SDH differs from Plesiochronous Digital Hierarchy (PDH) as it uses exact rates to transport data on the SONET / SDH are securely synchronized on the entire network by use of atomic clocks. The synchronization system permits inter-nation networks to operate synchronously thus helping to reduce significantly the amount of buffering required between network elements.
Protocol overview
Synchronous Optical Network and Synchronous Digital Hierarchy refers to standard protocols which transfer multiple digital bit streams simultaneously in optical fibers through the use of lasers or LEDs. In the case of low data transmissions rates, the data can be transmitted through an electrical interface. Both SONET and SDH were developed initially to aid in transport circuit mode communications from various sources. They were originally designed to offer support for a real time, the circuit-switched voice in PCM encoded format.
SONET differs from SDH in that it allows delivery of broad scope of protocols which include ATM, Ethernet, TCP/IP, and telephony traffic. It does not, therefore, qualify to be a native communication protocol and should thus be not confused as being connection-oriented. This protocol is majorly a multiplexed structure with its header intertwined between data in a sophisticated manner thus allowing very low latency. The SDH standard was defined initially by the European Telecommunications Standards Institute, later formalized to International Telecommunications Union (ITU) standards namely G.707, G.783, G.784, and G.803. The SONET protocol was then named by the Telcordia and American National Standards Institute (ANSI) standard T1.105.
Basic unit of transmission
The basic framing unit in an SDH is the STM-1 (Synchronous Transport Module, level 1). It operates at a speed of 155.520 megabits per second. SONET, on the other hand, refers to the unit as Synchronous Transport Signal 3, concatenated (STS-3c). SONET, however, offers a new transmission basic unit; the STS-1/ OC-1 which operates at 51.84 Mbit/s.
Framing
Frames in both SONET and SDH are represented through displaying them graphically as blocks of 90 columns, nine rows for STS-1 and a total of 270 columns and nine rows for the STM-1/STC-3c.
Transport overhead
It is used in the signaling and measurement of transmission error rates. It is composed of a section overhead, line overhead, an administrative unit pointer, a virtual path envelope, a payload overhead and a payload/user data (774 bytes for STM-0/STS-1, or 2,340 octets for STM-1/STS-3c)
10 Gigabit ethernet relationship with SONET and SDH
Gigabit Ethernet Alliance created LAN (10.3125 Gbit/s) and WAN (9,953,280 Kbit/s) PHY variants. The later condenses Ethernet data by use of lightweight SDH/SONET frame thus allowing compatibility with low-level equipment which has the ultimate design of carrying SDH/SONET signals. The LAN PHY condenses data by use of 64B/66B line coding.
The 10 Gigabit Ethernet, however, does not provide bitstream level interoperability with other SDH/SONET systems.
SONET/SDH data rates
Their data rates progression begins at 155 Mbits and gradually increases by multiples of four except 0C-24 which is an ANSI T1.105. The highest data transmission rate is the OC-768 or STM-256 circuit with operations of 38.5 Gbit/s.
Both SDH and SONET are extensively used nowadays. SONET is mostly utilized in the United States and Canada whereas SDH is employed in other countries in the world. Although standards of SONET were developed before SDH, it is referred and considered a variation because of SDHs immense global market penetration.
SONET/SDH network management protocols
They are used to configure and monitor SDH and SONET equipment remotely and locally.
References
https://en.wikipedia.org/wiki/Synchronous_optical_networking
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