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Traditional SDH/SONET
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SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical Networks) is a standard for telecommunications transport formulated by the International Telecommunication Union
(ITU). SDH/SONET based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fiber-optic cable, though SDH is also defined for use on radio relay
links, satellite links, and at electrical interfaces between equipment.
Introduced in 1992, SDH (and SONET, deployed in the United States, Canada, Korea, and Hong Kong) has been rapidly deployed throughout the world. It has been deployed at all the
levels in the network, from the access, to the core.
The motivation behind the introduction of SDH was as follows:
- Firstly, there was a need for a telecom standard that could facilitate multi-vendor interoperability. There was a significant amount spent on .interface. equipment, which
would enable multi-vendor equipment to communicate with each other. The introduction of the SDH standards ensured that all SDH network elements could communicate with each
other.
- The de-facto standard for Tele-communications was the Plesiochronous Digital Hierarchy (PDH). This meant that the timing would vary from equipment to equipment, because
their network clocks were not synchronized to a single source. In order to multiplex this type of signal, a process known as bit-stuffing is used. Bit-stuffing adds extra bits
to bring all input signals up to some common bit-rate, thereby requiring multi-stage multiplexing and demultiplexing. Because SDH is synchronous, it allows single-stage
multiplexing and demultiplexing. This single-stage multiplexing eliminates hardware complexity, thus decreasing the cost of equipment while improving signal quality.
- In PDH, the entire signal has to be demultiplexed to access a particular channel. And once the required signal was obtained, the other signals were re-multiplexed. In SDH,
only those channels that are required at a particular time are de-multiplexed, thus making it possible to add-drop individual channels.
SDH Hierarchy
| Bit-rate |
SDH |
PDH capacity |
| 51.84 Mbit/s |
STM-0 |
21 E1 |
| 155.52 Mbit/s |
STM-1 |
63 E1 |
| 622.08 Mbit/s |
STM-4 |
252 E1 |
| 2488.32 Mbit/s |
STM-16 |
1008 E1 |
| 9953.28 Mbit/s |
STM-64 |
4032 E1 |
| 39813.12 Mbit/s |
STM-256 |
16128 E1 |
Shortcomings
SDH/SONET has long served the Telecom industry. Its presence in Telecom networks is nearly ubiquitous. However, legacy SDH/SONET suffered from various shortcomings, which are
summarized below:
- Opex: Personnel costs make up a majority of operational expenses (OPEX). Service provisioning consists of the manual re-configuration required of network elements at all
layers. This requirement for manual configuration of network not only required more personnel, but also increased the time to provision.
- Inefficient Data transport: Data transport is characterized by a lack of statistical multiplexing functionality meaning data transmissions are forced into rigid 51.84
Mbps STS-1/VC-4 increments even though they often occupy less than 20 percent of the available bandwidth. This leads to scalability problems requiring either upgrading
existing rings via forklift change-outs of all infrastructure components, or adding dark fiber to deploy additional rings with additional equipment.
- Complex network: Having to procure redundant Layer 1, 2, and 3 devices to design and build-out a fully fault-tolerant metro network adds substantial capital expenditures
(CAPEX) to ATM (Asynchronous Transfer Mode) or IP over SDH/SONET MANs (Metro Access Networks). Separate Modems were required to map data onto E1.s. Also, the increased
complexity in the network meant added expenditure on the maintenance.
- Inefficient bandwidth usage: Legacy SDH/SONET utilized only 50% of the bandwidth, as the remaining was used to offer protection. If protection was not required, there was
no other option.
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