Optical Channel Layer

The Optical Channel Layer is the highest level layer and interacts directly with the clients system interface. It also provides the mechanism for transporting the client's digital signal through into the OTH compliant network. The optical channel trail from source to sink composes of two distinct logical signals:  

1.The optical client payload 

2.The  optical channel overhead ( the information added by OTH) 

To provide for end-to-end networking the following capabilities are included:  

•Optical Channel connection rearrangement 

•Processes for ensuring the integrity of the information added by the optical channel – This involves the introduction by OTH standards of bit-error rate evaluation and FEC (forward error correction) 

•Supervisory features for allowing network level management and control – this function involves the introduction of OTH standards for provisioning of connections, quality of service parameter exchange, and link protection schemes for network survivability  

The process required to create an optical channel from client data signal is:  

1.To create the optical channel from the client’s signal through interfacing with a transponder/transceiver in order to generate a continuous data stream that can be modulated onto a carrier wavelength 

2.The generation of management/maintenance signals for the OS overhead 

OTN Functional Architecture

Bearing in mind the goals of the OTH initiative, we can begin to visualize the structure and purpose of the OTN system. It would be predominantly optical delivery, certainly in its transmission path as it would be deployed over WDM. It would also retain one of WDM's advantages that of frame, protocol and bit-rate transparency independently of the client signals processing and transmission. Furthermore, we can see that the OTN model will have to provide telecommunications network management, monitoring and administrative control. Last, but not least, the OTN will provide necessary fault-tolerance, by providing protection mechanisms and link fail-over for redundancy.  

Therefore, the OTN must deliver the following functions on a per-client signal basis:  

•Transport 

•Routing 

•Multiplexing 

•Supervision 

•Survivability 

The OTH Goals

Some of the design goals of the OTH initiative were to provide the following:  

1.Develop a holistic system to handle rapid traffic growth 

2.Develop a system to handle transparently diverse signal types, frames, bit-rates and protocols 

3.To develop management OAM&P features through a supervisory information channel that did not negatively impact payload bandwidth 

4.Preserve the natural transparency and protocol independence of WDM and its inherent advantages 

5.To develop OTN standards – for optical termination, regeneration and cross-connects 

6.To work towards standards that would make full mesh OTN network practical 

Optical Transport Hierarchy

Up until the advent of the OTN initiative, the optical layer had been used simply as a physical medium over which to transmit data. This was primarily because of the impossibility to process optical data without first handling it over to the electrical domain. The OTH principle was seen by many as the first step in rectifying this shortcoming and in attempting to deliver some 'intelligence' to a passive system. Now, the OTH does not rewrite the laws of physics and it is still impossible to process data optically. However not to be deterred by such detail they opted for reserving a specific wavelength in order to use as a management or supervisor channel on which they could transport management information between equipment. Designers deemed this out-of-band supervisory channel a good solution as it did not interfere or unduly affect 

The payload of the underlying system was in contrast to SDH's overhead bytes. The OTH out-of-band supervisor channel would carry the same management information as SDH frames carried as overhead, but without using up precious cargo space in the frames. 

The OTH however would have to face come challenges as WDM differed quite considerable from the SDH framed protocol. For instance, there was no defined frame, DWM was light waves, without any frame structure or protocol and that was one of its strengths. Secondly, SDH handled one signal trail per path whereas WDM would be carrying several client signals per path and each would have to have its own individual management information with regards its own client’s state and fault. 

However, the challenge to delivering the project was easily outweighed by the potential benefits. If the principles of the OTH initiative could be applied to a working WDM out-of-band optical supervisory channel (OSC) then the potential was massive. Optical networks would be able to take on new topologies, such as partial and complete mesh networks using optical add/drop multiplexors, cross connects and optical switches in practical real world deployments. This would be due to the underlying management information being carried across the supervisory channel providing insight into the state of the network at every optical termination point, just like SDH. 

Furthermore, it would be possible to even implement protection and link fail-over as a network reliability measure, again due to having insight as to the link state between each optic termination link. However, the comparisons with SDH don't end there the OSC could also make provisioning of services possible, as it provides the ability to create, delete, and manage a service remotely, as can be done with SDH. 

In short, the OSC would do away with the WDM black-box approach to service management and provide all the SDH style visibility, manageability, flexibility and those precious OAM&P features for which everyone was clamoring.

OTH (Optical Transport Hierarchy)

The OTH (Optical Transport Hierarchy) was the initiative to bring intelligence and OAM&P features to WDM. OTH is the underlying optical multiplexing principles and techniques for aggregating several light wave signals onto a single multiplexed signal, which is then transported as a single signal across a single optical fiber. WDM's operation is independent of the original signals format, frames or bit-rate so it can handle any signal type. However, there are some physical restrictions in the WDM mode of operations.

1.       All the client wavelengths must be different to avoid interference.

2.       All the wavelengths must be sufficiently separated by a distance, a guard gap, to avoid signal cross talk.

WDM's advantage though is that it can take many different wavelengths (client signals) and multiplex them onto a single fiber, which had previously been only capable of handling one wavelength (client signal). This efficiency of resource usage is just one of WDM's beneficial features another is that it can carry transparently any client signal type. For instance client inputs can be from SDH, TDM, IP, ATM or any data stream as it is all transparent to WDM, it concerns itself only with carrying wavelengths.

