, it allows single stage multiplexing and demultiplexing.
- The provision of overhead and payload bytes – the overhead bytes permitting management of the payload bytes on an individual basis and facilitating centralized fault sectionalisation.
- The definition of a synchronous multiplexing format for carrying lower-level digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies the interface to digital switches, digital cross-connects, and add-drop multiplexers.
- The availability of a set of generic standards, which enable multi-vendor interoperability.
- The definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.
In brief, SDH defines synchronous transport modules (STMs) for the fiber-optic based transmission hierarchy. The driving force behind these developments is the growing demand for more bandwidth, better quality of service and reliability, coupled with the need to keep costs down in the face of increasing competition.
SDH Multiplexing
SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbit/s signals with required overhead to form a frame called Synchronous Transport Module at level one (STM-1). STM-1 frame is created by 9 segments of 270 bytes each. The first 9 bytes of each segment carry overhead information; the remaining 261 bytes carry payload.
The bit rate of a framed digital signal:
Bit rate = frame rate x frame capacity
The frame rate is 8,000 frames per second.
The frame capacity of a signal is the number of bits contained within a single frame.
Frame capacity = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame
The bit rate of the STM-1 signal is calculated as follows:
Bit rate = 8,000 frames/second x 19,440 bits/frame
= 155.52 Mbit/s
STM-1 frame is divided into two parts to physically segregate the layers, where each square represents an 8-bit byte. The first nine columns comprise the section overhead (SOH), while the remainder is called the virtual container at level four (VC-4). The SOH dedicates three rows for the regenerator section overhead (RSOH) and six rows for the multiplexer section overhead (MSOH). The VC-4 contains one column for the VC-4 path overhead (VC-4 POH), leaving the remaining 260 columns for payload data (149.76 Mbit/s).
Virtual Containers can have any phase alignment within the Administrative Unit, and this alignment is indicated by the Pointer in row four. The STM frame is transmitted in a byte-serial fashion, row-by-row, and is scrambled immediately prior to transmission to ensure adequate clock timing content for downstream regenerators.
Virtual Container
SDH supports a concept called virtual containers (VC). Through the use of pointers and offset values, VCs can be carried in the SDH payload as independent data packages. VCs are used to transport lower-speed tributary signals. VC can start (indicated by the J1 path overhead byte) at any point within the STM-1 frame. The start location of the J1 byte is indicated by the pointer byte values. Virtual containers can also be concatenated to provide more capacity in a flexible fashion.
Table below lists the names and some of the parameters of the virtual containers.
SDH Overhead
The SDH standard was developed using a client/server layer approach. The overhead and transport functions are divided into layers.
They are:
- Regenerator Section
- Multiplex Section
- Path
These layers have a hierarchical relationship, with each layer building on the services provided by all the lower layers.
The SDH signal is layered to divide the responsibility for transporting the payload through the network. Each SDH network element (NE) will interpret the incoming signal and generate its overhead layer, and communicates control and status information to the same layer of the other equipment. As the payload traverse through the SDH network, each layer is associated with specific NEs named regenerator section terminating equipment (RSTE), multiplexer section terminating equipment (MSTE), or path terminating equipment (PTE).
The figure below shows network with the layered functions. The lower-rate non-SDH signal enters the SDH network by a PTE then the POH is generated, PTE may be an Access Multiplexer (AM). The POH is removed when the payload exits the SDH network by a PTE, such as an access demultiplexer (AD). The POH is first-on, last-off, so the alarm and error information contained within this layer Represents end-to-end status.
Regenerator Section Overhead
The Regenerator Section Overhead contains only the information required for the elements located at both ends of a section. This might be two regenerators, a piece of line terminating equipment and a regenerator, or two pieces of line terminating equipment. The Regenerator Section Overhead is found in the first three rows of Columns 1 through 9 of the STM-1 frame .
 |
| Regenerator section overhead |
Multiplex Section Overhead
The Multiplex Section Overhead contains the information required between the multiplex section termination equipment at each end of the Multiplex section (that is, between consecutive network elements excluding the regenerators). The Multiplex Section Overhead is found in Rows 5 to 9 of Columns 1 through 9 of the STM-1 frame.
 |
| Multiplex section overhead |
Path Overhead (VC-4/VC-3)
The Path Overhead is assigned to, and transported with the Virtual Container from the time it’s created by path terminating equipment until the payload is demultiplexed at the termination point in a piece of path terminating equipment. The Path Overhead is found in Rows 1 to 9 of the first column of the VC-4 or VC-3.Byte by byte.
