Optical transport network architectures to support 5G

Standards 3 Mins Read

As the 5G era dawns, the need for robust transport network architectures has never been more critical. The advent of 5G brings with it a promise of unprecedented data speeds and connectivity, necessitating a backbone capable of supporting a vast array of services and applications. In this realm, the Optical Transport Network (OTN) emerges as a key player, engineered to meet the demanding specifications of 5G’s advanced network infrastructure.

Understanding OTN’s Role

The 5G transport network is a multifaceted structure, composed of fronthaul, midhaul, and backhaul components, each serving a unique function within the overarching network ecosystem. Adaptability is the name of the game, with various operators customizing their network deployment to align with individual use cases as outlined by the 3rd Generation Partnership Project (3GPP).

C-RAN: Centralized Radio Access Network

In the C-RAN scenario, the Active Antenna Unit (AAU) is distinct from the Distribution Unit (DU), with the DU and Central Unit (CU) potentially sharing a location. This configuration leads to the presence of fronthaul and backhaul networks, and possibly midhaul networks. The fronthaul segment, in particular, is characterized by higher bandwidth demands, catering to the advanced capabilities of technologies like enhanced Common Public Radio Interface (eCPRI).

C-RAN Deployment Specifics:

Large C-RAN: DUs are centrally deployed at the central office (CO), which typically is the intersection point of metro-edge fibre rings. The number of DUs within in each CO is between 20 and 60 (assume each DU is connected to 3 AAUs).
Small C-RAN: DUs are centrally deployed at the metro-edge site, which typically is located at the metro-edge fibre ring handover point. The number of DUs within each metro-edge site is around 5~10

D-RAN: Distributed Radio Access Network

The D-RAN setup co-locates the AAU with the DU, eliminating the need for a dedicated fronthaul network. This streamlined approach focuses on backhaul (and potentially midhaul) networks, bypassing the fronthaul segment altogether.

5G transport network architecture: D-RAN

NGC: Next Generation Core Interconnection

The NGC interconnection serves as the network’s spine, supporting data transmission capacities ranging from 0.8 to 2 Tbit/s, with latency requirements as low as 1 ms, and reaching distances between 100 to 200 km.

Transport Network Requirement Summary for NGC:

Parameter	Requirement	Comments
Capacity	0.8-2 Tbit/s	Each NGC node has 500 base stations. The average bandwidth of each base station is about 3Gbit/s, the convergence ratio is 1/4, and the typical bandwidth of NGC nodes is about 400Gbit/s. 2~5 directions are considered, so the NGC node capacity is 0.8~2Tbit/s.
Latency	1 ms	Round trip time (RTT) latency between NGCs required for DC hot backup intra-city.
Reach	100-200 km	Typical distance between NGCs.

Note: These requirements will vary among network operators.

The Future of 5G Transport Networks

The blueprint for 5G networks is complex, yet it must ensure seamless service delivery. The diversity of OTN architectures, from C-RAN to D-RAN and the strategic NGC interconnections, underscores the flexibility and scalability essential for the future of mobile connectivity. As 5G unfolds, the ability of OTN architectures to adapt and scale will be pivotal in meeting the ever-evolving landscape of digital communication.

References

https://www.itu.int/rec/T-REC-G.Sup67/en

OTN Secure Transport

Standards 4 Mins Read

In today’s world, where digital information rules, keeping networks secure is not just important—it’s essential for the smooth operation of all our communication systems. Optical Transport Networking (OTN), which follows rules set by standards like ITU-T G.709 and ITU-T G.709.1, is leading the charge in making sure data gets where it’s going safely. This guide takes you through the essentials of OTN secure transport, highlighting how encryption and authentication are key to protecting sensitive data as it moves across networks.

The Introduction of OTN Security

Layer 1 encryption, or OTN security (OTNsec), is not just a feature—it’s a fundamental aspect that ensures the safety of data as it traverses the complex web of modern networks. Recognized as a market imperative, OTNsec provides encryption at the physical layer, thwarting various threats such as control management breaches, denial of service attacks, and unauthorized access.