At this point, it will be good to introduce the term Lambdas as this term is often used to describe a single wavelength. Consequently, the words wavelength and lambdas will be used inter-changeably in this book as the terms are synonymous.  On a similar note, the term OTH (Optical Transport Hierarchy) is used to describe the efforts to the OTH party to standardize the OTN network. However it can also be considered to be interchangeable with the term G.709, which was the resultant standard.

The OTH goal was to apply the principles of OTH design, learned from SDH, with the technology of WDM to provide the basis for a manageable fully meshed optical transport network, the OTN.

Narrowband-Internet of Things (NB-IoT)

The IoT and 5G massive Machine-Type Communications (mMTC) will be a large market in 5G. NB-IoT is a Low Power Wide AreaNetwork (LPWAN) radio technology 4G standard for IoT sensorsand devices to communicate over cellular networks. However,unlike many other characteristics of 5G, NB-IoT is not necessarily designed for speed.

Cat-NB1 uses a channel bandwidth of just200kHz, more akin to the old Global System for Mobile Communications (GSM) standard than the 20MHz of LTE and 100MHz of5G NR.Cat-NB1 and its 5G derivative is the antithesis of 5G enhanced Mobile Broadband (eMBB). It uses the narrowest bandwidth andslowest data rate to lower costs and enable a ten-year (or more) battery life.

Kbps stands for kilobits per second — and one million kilobits equalsone gigabit — a popular measure of speed back when modems used tone-modulated phone signals (accompanied by annoyingscreeching sounds) and people still used the yellow pages.

NB-IoT is designed primarily for IoT sensors and devices. Thelong range, using bands less than 1GHz, allows a vast network of low power sensors, each sending small data reports, to be aggregated efficiently. For these devices, it isn’t critical that the data they transmit is received with ultra-low latency, or that every transmission be acknowledged by the receiver. Thus, NB-IoT is an ideal technology for massive IoT use cases

Medium Access Control (MAC) Protocol

The MAC layer provides services through SAPs to the upper layer as all other sublayers of layer 2. The layer above MAC is the RLC layer; the lower layer is the physical layer which provides services to the MAC. In the case of the MAC, SAPs to the RLC layer are logical channels. Logical channels are used by higher layers to differentiate between logical connections which may use different metrics, for example, in terms of quality or delay, and so on. Furthermore, logical channels are used to distinguish control plane connections, either CCCHs or DCCHs, from user plane connections (DTCHs).

Services provided by the physical layer to the MAC layer are granted via another type of SAP. SAPs between the MAC and the physical layer are transport channels. Transport channels match data units to physical channels in which data is supposed to be transmitted. One exception is the PCH which is multiplexed into the PDSCH identified with the P-RNTI =0xFFFE.

Multiplexing of data units from logical channels to transport channels is one of the tasks of the MAC layer. Logical channels are differentiated with LCIDs. Tables 1 and 2 show the defined LCIDs and their values for DL and UL respectively. A CCCH always has LCID =0. Other UE dedicated channels start with LCID = 1.

Table 1: Values of LCID for DL-SCH. 

Index	LCID values
00000	CCCH
00001–01010	Identity of the logical channel
01011–11011	Reserved
11100	UE contention resolution identity
11101	Timing advance command
11110	DRX command
11111	Padding

Table 1.21: Values of LCID for UL-SCH. 

Index	LCID values
00000	CCCH
00001–01010	Identity of the logical channel
01011–11001	Reserved
11010	Power headroom report
11011	C-RNTI
11100	Truncated BSR
11101	Short BSR
11110	Long BSR
11111	Padding

A MAC PDU consists of a MAC payload part and a MAC header part. The MAC payload conveys multiple units of MAC control elements and MAC SDUs from higher layers. Therefore, the MAC header is also divided into sub-headers depending on the units carried in the MAC payload as MAC sub-headers describe the MAC payload units. There are various possible combinations of MAC control elements, MAC SDUs, and MAC padding derivatives. An example of a MAC PDU with a combination of MAC sub-headers, MAC control elements, and MAC SDUs in the payload section is depicted in Figure 1.

Figure 1: Example of MAC PDU consisting of MAC header, MAC control elements, MAC SDUs, and padding (TS36.321). Reproduced with permission from © 3GPP^™

Logical channels which are multiplexed to transport channels are prioritized by the scheduling algorithm. The scheduling algorithm decides what to schedule on which physical resources as described in detail for DL scheduling. There is only one MAC entity per UE; thus, the UL within the UE has one MAC entity and the eNB executes multiple parallel MAC entities in the DL direction in case the eNB has to handle multiple UEs.

The MAC layer implements a soft combining N -process stop-and-wait FEC and detection mechanism, or HARQ (Hybrid Automatic Repeat Request). Transport blocks are protected with a FEC algorithm known as turbo codes. Soft combining means not correctly decoded blocks are not acknowledged in order to conduct a retransmission, but the previous received not decoded block is held in a soft buffer to be recombined with the new retransmission. This process of soft combining two or more receptions increases the chance that the last received retransmission can be decoded error-free.