Question

Q3) Describe: i) SONET/SDH rings ii SONET/SDH networks.i) Frame format of SONET/SDH. Q4) Explain the Optical Transport Networks (OTN) and the main advantages over SONET/SDH systems.

Answer 1

SONET - Synchronous Optical Networking / SDH - Synchronous Digital Hierarchy

SONET/SDH

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower data rates can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH) system for transporting larger amounts of telephone calls and data traffic over the same fiber without synchronization problems. SONET generic criteria are detailed in Telcordia Technologies Generic Requirements document GR-253-CORE.Generic criteria applicable to SONET and other transmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found in Telcordia GR-499-CORE.

SONET and SDH, which are essentially the same, were originally designed to transport circuit mode communications (e.g., DS1, DS3) from a variety of different sources, but they were primarily designed to support real-time, uncompressed, circuit-switched voice encoded in PCM format.The primary difficulty in doing this prior to SONET/SDH was that the synchronization sources of these various circuits were different. This meant that each circuit was actually operating at a slightly different rate and with different phase. SONET/SDH allowed for the simultaneous transport of many different circuits of differing origin within a single framing protocol. SONET/SDH is not itself a communications protocol per se, but a transport protocol.

Due to SONET/SDH's essential protocol neutrality and transport-oriented features, SONET/SDH was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. It quickly evolved mapping structures and concatenated payload containers to transport ATM connections. In other words, for ATM (and eventually other protocols such as Ethernet), the internal complex structure previously used to transport circuit-oriented connections was removed and replaced with a large and concatenated frame (such as OC-3c) into which ATM cells, IP packets, or Ethernet frames are placed.

Racks of Alcatel STM-16 SDH add-drop multiplexers

Both SDH and SONET are widely used today: SONET in the United States and Canada, and SDH in the rest of the world. Although the SONET standards were developed before SDH, it is considered a variation of SDH because of SDH's greater worldwide market penetration.

The SDH standard was originally defined by the European Telecommunications Standards Institute (ETSI), and is formalized as International Telecommunications Union (ITU) standards G.707,G.783,G.784,and G.803.The SONET standard was defined by Telcordia and American National Standards Institute (ANSI) standard T1.105.

Frame format of Sonet/Sdh

SONET is not that different from other technologies, but hardware was manufactured to provide better configuration and reliable services to its users. SONET may use a re-generator for long haul distances. This device boosts signals that have already traveled for a long distance. Signals are transmitted into electrical signals and then re-generated into high-power signals. Add drop multiplexers (ADMs) are also common parts of SONET. ADMs are designed to fully support the network architecture of SONET.SONET is not that different from other technologies, but hardware was manufactured to provide better configuration and reliable services to its users. SONET may use a re-generator for long haul distances. This device boosts signals that have already traveled for a long distance. Signals are transmitted into electrical signals and then re-generated into high-power signals. Add drop multiplexers (ADMs) are also common parts of SONET. ADMs are designed to fully support the network architecture of SONET.

This page of SDH tutorial covers SDH frame structure and explains STS-1 SONET/SDH frame including interleaving concept, transport overhead and payload overhead part of the SDH frame in depth.

Synchronous Pavload Envelope (SPE J1 B1|E1 D1 D2 D3 C2 H1 H2 B2 K1 D4 D5 7 D8 D10 D11 D12 Z4 9 Rows H3 G1 K2 F2 D6 H4 D9 Z3 MO N1 Transport OVH Payload OVH Section overhead Line Overhead Payload overhead Data

The figure describes basic SONET STS-1 frame consisting of 9 rows and 90 columns. SONET frame is composed of 810 octets (bytes). Transmission is carried out row wise from left to right and from top to bottom. Bits are transmitted serially.

The STS-1 frame of SDH is composed of section overhead, transport overhead, payload overhead and data part. The frame starts with fixed A1/A2 bit pattern of 0xf628 used for bit/octet synchronization. SONET/SDH is referred as octet synchronous. The first three columns of SONET frame is referred as transport overhead. The next 87 columns of the frame are referred as Synchronous payload envelope (SPE). Payload overhead is part of SPE.

STS-1 data rate is about 51.84 Mbps. Let us examine how this has been achieved. Every SONET/SDH frame repeats once every 125 micro-sec. 90 columns in 9 rows and 8000 times per second and 8 bits per octet give us data rate of 51.84 Mbps. STS is the abbreviation of Synchronous Transport Signal. STS-1 is referred as OC-1(Optical Carrier) after scrambling is done on STS-1.

SDH/SONET Digital rate hierarchy table

SONET Rate Name	SDH name	Line Rate (Mbps)	Synchronous payload envelope rate(Mbps)	Transport Overhead rate(Mbps)
STS-1	None	51.84	50.112	1.728
STS-3	STM-1	155.52	150.336	5.184
STS-12	STM-4	622.08	601.344	20.736
STS-48	STM-16	2488.32	2405.376	84.672
STS-192	STM-64	9953.28	9621.504	331.776
STS-768	STM-256	39813.12	38486.016	1327.104

Interleaving in SONET/SDH

STS-3 frame is formed using three STS-1 frames with the help of interleaving technique. The interleaving is octet type i.e. A1 octet from 1st,2nd and 3rd STS-1 frame is taken first then A2 octet from all these three frames are taken and transmitted.

Transport Overhead

Framing octets (A1,A2) :
These two octets are used to determine start of the SDH frame. A1 is 0xf6 and A2 is 0x28 hexadecimal values.

Section Trace (J0) :
It is used allow connected sections to verify whether the connection is still alive and with the right terminations or not.

Pariry (B1) :
This parity octet is used by the receiver for bit error rate estimation. As this is of 8 bits, 8 parities are calculated.