Conceptualizing Secure Transport

OTN secure transport can be visualized through two conceptual approaches. The first, and the primary focus of this guide, involves the service requestor deploying endpoints within its domain to interface with an untrusted domain. The second approach sees the service provider offering security endpoints and control over security parameters, including key management and agreement, to the service requestor.

OTN Security Applications

As network operators and service providers grapple with the need for data confidentiality and authenticity, OTN emerges as a robust solution. From client end-to-end security to service provider path end-to-end security, OTN’s applications are diverse.

Client End-to-End Security

This suite of applications ensures that the operator’s OTN network remains oblivious to the client layer security, which is managed entirely within the customer’s domain. Technologies such as MACsec [IEEE 802.1AE] for Ethernet clients provide encryption and authentication at the client level.Following are some of the scenerios.

Client end-to-end security (with CPE)

Client end-to-end security (without CPE)

DC, content or mobile service provider client end-to-end security

Service Provider CPE End-to-End Security

Service providers can offer security within the OTN service of the operator’s network. This scenario sees the service provider managing key agreements, with the UNI access link being the only unprotected element, albeit within the trusted customer premises.

Service provider CPE end-to-end security

OTN Link/Span Security

Operators can fortify their network infrastructure using encryption and authentication on a per-span basis. This is particularly critical when the links interconnect various OTN network elements within the same administrative domain.

Second Operator and Access Link Security

When services traverse the networks of multiple operators, securing each link becomes paramount. Whether through client access link security or OTN service provider access link security, OTN facilitates a protected handoff between customer premises and the operator.

Multi-Layered Security in OTN

OTN’s versatility allows for multi-layered security, combining protocols that offer different characteristics and serve complementary functions. From end-to-end encryption at the client layer to additional encryption at the ODU layer, OTN accommodates various security needs without compromising on performance.

Final Observations

OTN security applications must ensure transparency across network elements not participating as security endpoints. Support for multiple levels of ODUj to ODUk schemes, interoperable cipher suite types for PHY level security, and the ability to handle subnetworks and TCMs are all integral to OTN’s security paradigm.

This blog provides a detailed exploration of OTN secure transport, encapsulating the strategic implementation of security measures in optical networks. It underscores the importance of encryption and authentication in maintaining data integrity and confidentiality, positioning OTN as a critical component in the infrastructure of secure communication networks.

By adhering to these security best practices, network operators can not only safeguard their data but also enhance the overall trust in their communication systems, paving the way for a secure and reliable digital future.

References

More Detail article can be read on ITU-T at

https://www.itu.int/rec/T-REC-G.Sup76/en

BER and Q Factor

Standards 3 Mins Read

Signal integrity is the cornerstone of effective fiber optic communication. In this sphere, two metrics stand paramount: Bit Error Ratio (BER) and Q factor. These indicators help engineers assess the performance of optical networks and ensure the fidelity of data transmission. But what do these terms mean, and how are they calculated?

What is BER?

BER represents the fraction of bits that have errors relative to the total number of bits sent in a transmission. It’s a direct indicator of the health of a communication link. The lower the BER, the more accurate and reliable the system.

ITU-T Standards Define BER Objectives

The ITU-T has set forth recommendations such as G.691, G.692, and G.959.1, which outline design objectives for optical systems, aiming for a BER no worse than 10⁻¹² at the end of a system’s life. This is a rigorous standard that guarantees high reliability, crucial for SDH and OTN applications.

Measuring BER

Measuring BER, especially as low as 10⁻¹², can be daunting due to the sheer volume of bits required to be tested. For instance, to confirm with 95% confidence that a system meets a BER of 10⁻¹², one would need to test 3×10¹² bits without encountering an error — a process that could take a prohibitively long time at lower transmission rates.

