Optical Transport Module (OTM-n.m, OTM-nr.m)

The OTM-n is the output from the OMS layer and is actually the signal that will be output onto the fiber. The OTM-m signal is the result of multiplexing all of the OCh payloads with the OSC signal (OOS). 

The OSC signal contains information related to:  

•The state of the clients' signals 

•Maintenance and operation functions of the OMS 

•Maintenance and operation functions of the OTS 

•General management communication between network equipment  

Putting it all together we have the visual representation below

The OTM also has a reduced version represented as OTM-nr.m where r represents reduced functionality. The primary difference between the two OTM-n.m and OTM-nr.m is that the reduced functionality version has no OSC. 

Optical Channel (OCh, OChr)

Now that the system has created all the information required and prepared it in a container of appropriate size for it to cross an OTH network and if necessary, for it to be regenerated, it is a requirement to provide an additional transparent network connection. The requirement for this transparent connection is due to the fact that to be able to direct and process client information - which is inside an OTUk remember – the system must be able to access the individual wavelength state without performing a full optical to electrical conversion. 

The way the OTH standard handles this is to use the OCh OH but instead of inserting it into the client carrier wavelength they utilize another wavelength channel altogether and this is the purpose of the OSC (optical supervisor channel) that has been mentioned in passing on several occasions. 

The OSC is carried on a frequency out with the carrier wavelength spectrum so that it does not interfere with the carrier wavelengths. Furthermore all vendor active network equipment has electrical access to this OSC channel. The OSC is the only signal, which all OTH compliant equipment can access.

Consequently if any piece of equipment on the optical path detects a client signal failure (loss of signal) it will process the corresponding OCh OH to inform the other system equipment about the state of the client wavelength. 

Note: There are two versions of the optical channel OCh & OChr.  The non-standard version is called OChr, where 'r' represents the reduced version that does not insert an OCh OH into the OSC wavelength.

Optical Channel Transport Unit (OTUk, OTUkV)

With the introduction of an overhead into each client signal as is representative of that signal only, it becomes necessary to introduce a further OH that will provide information to equipment handling regeneration. The information the equipment uses to perform this task is inserted into the OTU overhead (OTU OH). This information relates to the maintenance and operation functions to support the optical channel section. 

These functions include:  

•Functionalities of section monitoring 

•General communication channel 

•Reserved bits 

This unit also contains information that defines the use of a FEC (forward error control) as a way to improve signal quality and reliability. This unit is represented as follows: 

ODUk	OTUk
ODU1	OTU1
ODU2	OTU2
ODU3	OTU3
ODU4	OTU4

The OTU terminates at the point of assembly/disassembly and this is identical to the regeneration overhead in SDH. Note: The standard defines that the use of several types of FEC is possible though the only one standardized at the time was RS (255,239) the type of FEC used now though is based on Reed-Solomon and it is used to define the OTUk. Other possibilities of FEC are inserted into the OTUkV. 

ODUflex

ODUflex was introduced into the G.709 standards in 2009 

Two flavors of ODUflex standardization  

1.Circuit ODUflex 

 •Supports any possible client bit rate as service in circuit transport networks 

•CBR clients use a bit-sync mapping into ODUflex (239/238xthe client rate)  

2.Packet ODUflex 

•Supports variable length packet trunks for transporting packet flows using layer 0 switching 

•Theoretically can be of any size but in practice is usually set to be multiples of the lowest tributary slot size in the network  In order to support ODUflex a new ODU was created called ODU2e ODU2e Definition New Low Order (LO) tier of the hierarchy (Oct 2009) to transport “proprietary” 10G signals  

•Serves as a logical wrapper for 10GBASE-R when carried over a standardized physical layer of OTU3 or OTU4 

•Part of compromise made to enable standards progress - most commonly deployed 

•“proprietary” transparent mapping of 10GBASE-R 

•Over-clocked physical OTU2e signal remains in G.sup43 

•Can map ´10 into OPU4 (which is sized to carry 100GBASE-R) 

•Can map as ODUflex in 9´1.25G OPU3 tributary slots (up to 3´ODU2e per OPU3)  

OPU2e can carry:  

•10GBase-R 

•Transcoded FC-1200 

ODU1 and ODU2 Definition

 ODU1 Definition 

Original tier of the hierarchy to transport 2.5G signals  

•ODU1 = 2.498775Gbit/s 

•OTU1 = 2 666057Gbit/s 

 OTU1 can be used as a higher order ODU to carry lower order ODU0s  

•Divided into 2x1.25G tributary slots: 

•ODU0 maps into 1 tributary slot 

 OPU1 can carry:  

•STS-48 

•STM-16 

•FC-200 

 ODU2 Definition 

Original tier of the hierarchy to transport 10G signals  

•ODU2 = 10.037273Gbit/s 

•OTU2 = 10 709224Gbit/s 10.709224Gbit/s 

Can be used as a higher order ODU to carry lower order ODUs  

•Divided into 4´2.5G or 8´1.25G tributary slots: 

•ODU0 maps into 1 tributary slot 

•ODU1 maps into 1 2.5G or 2 1.25G tributary slot(s)  OPU2 can carry:  

•STS-192 

•STM-64 

OMS/OCh Adaptation

The bidirectional OMS/OCh adaptation (OMS/OCh_A) functions are performed by a co-located pair of sources and sink OMS/OCh adaptation functions. The OMS/OCh adaptation source (OMS/OCh_A_So) performs the following processes between its input and its output:   

•modulation of an optical carrier signal by means of a defined modulation scheme 

•wavelength (or frequency) and power allocation to the optical carrier 

•optical channel multiplexing to form an optical multiplex  

The OMS/OCh adaptation sink (OMS/OCh_A_Sk) performs the following processes between its input and its output:  

•optical channel demultiplexing according to carrier wavelength (or frequency) 

•termination of the optical carrier and recovery of the optical transport unit 

ODU0 Definition

Smallest container defined in G.709 (OTN Standard) 1.25G container size (specifically 1.244160 Gbit/s 20ppm) Established in October 2009 for transport of 1000BASE-X (Gigabit Ethernet) Sized to fit existing OTN hierarchy  

•2 into ODU1 

•8 into ODU2 

•32 into ODU3 

•80 into ODU4 

 ODU0 can carry:  

•1000Base-X (1GbE) 

•STM-1 

•STM-4 

•FC-100 

No OTU0 physical layer Only a lower order wrapper for 1000BASE X mapped into standardized physical layers