Order Wire (E1) :
This is not used today. This was used by technicians to test the system while installation.

Section User Channel (F1) :
This is used by the network service provider. The octet is carried over from section to section within the line.

Section Data Communication Channel (D1,D2,D3) :
These octets form a communication channel to send administrative messages. These are considered as single 192 kbps message. Used for maintenance, control, alarm, monitor, administration and the other need of communication between section terminating equipments.

Pointers and Pointer Action (H1, H2, H3) :
These are used to point to the payload (SPE). They provide flags to indicate about changes to payload location and provide location for the data.

Line Parity (B2) :
B2 Octet is used for bit error rate estimation.

Automatic Protection Switching (APS) channel (K1,K2) :
These two octets are used for APS signalling between line level entities. APS stands for Automatic Protection Switching.

Line Data Communications Channel (D4-D12) :
D4 to D12 octets form communication channel to send administrative messages. Used for line data communication and consider as single 576kbps message based channel. Used for maintenance, control, monitoring, administration, alarms as well as communication need between line terminating entities.

Synchronization messaging (S1) :
It is used for transporting synchronization status messages and defined for STS-1 of the STS-N signal. Bits 5 to 8 are used for this purpose.

STS-1 REI(M0) :
This octet sends no. of errors detected by B octets back to the transmitter. This helps in knowing line status as well as receiver status.

STS-N REI (M1) :
The function is same as listed in M0 above.

OrderWire (E2) :
The function is same as listed above for E1.

Payload Overhead

The first column in synchronous payload envelope (SPE) is referred as Payload overhead (POH). It consists of Path Trace (J1), Path BIP-8(B3), STS Path Signal Label (C2), Path Status (G1), Path user channel (F2), Multi-frame indicator (H4), growth octets(Z3,Z4) and N1 fields.

Path Trace (J1):
It helps two ends to verify the connection status (live or not) and check whether it is connected with right terminations. It is used to transmit STS path Access point identifier repetitively, Hence path receiving terminal can verify its continued connection with intended transmitter. 64 byte frame is used for the purpose.

Path BIP-8(B3):
It is used by the receiver for BER estimation. It is calculated over all the bits of previous STS SPE before the scrambling process.

STS Path Signal Label (C2):
This indicates type of traffic carried in the payload part of the SDH frame.

Path Status (G1):
It is used to convey path terminating status/performance back to the transmitter (Originating STS PTE). PTE stands for Path Terminating Equipment.

Path user channel (F2):
It is used for user communication similar to F1 octet in transport overhead.

Multi-frame indicator (H4):
It provides generalized multi-frame indicator for the payloads. The first purpose of this indicator is for VT structured payload. The second purpose is for support of virtual concatenation of STS-1 SPEs.

Growth octets (Z3,Z4):
Reserved for future use.

N1 fields:
This octet is used to allocate support for tandem connection maintenance and tandem connection data link.

What is OTN?

Also commonly called ‘digital wrapper,’ OTN—or Optical Transport Networking—is a next-generation, industry-standard protocol that provides an efficient and globally accepted way to multiplex different services onto optical light paths.

Telecommunications industry and service provider networks must quickly evolve to deal with an explosion of digital traffic driven by multimedia services, mobile applications, social media, VoIP, and cloud computing. Plus, there’s an ever-growing array of bandwidth-hungry applications.

Network traffic used to be all about voice calls carried over circuit-based networks in a predictable network connection between pairs of endpoints. Today, most network traffic is packet-based, generated by a multitude of services and applications in bursty, unpredictable traffic patterns, with widely varying and more stringent demands on bandwidth and data transmission performance.

OTN wraps each client payload transparently into a container for transport across optical networks, preserving the client’s native structure, timing information, and management information. The enhanced multiplexing capability of OTN allows different traffic types—including Ethernet, storage, and digital video, as well as SONET/SDH—to be carried over a single Optical Transport Unit frame.

Because OTN is a fully transparent protocol, adapting existing services is pretty straightforward. OTN leaves current OSS/BSS solutions intact, utilizes all available tools and automation, and requires little to no retraining. OTN’s cost-effectiveness, ease of implementation, and simplicity offer companies a straightforward, painless solution to evolving network needs.

OTN-based backbones and metro cores offer significant advantages over traditional WDM transponder-based networks, including increased efficiency, reliability, and wavelength–based private services. The IP-over-OTN infrastructure also offers better management and monitoring, reduced hops, increased protection of services, and reduced costs for equipment acquisition. In addition to scaling the network to 100G and beyond, OTN plays a key role in making the network an open and programmable platform, enabling transport to become as important as computing and storage in intelligent data center networking.

OTN is a digital wrapper that provides an efficient and globallyaccepted way to multiplex different services onto optical light paths.

OTN has a number of advantages, including:

• Reduction in transport costs: With multiple clients transported on a single wavelength, OTN provides an economical mechanism to fill optical network wavelengths.
• Efficient use of optical spectrum: OTN facilitates efficient use of DWDM capacity by ensuring consistent fill rates across a network using OTN switches at fiber junctions.
• Determinism: OTN dedicates specific and configurable bandwidth to each service, group of services, or network partition, guaranteeing network capacity and managed performance for each client and no contention between concurrent services or users.
• Virtualized network operations: New virtualization techniques such as Optical Virtual Private Networks (O-VPNs) provide a dedicated set of network resources to a client, independent of the rest of the network.
• Flexibility: OTN networks enable operators to employ the technologies they need now while enabling the adoption of new technologies as business requirements dictate.
• Secure by design: OTN networks ensure a high level of privacy and security through hard partitioning of traffic onto dedicated circuits.
• Robust yet simple operations: OTN network management data is carried on a separate channel, completely isolated from user application data, so settings are much more difficult to access and modify through a client interface port.