The frame consists of two parts, the transport overhead and the path virtual envelope.
Transport overhead
The transport overhead is used for signaling and measuring transmission error rates, and is composed as follows:
Section overhead - called RSOH (regenerator section overhead) in SDH terminology: 27 octets containing information about the frame structure required by the terminal equipment.
Line overhead - called MSOH (multiplex section overhead) in SDH: 45 octets containing information about alarms, maintenance and error correction as may be required within the network.
Pointer – It points to the location of the J1 byte in the payload.
Path virtual envelope
Data transmitted from end to end is referred to as path data. It is composed of two components:
Payload overhead (POH): 9 octets used for end to end signaling and error measurement.
Payload: user data (774 bytes for STM-0/STS-1, or 2340 octets for STM-1/STS-3c)
For STS-1, the payload is referred to as the synchronous payload envelope (SPE), which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of 756 bytes.
The STS-1 payload is designed to carry a full PDH DS3 frame. When the DS3 enters a SONET network, path overhead is added, and that SONET network element (NE) is said to be a path generator and terminator. The SONET NE is said to be line terminating if it processes the line overhead. Note that wherever the line or path is terminated, the section is terminated also. SONET regenerators terminate the section but not the paths or line.
An STS-1 payload can also be subdivided into 7 VTGs (virtual tributary groups). Each VTG can then be subdivided into 4 VT1.5 signals, each of which can carry a PDH DS1 signal. A VTG may instead be subdivided into 3 VT2 signals, each of which can carry a PDH E1 signal. The SDH equivalent of a VTG is a TUG2; VT1.5 is equivalent to VC11, and VT2 is equivalent to VC12.
Three STS-1 signals may be multiplexed by time-division multiplexing to form the next level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbps. The multiplexing is performed by interleaving the bytes of the three STS-1 frames to form the STS-3 frame, containing 2,430 bytes and transmitted in 125 microseconds.
Higher speed circuits are formed by successively aggregating multiples of slower circuits, their speed always being immediately apparent from their designation. For example, four STS-3 or AU4 signals can be aggregated to form a 622.08 Mbps signal designated as OC-12 or STM-4.
The highest rate that is commonly deployed is the OC-192 or STM-64 circuit, which operates at rate of just under 10 Gbps. Speeds beyond 10 Gbps are technically viable and are under evaluation. [Few vendors are offering STM-256 rates now, with speeds of nearly 40Gbps]. Where fiber exhaustion is a concern, multiple SONET signals can be transported over multiple wavelengths over a single fiber pair by means of wavelength-division multiplexing, including dense wavelength division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM circuits are the basis for all modern transatlantic cable systems and other long-haul circuits.
SONET/SDH and relationship to 10 Gigabit Ethernet
Another circuit type amongst data networking equipment is 10 Gigabit Ethernet (10GbE). This is similar to the line rate of OC-192/STM-64 (9.953 Gbps). The Gigabit Ethernet Alliance created two 10 Gigabit Ethernet variants: a local area variant (LAN PHY), with a line rate of exactly 10,000,000 kbps and a wide area variant (WAN PHY), with the same line rate as OC-192/STM-64 (9,953,280 kbps). The Ethernet wide area variant encapsulates its data using a light-weight SDH/SONET frame so as to be compatible at low level with equipment designed to carry those signals.
However, 10 Gigabit Ethernet does not explicitly provide any interoperability at the bitstream level with other SDH/SONET systems. This differs from WDM system transponders, including both coarse and dense WDM systems (CWDM, DWDM) that currently support OC-192 SONET signals, which can normally support thin-SONET framed 10 Gigabit Ethernet.
SONET/SDH data rates
SONET/SDH Designations and bandwidths
SONET Optical Carrier Level
SONET Frame Format
SDH level and Frame Format
Payload bandwidth (kbps)
Line Rate (kbps)
OC-1
STS-1
STM-0
50,112
51,840
OC-3
STS-3
STM-1
150,336
155,520
OC-12
STS-12
STM-4
601,344
622,080
OC-24
STS-24
–
1,202,688
1,244,160
OC-48
STS-48
STM-16
2,405,376
2,488,320
OC-192
STS-192
STM-64
9,621,504
9,953,280
OC-768
STS-768
STM-256
38,486,016
39,813,120
OC-3072
STS-3072
STM-1024
153,944,064
159,252,480
In the above table, payload bandwidth is the line rate less the bandwidth of the line and section overheads. User throughput must also deduct path overhead from this, but path overhead bandwidth is variable based on the types of cross-connects built across the optical system.
Note that the data rate progression starts at 155Mb/s and increases by multiples of 4. The only exception is OC-24 which is standardised in ANSI T1.105, but not a SDH standard rate in ITU-T G.707. Other rates such as OC-9, OC-18, OC-36, and OC-96, and OC-1536 are sometimes described, but it is not clear if they were ever deployed, and are certainly not common, and are not standards compliant.