The Q Factor

The Q factor measures the signal-to-noise ratio at the decision point in a receiver’s circuitry. A higher Q factor translates to better signal quality. For a BER of 10⁻¹², a Q factor of approximately 7.03 is needed. The relationship between Q factor and BER, when the threshold is optimally set, is given by the following equations:

The general formula relating Q to BER is:

A common approximation for high Q values is:

For a more accurate calculation across the entire range of Q, the formula is:

Practical Example: Calculating BER from Q Factor

Let’s consider a practical example. If a system’s Q factor is measured at 7, what would be the approximate BER?

Using the approximation formula, we plug in the Q factor:

This would give us an approximate BER that’s indicative of a highly reliable system. For exact calculations, one would integrate the Gaussian error function as described in the more detailed equations.

Graphical Representation

The graph typically illustrates these relationships, providing a visual representation of how the BER changes as the Q factor increases. This allows engineers to quickly assess the signal quality without long, drawn-out error measurements.

Concluding Thoughts

Understanding and applying BER and Q factor calculations is crucial for designing and maintaining robust optical communication systems. These concepts are not just academic; they directly impact the efficiency and reliability of the networks that underpin our modern digital world.

References

https://www.itu.int/rec/T-REC-G/e

How Layer 0(zero) is so efficient with “Latency” -One of the key parameter for Operator.

Standards 1 Min Read

Layer 0(zero) merely introduces any latency to the network.

It’s mostly the electronics that imparts latency in the network.
The more we play with bit streams/signals ;more will be the latency.
Most of the latency optimization happens in layer 1-3.

Future Transport Network Requirements:

OTN maintenance signal interaction

Standards 1 Min Read

The maintenance signals defined in [ITU-T G.709] provide network connection status information in the form of payload missing indication (PMI), backward error and defect indication (BEI, BDI), open connection indication (OCI), and link and tandem connection status information in the form of locked indication (LCK) and alarm indication signal (FDI, AIS).

Interaction diagrams are collected from ITU G.798 and OTN application note from IpLight

Tandem Connection Monitoring (TCM)

Standards 3 Mins Read

Tandem Connection Monitoring (TCM)

Tandem system is also known as cascaded systems.

SDH monitoring is divided into section and path monitoring. A problem arises when you have “Carrier’s Carrier” situation where it is required to monitor a segment of the path that passes another carrier network.

Tandem Connection Monitoring

Here Operator A needs to have Operator B carries his signal. However he also needs a way of monitoring the signal as it passes through Operator B’s network. This is what a “Tandem connection” is. It is a layer between Line Monitoring and Path Monitoring. SDH was modified to allow a single Tandem connection. ITU-T rec. G.709 allows 6.

TCM1 is used by the User to monitor the Quality of Service (QoS) that they see. TCM2 is used by the first operator to monitor their end-to-end QoS. TCM3 is used by the various domains for Intra domain monitoring. Then TCM4 is used for protection monitoring by Operator B.

There is no standard on which TCM is used by whom. The operators have to have an agreement, so that they do not conflict.

TCM’s also support monitoring of ODUk connections for one or more of the following network applications (refer to ITU-T Rec. G.805 and ITU-T Rec. G.872):

– optical UNI to UNI tandem connection monitoring ; monitoring the ODUk connection through the public transport network (from public network ingress network termination to egress network termination)

– optical NNI to NNI tandem connection monitoring; monitoring the ODUk connection through the network of a network operator (from operator network ingress network termination to egress network termination)

– sub-layer monitoring for linear 1+1, 1:1 and 1:n optical channel sub-network connection protection switching, to determine the signal fail and signal degrade conditions

– sub-layer monitoring for optical channel shared protection ring (SPRING) protection switching, to determine the signal fail and signal degrade conditions

– Monitoring an optical channel tandem connection for the purpose of detecting a signal fail or signal degrade condition in a switched optical channel connection, to initiate automatic restoration of the connection during fault and error conditions in the network

– Monitoring an optical channel tandem connection for, e.g., fault localization or verification of delivered quality of service

A TCM field is assigned to a monitored connection. The number of monitored connections along an ODUk trail may vary between 0 and 6. Monitored connections can be nested, overlapping and/or cascaded.

