ITU standards related to transport networks

Functional Elements

Modeling is the mapping of network elements onto functional elements. G.805 defines a few basic functional elements for use on the data plane.

G.805 — Generic functional architecture of transport networks

Defines basic terminology to describe network connections. Focuses on connections rather then networks. The strength is that all definitions are technology-independent, and has a good concept of the interaction between network layers. The drawbacks are that the descriptive text of the formal definition is terrible, and the mathematical model in the Appendix is even worse. Also the text does not give examples of real networks (probably on purpose), which makes the text hard to read. The model itself is very good, and should be (and is) the basis of most, if not all, models of connections. It is very important to note that one single element can be applied on different scales. For example, a CP (connection point) may describe either an interface on a device, or a device as a whole.

Defines connection related: Link Connection (LC), Network Connection (NC) (end-to-end), Trail, Subnetwork, Subnetwork Connection (SNC), Connection Point (CP), Termination Connection Point (TCP), Adaptation (conversion between layers), termination (adding reliability), multiplexing (channels), Tandem.

Defines network related: Layer network, subnetwork, link, access group.

Important figures: 1/G.805 defines basic building blocks. Note that circles in a Connection Point (CP) always lie strictly inside the CP-circle, while for a Subnetwork (SN), the CP always lie exactly on the SN-circle. This allows you to quickly recognize a CP from a SN. Table 1/G.805 is great to quickly get a feeling what element (AP, CP or TCP) to use in which circumstances. Figure 11/G.805 defines the concept of channels (multiplexing). Though a side-note of a picture, Figure 16/G.805 does give an example of multiplexing with a CP with two links. Simularly, figures 23 and 24/G.805 does show an example where a non-basic block is added to the picture.

I.326 — Functional architecture of transport networks based on ATM

Describes ATM terminology (VP, VC, etc.) using G805 functional blocks. This is not very interesting unless you are familiar with ATM as defined in ITU I.311. However, appendix I defines Multipoint connections, which is an addition to G.805. Optionally, this can be used to describe networks rather then connections, although that is not intended by the authors.

Defines: Multipoint connection Point (MPCP)

Important figures: Figure A.1/I.326 shows that a TCP can contain other functional elements.
Figure I.1/I.326 defines the basic types of multipoint connections

G.809 — Functional architecture of connectionless layer networks

Rewrite of G.805, but for connection-less instead of for connection-oriented (transport) networks. The mapping is virtually one-to-one, though in G.809, all definitions are for unidirectional datastreams, while in G.805, most definitions are bidirectional.

Defines: Access group (for connection-less layer networks), Flow (Equivalent of 805 Unidirectional connection), Flow point (Equivalant of unidirectional connection point), Flow Point Pool Link (equivalent of Link), Link Flow (Equivalent of Link Connection), Network Flow (Equivalent of Network Connection), Flow Domain (Equivalent of Subnetwork), Flow Domain Flow (Equivalent of Subnetwork connection), Termination Flow Point (Equivalent of unidirectional TCP), Connectionless Trail (Equivalent of Trail), Flow Termination (Equivalent of Trail Termination)

Important figures: 6/G.809: connection-oriented layer on top of connection-less layer and visa-versa, Table A.1/G.809: Difference in Network Characteristics between connection-oriented and connection-less networks, B.1/G.809: Mapping of terms between G.805 and G.809.

G.806 — Characteristics of transport equipment – Description methodology and generic functionality

Partly applies G.805 elements to a specific technology: mostly SDH and SDH framing over PDH, sometimes also OTN, but never Ethernet, DWDM, etc. The whole document is rather SDH-specific. It does define a number of (generic) naming schemes for specific adaption functions, fault states, etc. The strenght is that it applies the abstract terms from G.805 to a real technology. The weakness is that it is very SDH-specific, and especially the later half the document is hard to decipher without detailed knowledge about SDH.

Defines: path (= a trail in a path layer, which is not defined), section (= a trail in a section layer, which is not defined), Trail Termination function. Alse defines detailed naming scheme for fault detection (section 5.8), signal quality, etc.

Important Figures: Table 5-1/G.806: Naming scheme of some transmission layers (very much SDH/PDH based). Figure 5-3/G.806: Symbols for Interworking on the same layer. Table 5-1/G.806: Naming scheme for Transmission layers. Section 5.8: Naming scheme for fault detection. Figure 5-4/G.806: Example usage of naming scheme.

Automatically Switched Optical Network (ASON)

G.807/Y.1302 — Requirements for automatic switched transport networks (ASTN)


G.8080/Y.1304 — Architecture for the automatically switched optical network (ASON)

Optical Transport Networks (OTN)

G.872 — Architecture of optical transport networks

OpticalTransportNetworks is a whole topic in itself, and is a technology aimed at DWDM, but wants to be generic enough to be used below SDH as well. SURFnet intends to use it in SURFnet6. In relation with G.805, it does further define the multi-domain aspects of the network, by defining inter- and intradomain interfaces, where a domain is an administrative domain. However, management domains defined in G.805 and G.872 are not compatible with our view. In G.805, network elements may belong to multiple management domains, and no management domain may cross the border of a administrative domain. Both statements do not hold true for our view of management domains. Administrative domains, are however compatible with our view.

Defines: OTN, Interdomain interface, Intradomain interface, path layer network, port, tandem connection, matrix connection,

G.709/Y.1331 — Interfaces for the Optical Transport Network (OTN)

Whereas G.872 illustrates the network architecture on OTN, G.709 focuses on structure, interfaces, and mapping.


Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are TDM technologies. SDH is an ITU-T standard based on the SONET standard defined by ANSI.

G.708 — Sub STM-0 network node interface for the synchronous digital hierarchy (SDH)


G.707/Y.1322 — Network node interface for the synchronous digital hierarchy (SDH)


G.7041/Y.1303 — Generic framing procedure (GFP)


G.7042/Y.1305 — Link capacity adjustment scheme (LCAS) for virtual concatenated signals

LCAS is a method to dynamically increase or decrease the bandwidth of virtual concatenated containers

G.7043 — Virtual concatenation of plesiochronous digital hierarchy (PDH) signals

Virtual Concatenation (VCAT) allows the use of non-consequetive timeslots in SONET. Thus if you need 7 VC-4s for a 1 Gb/s Ethernet connection, you don’t need the VC-4 to be in consequetive timeslots. For example, they can go in slots 1,4,5,11,63,12,13 rather then 8,9,10,11,12,13,14.

G.831 — Management capabilities of transport networks based on the synchronous digital hierarchy (SDH)

Defines: Access Point Identifier (API)

G.803 — Architecture of transport networks based on the synchronous digital hierarchy (SDH)

G.808.1 — Generic protection switching – Linear trail and subnetwork protection

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