SDH Performance Monitoring
Each layer in the SDH signal provides alarm and error monitoring capabilities between various terminating points in the network. Similar to 2 Mbit/s signals, parity is calculated and stored in the transmitted signal. The parity is recalculated by the receiver and verified against the stored value to determine if an error occurred during transmission.
Each layer in the SDH signal has its own Bit Interleaved Parity (BIP) calculation. If CRC framing is used at the 2 Mbit/s level, a Remote End Block Error (REBE) may be returned to the sender when a bit error is detected in the framed 2 Mbit/s PDH signal. SDH uses the same algorithm, using a layered approach. If a MSTE receives some number of multiplexer section BIP errors, it transmits the same number of multiplexer section Far End Block Errors (FEBEs) back to the originator. PTEs use the same approach in the path layer of overhead. Like 2, 34, and 140 Mbit/s PDH signals, the SDH signal also contains Alarm Indication Signals (AISs) and Remote Alarms, except that a SDH Remote Alarm is called Remote Defect Indication (RDI), and is layered like all of the other SDH results
SDH Timing Compensation
The SDH signal was designed to be timing-tolerant to support Plesiochronously timed, lower-rate signals and slight timing differences between synchronously timed NEs. Two mechanisms allow for robust timing compensation: variable bit justification of the lower-rate signal, and a technique called pointer adjustments between synchronous elements in the SDH network.
Pointer adjustments allow the VC-4 to float with respect to the SDH frame. This means that a single VC-4 payload frame typically crosses the STM-1 frame boundary. The pointer is contained in the H1 and H2 bytes of the MSOH, and it is a count of the number of bytes the VC-4 POH J1 byte is away from the H3 bytes, not including the section overhead bytes. A valid VC-4 pointer can range from 0 to 782.
When timing differences exist, dummy bytes can be inserted into the VC-4 without affecting the data. Since the pointer is adjusted to indicate where the real POH starts, the receiving end can effectively recover the payload (i.e., ignore the dummy bytes). When justified bytes are used, they are always in the same location, regardless of where the POH starts. H3 bytes are called negative justification bytes and carry real payload data for one frame during a pointer decrement. The three bytes following the last H3 byte in the VC-4 are called positive justification bytes and carry three dummy bytes of information for one frame during a pointer increment.
If there is no timing difference between two nodes, the incoming STM-1 payload bit rate is identical to the transmitting source that drives the outgoing STM-1 frame rate, so no pointer adjustments are needed. Figure below shows a SDH node that has an incoming frequency f1 and an outgoing frequency f2. If f1 is less than f2, there is a constant lack of payload data to place into the outgoing SDH signal. To compensate, three dummy bytes are placed into the positive stuff bytes and the data is moved to the right by three bytes, so the VC-4 pointer is incremented by one. On the other hand, if f1 is greater than f2 then three extra VC-4 payload bytes are stored into the negative stuff bytes, H3, in the MSOH for one frame, while the entire payload data is moved to the left by three bytes and the pointer is decreased by one. The only equipment that can perform path pointer adjustments is MSTE, since the pointer value is contained in the MSOH. Also, path pointer adjustments are not performed by PTE (where the payload data enters the SDH network) even though there are potential timing differences at these locations as well. The timing differences at PTEs are due to Plesiochronously-timed tributary signals and are corrected by traditional bit justification techniques.
The multiplexing principles of SDH follow, using these terms and definitions:
- Mapping – A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information.
- Aligning – This process takes place when a pointer is included in a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located.
- Multiplexing – This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.
- Stuffing – As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all the various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this spare capacity is filled with “fixed stuffing” bits that carry no information, but are required to fill up the particular frame.
The ITU-T recommended SDH multiplexing structure is shown below in the figure.
At the lowest level, containers (C) are input to virtual containers (VC). The purpose of this function is to create a uniform VC payload by using bit-stuffing to bring all inputs to a common bit-rate ready for synchronous multiplexing. Various containers (ranging from VC-11 at 1.728 Mbit/s to VC-4 at 150.336 Mbit/s) are covered by the SDH hierarchy.
Next, VCs are aligned into tributary units (TUs), where pointer processing operations are implemented. These initial functions allow the payload to be multiplexed into TU groups (TUGs). As Figure 4 illustrates, the xN label indicates the multiplexing integer used to multiplex the TUs to the TUGs. The next step is the multiplexing of the TUGs to higher level VCs, and TUG-2 and TUG-3 are multiplexed into VC-3 (ANSI mappings) and VC-4. These VCs are multiplexed with fixed byte-stuffing to form administration units (AUs) which are finally multiplexed into the AU group (AUG) as shown in the figure below. This payload then is multiplexed into the Synchronous Transport Module (STM).
