Payload or Application Rate SONET VC Group
Ethernet (frame mapped) 10 Mb/s VT1.5-7v
Fast Ethernet (frame mapped) 100 Mb/s VT1.5-64v
Gigabit Ethernet (frame mapped) 1000 Mb/s STS-3c-7v or STS-1-21v
Fiber Channel or FICON 850 Mb/s STS-3c-6v
ESCON 260 Mb/s STS-1-4v
GbE Ethernet (phy 8b/10b coded) 1250 Mb/s STS-1-25v
Infiniband (frame mapped) 2.5 Gb/s STS-3c-14v
DVB-ASI (digital video) 216 Mb/s STS-3c-2v
general purpose "bit pipe" at 160 Mb/s SONET STS-1-4v
general purpose "bit pipe" at 425 Mb/s SONET STS-3c-3v
general purpose "bit pipe" at 850 Mb/s SONET STS-3c-6v
2.5.3 Link Capacity Adjustment Scheme (LCAS)
LCAS is the signaling protocol through which the two end-nodes of a SONET-formatted lightpath can dynamically manage and reconfigure the VCG configuration of the SONET path between them, allowing them to dynamically resize the bit pipes allocated to various data flows between them as well as accommodate variable numbers of TDM circuit connections. Figure 2-9 portrays how LCAS (in conjunction with GFP and VC) opens up the SONET line-rate such as OC-48 or OC-192 to highly flexible and mixed use for both data and TDM services.
In the example, an OC-48 coming in from the left has two STS-3 and three STS-1s configured for TDM traffic. The remaining OC-48
bandwidth is available as an STS-3c-14v providing a clear bit pipe of 2.1 Gb/s under GFP for statistically multiplexed access by any desired data applications. The GFP portion of the OC-48 SONET bandwidth can be used by any combination of PDU (frame-mapped) or transparent data signal flows, subject only to suitable traffic engineering. Outgoing from the site, on the right, the balance of TDM and data may be different, so the VCG is configured accordingly. In this case the TDM component has gone down by three STS-1s, so the VCG capacity is raised to STS-3c-15v. The LCAS protocol is used to make this adjustment. GFP lets any or all of the SONET SPE function as a transparent bit-pipe for pure statistical multiplexing of any type of packet data traffic and/or transport of the intact physical layer signal of various LAN interfaces.
Figure 2-9. How SONET flexibly integrates data and TDM using GFP, VC and LCAS.
There are at least two senses in which the GFP, VC and LCAS are not only relevant as background to work in transport networking, but are directly relevant to subsequent problems in this book. One is that the simple model of aggregation of total demands in determining the number of wavelength or OC-n paths required between node pairs is made even more directly applicable to network planning. With GFP, VC, and LCAS it is not necessary to forecast precisely by application type how many paths will be needed on each node pair, only what the aggregate of all requirements is expected to be. The second is that GFP, VC and LCAS make it especially easy to adapt the configuration of each OC-n link or path to meet time-evolving requirements. In other words, the feasibility of rearranging the logical configuration of already deployed transport capacity is enhanced, enabling the application of incremental growth and rearrangement applications of the planning models that follow. For more detail on GFP, virtual concatenation and LCAS see [BoRo02] [CoMa02], and the references therein.
[ Team LiB ]
[ Team LiB ]
2.6 Optical Service Channels and Digital Wrapper
An early view of all-optical networking was that of a fully transparent network where lightpaths would follow the same wavelength assignment end-to-end without conversion into the electrical domain anywhere en route. Paths through the network itself would be completely independent of the rates and formats of any payload. Only the end nodes of each path would have to agree on the payload structure. But this also implies that only the end nodes can monitor the integrity of the lightpath and its payload. Such complete
transparency does not turn out to be workable in practice. An important attribute of SONET that turns out to be equally essential in optical networking is to have some form of channel-associated overhead signaling. SONET section, line and path overheads facilitate fault isolation, protection switching, signal identification tracing, general purpose in-band data communication channels for remote operations, maintenance and control, and so on. These are enormously useful capabilities. Yet when we contemplate an OTN, how do we achieve the same kind of signaling functionality? Optical Service Channel and Digital Wrapper are two basic options to provide the required overhead capabilities in optical networks.
2.6.1 Optical Service Channel (OSC)
The Optical Service Channel (OSC) is a designated wavelength that bears all link-associated status and signaling information. Only this designated channel need be electrically detected and processed on every fiber link, allowing other wavelength channels to pass transparently through an associated WP OXC without the expense of leaving the optical domain. The OSC approach thus permits a network to stay in the pure WP mode of operation on all other bearer optical channels. The OSC concept is summarized in Figure 2-10.
Figure 2-10. Optical Service Channel Concept: transparent optical cross-connection for most
channels, opaque processing of the OSC for link related OAM&P functions.
Most vendors provide an OSC carrier and supervisory data interface function as a specialized laser transmit/receive card that feeds into the WDM mux/demux for the corresponding fiber. Thus, an OSC can be established as desired over any DWDM fiber span. The supervisory wavelength may be out of the DWDM band (at 1310 nm, say) or can be a designated wavelength within the 1500 nm band.
Since this channel is fully terminated and processed at each node, it is not crucial which wavelength is used as the processed outgoing contents will always be placed on a new outgoing optical source. Adjacent nodes need only know which wavelength is bearing the OSC function. Some optical networking product lines have levered off the necessity of the OSC being present to use 1310 nm optics that are lower in cost and completely out of the DWDM band, but can still support an OC-48. The 1310 nm OSC channel is by default routed through all nodes of the network in a ring-like way and bears a standard OC-48 frame that is fully terminated and processed at every node.