The next logical rate of 160 Gb/s OC-3072/STM-1024 has not yet been standardised, due to the cost of high-rate transceivers and the ability to more cheaply multiplex wavelengths at 10 and 40 Gb/s.
Physical layer
The physical layer actually comprises a large number of layers within it, only one of which is the optical/transmission layer (which includes bitrates, jitter specifications, optical signal specifications and so on). The SONET and SDH standards come with a host of features for isolating and identifying signal defects and their origins.
SONET/SDH network management protocols
SONET equipment is often managed with the TL1 protocol. TL1 is a traditional telecom language for managing and reconfiguring SONET network elements. TL1 (or whatever command language a SONET Network Element utilizes) must be carried by other management protocols, including SNMP, CORBA and XML.
There are some features that are fairly universal in SONET Network Management. First of all, most SONET NEs have a limited number of management interfaces defined. These are:
Electrical interface. The electrical interface (often 50 Ω) sends SONET TL1 commands from a local management network physically housed in the Central Office where the SONET NE is located. This is for "local management" of that NE and, possibly, remote management of other SONET NEs.
Craft interface. Local "craftspersons" can access a SONET NE on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be hooked-up to a console server, allowing for remote out-of-band management and logging.
SONET and SDH have dedicated data communication channels (DCC)s within the section and line overhead for management traffic. Generally, section overhead (regenerator section in SDH) is used. According to ITU-T G.7712, there are three modes used for management:
IP-only stack, using PPP as data-link
OSI-only stack, using LAP-D as data-link
Dual (IP+OSI) stack using PPP or LAP-D with tunneling functions to communicate between stacks.
An interesting fact about modern NEs is that, to handle all of the possible management channels and signals, most NEs actually contain a router for routing the network commands and underlying (data) protocols.
The main functions of network management include:
Network and NE provisioning. In order to allocate bandwidth throughout a network, each NE must be configured. Although this can be done locally, through a craft interface, it is normally done through a network management system (sitting at a higher layer) that in turn operates through the SONET/SDH network management network.
Software upgrade. NE software upgrade is in modern NEs done mostly through the SONET/SDH management network.
Performance management. NEs have a very large set of standards for Performance Management. The PM criteria allow for monitoring not only the health of individual NEs, but for the isolation and identification of most network defects or outages. Higher-layer Network monitoring and management software allows for the proper filtering and troubleshooting of network-wide PM so that defects and outages can be quickly identified and responded to.
Equipment
With recent advances in SONET and SDH chipsets, the traditional categories of NEs are breaking down. Nevertheless, as network architectures have remained relatively constant, even newer equipment (including "Multiservice Provisioning Platforms") can be examined in light of the architectures they will support. Thus, there is value in viewing new (as well as traditional) equipment in terms of the older categories.
Regenerator
Traditional regenerators terminate the section overhead, but not the line or path. Regenerators extend long haul routes in a way similar to most regenerators, by converting an optical signal that has already traveled a long distance into electrical format and then retransmitting a regenerated high-power signal.
Since the late 1990s, regenerators have been largely replaced by optical amplifiers. Also, some of the functionality of regenerators has been absorbed by the transponders of wavelength-division multiplexing systems.
Add-drop multiplexer
Add-drop multiplexers (ADMs) are the most common type of NEs. Traditional ADMs were designed to support one of the network architectures, though new generation systems can often support several architectures, sometimes simultaneously. ADMs traditionally have a "high speed side" (where the full line rate signal is supported), and a "low speed side", which can consist of electrical as well as optical interfaces. The low speed side takes in low speed signals which are multiplexed by the NE and sent out from the high speed side, or vice versa.
Digital cross connect system
Recent digital cross connect systems (DCSs or DXCs) support numerous high-speed signals, and allow for cross connection of DS1s, DS3s and even STS-3s/12c and so on, from any input to any output. Advanced DCSs can support numerous subtending rings simultaneously.
Network architectures
Currently, SONET (and SDH) have a limited number of architectures defined. These architectures allow for efficient bandwidth usage as well as protection (i.e. the ability to transmit traffic even when part of the network has failed), and are key in understanding the almost worldwide usage of SONET and SDH for moving digital traffic. The three main architectures are:
Linear APS (automatic protection switching), also known as 1+1: This involves 4 fibers: 2 working fibers (1 in each direction), and two protection fibers. Switching is based on the line state, and may be unidirectional, with each direction switching independently, or bidirectional, where the NEs at each end negotiate so that both directions are generally carried on the same pair of fibers.
