OTN

Politecnico di Milano - Academic Year 2012/2013 - Mario Martinelli - Optical Communication Course - Lecture Slide -Only for Course Use - Proprietary Material - No publication or reproduction is permitted without written consensus of the Author-

3. The Optical Transport Network, OTN

The enormous grown of the traffic due to IP pushes toward the design of transport networks less opaque of SONET/SDH with respect to WDM layer. In other term it becomes soon mandatory to have a transport protocol suitable to manage the WDM colours. In fact, SONET/SDH was born essentially grey, thus there are not native information about the colours in the OH of both the protocols. In the 1999 with the Reccomandation ITU G.872 the new protocol named Optical Transport Network or OTN was born. The new protocol:

Is transparent for the client SONET/SDH as we ll as for the G-Ethernet, ATM, IP and MPLS;

Delivers path with granularity 2.5,10, and 40 Gb/s; I mprove the OAM function for all the clients.

The Optical Transport Module collects and organizes the vital information to move safely the client traffic into the Transport Net. It is divided in Analogic and Digital zones.


3. The Optical Transport Network, OTN The most important entity of the Optical Transport Module is the Optical Channel that can be defined as a optical circuit (the Optical Channel Carrier OCC) provided of frame (the Optical Transport Unit OTU). By means of the OCh the information of the client can journey through the network including the optical subsystems (Optical Amplifier, Optical Add-Drop Multiplexer, Optical Cross-Connect, WDM and so on).



Convoy: the OTU

Rail: OCC or the or the colour

In the railway paradigm, the Optical Channel is like the junction between the convoy and the rail



Optical channel green

Optical channelviolet

Optical channelblu



All the differentOch are merged In the same core of the optical fiber


3. The Optical Transport Network, OTN All the information about the allocation (the OCC) , abse nce or presence of clients, characteristic of the main sub-systems and components of the net, are contained in the Non-Associated Over-Head (thus, over-head does not associated to a par ticular ) and will be transmitted by appropriated out-of-band signalling. Viceversa, the information related to the specific OCh are allocated in the Associated Over Head of the OTU. NAOH and AOH together form the Optical Transport Module OTM. In the real networks, we are in presence of many optical circuits and many electrical clients: the result is an increasing of the OTM complexity which presents an analog and a digital sub-section, both organized with the layers paradigm.



The analogical section of the OTM is organized like the SONET OH. In fact we have three layers: Optical Transport Section, OTS, that involves the optical fiber, the optical amplifier and the optical tx/rx; Optical Multiplexing Section, OMS, that provide for the function of the multiplexing/demultiplexing (the change of the wavelength); Optical Channel that can be delivered or extracted by menas the Optical Add/Drop Multiplexers, OADM.



O/EWC



In the Figure is shown the analogy between the layers of the SONET and of the analog section of the OTM



The Digital Section of the OTM must guarantee the correct mapping of the synchronous as well as a-synchronous clinets. The Frame organization of the digital section of the OTM is similar to that of SONET with one main difference: the frames at different bit-rate have different temporal length (while in SONET/SDH the temporal length is always 125 s) Also the Digital Section of the OTM is organized in three sub-layer: The Optical Payload Unit, OPU The Optical Data Unit, ODU The Optical Transport Unit, OTU (that we have seen is joint with the OCC)



-OPU (Optical Payload Unit): adapts the different tributaries (both synchronous as a-synchronous) ; -ODU (Optical Data Unit): manages the electrical connection end-to-end; -OTU (Optical Transport Unit): inserts the FEC and provides for the signal monitoring.



The hierarchic levels of OTM are only three: 1 corresponding to 2,5 Gb/s 2 corresponding to 10 Gb/s 3 corresponding to 40 Gb/s In correspondence we have the payload OPU-k where k=1,2,3



As example let us consider the OPU-1. The frame is constituted by 3807 columns and 4 rows equivalent to 15228 bytes. The temporal length is fixed in order to recovery exactly the bytes transported by STM-16 (thus 90x9x3x16= 38880 bytes). Hence

!

152283888012510

"6 sec = 48.971 sec

The same rule is valid also for OPU-2 and OPU-3 with the sole exception of more bytes added in OPU-2 (16x4=64) and in OPU-3 (16x4x2=128) in order to fill the frame. Hence we have for OPU-2

!

15164155520125 sec =12.191 sec and for OPU-3 , 3,035 s.



Hence, the whole structure of the OTM involving both the Analog and the Digital section looks like the figure below



In the figure below the frame of SONET is compared with the frame of OTN



The whole vision of the traffic management in the OTN net is schematized in the figure below. It can be noticed that the digital information carried out by the optical channel in the network can be associated to frame with different colours. In other terms, the OTN allows the circuit switching of the colours.

OTN

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