In this way the necessity of o/e conversion for the OSC channel is used to also establish an internal LAN using the SONET data communications channel (DCC) for communication from a control center to all nodes, as well as monitoring optical integrity of each link and so on. But the OC-48 SPE, itself a non-trivial amount of bandwidth, also serves to provide for a number of "milk run" IP flows or DS-1 requirements functioning as a logical ring of OC-48 add/drop multiplexers.
2.6.2 Digital Wrapper
The Digital Wrapper (DW) approach [Brun00] is more SONET-like in the sense that every wavelength channel (not just one designated OSC per link) has overheads added to it. These overheads also adopt a section-, line- and path-like hierarchy and are meant to be detected and processed electrically at every node. Digital Wrapper is therefore much more oriented to opaque optical networks using o-e-o cross-connects. DW even implements some capabilities that were proposed but not adopted for SONET, most notably Forward Error Correction (FEC). DW also provides a framework for carrying high rate unchannelized packet or cell-based IP traffic, GbE or ATM cell flows directly over the optical layer of the network. Digital wrapper is sometimes qualitatively explained as "adding a wrapper around a wavelength." This of course has no direct physical meaning but expresses the concept that the wrapper helps safeguard the payload (especially since it includes FEC), and contains various other data about the payload itself. The notable thing is that DW is implemented in the electrical domain at the full payload bit rate as a specialized sequence of additional headers and trailers added to blocks of the payload bit sequence. The digital wrapper adds about 7 percent increase in the signal rate (relative to the payload rate alone) to include new overheads for restoration signaling, framing and FEC. The augmented electrical payload then drives a laser to put the composite
"wrapped" payload signal onto a wavelength. DW recreates virtually all of the overhead signaling that SONET provided, but does so for wavelength channels and without asserting its structure on the payload. One of the most important features of DW that SONET never did incorporate is a standardized way for protecting any lightpath payload with FEC. FEC is a feature most manufacturers had implemented in some proprietary way in their OC-192 systems.
However, the DW technique for adding overhead is different from SONET in important ways. Whereas SONET defined the frame structure, frame timing, and overheads, requiring payloads to be adapted or "mapped into" the SONET SPE, DW reverses the relationship by conforming ("wrapping") itself to the payload in a more transparent way. Any payload data sequence will appear to see a clear serial channel at a rate that is ~93.3 percent of the clock rate selected to drive the laser. This allows arrangements where the payload rate and timing is not adapted into a set OC-n rate, but rather the payload rate is independent and the clock rate for the digitally wrapped composite signal is raised slightly as needed to add the DW overhead. This eases the otherwise complex process of synchronizing/desynchronizing a constant bit-rate payload signal with a custom SONET payload mapping structure. It also allows SONET-framed signals themselves to be payloads of the DW, in which case they obtain the benefits of FEC which SONET itself never provided. The overhead consists of Optical Channel (OCh) overheads preceding a block of payload data, followed by the FEC check bit data. The FEC scheme used is an adaptation of the Reed-Solomon (255, 239) FEC coding already developed for undersea fiber optic applications (ITU G.975) in which 16 bits of FEC check data protect against single byte errors in a 255 bit block (including the check bits). The composite DW signal that is applied to a wavelength is referred to as an LnGm "lambda signal" where n.m describes the aggregate bit rate. These conceptual aspects of digital wrapper are summarized in Figure 2-11. Among the channel associated overhead capabilities that Digital Wrapper provides are:
Path trace identification
Forward and backward failure indications General purpose DCC channel
Operator specific order wire channels DW framing
Bit interleaved parity checking (BIP) Overheads reserved for future applications
A push-pop data structure recording performance information at each section along a path Protection switching protocol bytes analogous to those in SONET K1-K2 bytes
Figure 2-11. Concept of digital wrapper for wavelength-path transport of arbitrary clear-channel payloads, but with rich overhead functionality.
What is most significant about the advent of DW is that from a network planning and network operations standpoint an optical network with o-e-o cross-connects terminating and processing DW on every lightwave channel is essentially a perfect logical analogy to a SONET mesh network based on B-DCS nodes. First, wavelength assignment is not an issue; we need only address the pure capacity planning problem.
Secondly, the channel-associated signaling overheads that DW provides allow the same or better capabilities in fault sectionalization and support for distributed restoration and a variety of distributed protection preplanning schemes such as follow in the book.
Figure 2-12 gives an overview of how these new developments for data-centric payloads and optical network channel overhead schemes interrelate. The figure attempts to show all the ways a payload can wind up riding on a lightpath and the options for overhead signaling that may also apply to the lightpath. Still present for traditional PDH or SONET TDM payloads is the option of mapping into a normal SONET STS-n structure applied directly to a DWDM wavelength path. DW can optionally be applied outside the SONET frame if the benefit of FEC is desired. The clustering on LAN/SAN and IP-centric data payloads in the upper left can either be transported over existing SONET transport using GFP, in which case the added benefits of virtual concatenation and LCAS are available to efficiently size the bandwidth allocations of a SONET OC-N to a mixture of TDM and data flows. GFP allows the allocated VCG bandwidth to be used in a stat-muxed packet traffic mode or it can also carry these sources "transparently," moving the physical layer signal of the input intact. Alternately, if the bit rate or application warrant a dedicated wavelength path, the latter payloads can also be adapted for direct wavelength transport by
digital wrapper. In addition, a specific adaptation of OC-192 technology leads to a pure Ethernet transport option (10 GbE). In addition, Digital Wrapper, basic SONET framing, and OSC all provide options for network status monitoring, fault location and implementation of mesh-based survivability schemes or signaling for topology and resource discovery. All this goes hand in hand with the capacity design methods that follow, as these are the basic ways through which capacity, for both working and protection, can be managed, monitored, provisioned, configured, and cross-connected for mesh-based survivability.