UPSR (unidirectional path-switched ring): In a UPSR, two redundant (path-level) copies of protected traffic are sent in either direction around a ring. A selector at the egress node determines the higher-quality copy and decides to use the best copy, thus coping if deterioration in one copy occurs due to a broken fiber or other failure. UPSRs tend to sit nearer to the edge of a network and, as such, are sometimes called "collector rings". Because the same data is sent around the ring in both directions, the total capacity of a UPSR is equal to the line rate N of the OC-N ring. For example if we had an OC-3 ring with 3 STS-1s used to transport 3 DS-3s from ingress node A to the egress node D, then 100% of the ring bandwidth (N=3) would be consumed by nodes A and D. Any other nodes on the ring, say B and C could only act as pass through nodes. The SDH analog of UPSR is subnetwork connection protection (SNCP); however, SNCP does not impose a ring topology, but may also be used in mesh topologies.
BLSR (bidirectional line-switched ring): BLSR comes in two varieties, 2-fiber BLSR and 4-fiber BLSR. BLSRs switch at the line layer. Unlike UPSR, BLSR does not send redundant copies from ingress to egress. Rather, the ring nodes adjacent to the failure reroute the traffic "the long way" around the ring. BLSRs trade cost and complexity for bandwdith efficiency as well as the ability to support "extra traffic", which can be pre-empted when a protection switching event occurs. BLSRs can operate within a metropolitan region or, often, will move traffic between municipalities. Because a BLSR does not send redundant copies from ingress to egress the total bandwidth that a BLSR can support is not limited to the line rate N of the OC-N ring and can actually be larger than N depending upon the traffic pattern on the ring. The best case of this is that all traffic is between adjacent nodes. The worst case is when all traffic on the ring egresses from a single node, i.e. the BLSR is serving as a collector ring. In this case the bandwidth that the ring can support is equal to the line rate N of the OC-N ring. This is why BLSRs are seldom if ever deployed in collector rings but often deployed in inter-office rings. The SDH equivalent of BLSR is called Multiplex Section-Shared Protection Ring (MS-SPRING).
Synchronization
Clock sources used by synchronization in telecommunications networks are rated by quality, commonly called a 'stratum' level. Typically, a network element (NE) uses the highest quality stratum available to it, which can be determined by monitoring the synchronization status messages(SSM) of selected clock sources.
As for synchronization sources available to an NE, these are:
Local external timing. This is generated by an atomic Caesium clock or a satellite-derived clock by a device in the same central office as the NE. The interface is often a DS1, with sync status messages supplied by the clock and placed into the DS1 overhead.
Line-derived timing. An NE can choose (or be configured) to derive its timing from the line-level, by monitoring the S1 sync status bytes to ensure quality.
Holdover. As a last resort, in the absence of higher quality timing, an NE can go into "holdover" until higher quality external timing becomes available again. In this mode, an NE uses its own timing circuits as a reference.
Timing loops
A timing loop occurs when NEs in a network are each deriving their timing from other NEs, without any of them being a "master" timing source. This network loop will eventually see its own timing "float away" from any external networks, causing mysterious bit errors and ultimately, in the worst cases, massive loss of traffic. The source of these kinds of errors can be hard to diagnose. In general, a network that has been properly configured should never find itself in a timing loop, but some classes of silent failures could nevertheless cause this issue
Next-generation SONET/SDH
SONET/SDH development was originally driven by the need to transport multiple PDH signals like DS1, E1, DS3 and E3 along with other groups of multiplexed 64 kbps pulse-code modulated voice traffic. The ability to transport ATM traffic was another early application. In order to support large ATM bandwidths, the technique of concatenation was developed, whereby smaller multiplexing containers (eg, STS-1) are inversely multiplexed to build up a larger container (eg, STS-3c) to support large data-oriented pipes.
One problem with traditional concatenation, however, is inflexibility. Depending on the data and voice traffic mix that must be carried, there can be a large amount of unused bandwidth left over, due to the fixed sizes of concatenated containers. For example, fitting a 100 Mbps Fast Ethernet connection inside a 155 Mbps STS-3c container leads to considerable waste. More important is the need for all intermediate NEs to support the newly introduced concatenation sizes. This problem was later overcome with the introduction of Virtual Concatenation.
(Virtual concatenation (VCAT) allows for a more arbitrary assembly of lower order multiplexing containers, building larger containers of fairly arbitrary size (e.g. 100 Mbps) without the need for intermediate NEs to support this particular form of concatenation. Virtual Concatenation increasingly leverages X.86 or Generic Framing Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the virtually concatenated container.
Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the bandwidth via dynamic virtual concatenation, multiplexing containers based on the short-term bandwidth needs in the network.
The set of next generation SONET/SDH protocols to enable Ethernet transport is referred to as Ethernet over SONET/SDH (EoS).