ODUk monitored connections

Monitored connections A1-A2/B1-B2/C1-C2 and A1-A2/B3-B4 are nested, while monitored connections B1-B2/B3-B4 are cascaded.

Overlapping monitored connections are also supported.

Overlapping ODUk monitored connections

How OTN supercedes over SDH/SONET

Standards 4 Mins Read

Here we will discuss what are the advantages of OTN(Optical Transport Network) over SDH/SONET.

The OTN architecture concept was developed by the ITU-T initially a decade ago, to build upon the Synchronous Digital Hierarchy (SDH) and Dense Wavelength-Division Multiplexing (DWDM) experience and provide bit rate efficiency, resiliency and management at high capacity. OTN therefore looks a lot like Synchronous Optical Networking (SONET) / SDH in structure, with less overhead and more management features.

It is a common misconception that OTN is just SDH with a few insignificant changes. Although the multiplexing structure and terminology look the same, the changes in OTN have a great impact on its use in, for example, a multi-vendor, multi-domain environment. OTN was created to be a carrier technology, which is why emphasis was put on enhancing transparency, reach, scalability and monitoring of signals carried over large distances and through several administrative and vendor domains.

The advantages of OTN compared to SDH are mainly related to the introduction of the following changes:

Transparent Client Signals:

In OTN the Optical Channel Payload Unit-k (OPUk) container is defined to include the entire SONET/SDH and Ethernet signal, including associated overhead bytes, which is why no modification of the overhead is required when transporting through OTN. This allows the end user to view exactly what was transmitted at the far end and decreases the complexity of troubleshooting as transport and client protocols aren’t the same technology.

OTN uses asynchronous mapping and demapping of client signals, which is another reason why OTN is timing transparent.

Better Forward Error Correction:

OTN has increased the number of bytes reserved for Forward Error Correction (FEC), allowing a theoretical improvement of the Signal-to-Noise Ratio (SNR) by 6.2 dB. This improvement can be used to enhance the optical systems in the following areas:

Increase the reach of optical systems by increasing span length or increasing the number of spans.
Increase the number of channels in the optical systems, as the required power theoretical has been lowered 6.2 dB, thus also reducing the non- linear effects, which are dependent on the total power in the system.
The increased power budget can ease the introduction of transparent optical network elements, which can’t be introduced without a penalty. These elements include Optical Add-Drop Multiplexers (OADMs), Photonic Cross Connects (PXCs), splitters, etc., which are fundamental for the evolution from point-to-point optical networks to meshed ones.
The FEC part of OTN has been utilised on the line side of DWDM transponders for at least the last 5 years, allowing a significant increase in reach/capacity.

Better scalability:

The old transport technologies like SONET/SDH were created to carry voice circuits, which is why the granularity was very dense – down to 1.5 Mb/s. OTN is designed to carry a payload of greater bulk, which is why the granularity is coarser and the multiplexing structure less complicated.

Tandem Connection Monitoring:

The introduction of additional (six) Tandem Connection Monitoring (TCM) combined with the decoupling of transport and payload protocols allow a significant improvement in monitoring signals that are transported through several administrative domains, e.g. a meshed network topology where the signals are transported through several other operators before reaching the end users.

In a multi-domain scenario – “a classic carrier’s carrier scenario”, where the originating domain can’t ensure performance or even monitor the signal when it passes to another domain – TCM introduces a performance monitoring layer between line and path monitoring allowing each involved network to be monitored, thus reducing the complexity of troubleshooting as performance data is accessible for each individual part of the route.

Also a major drawback with regards to SDH is that a lot of capacity during packet transport is wasted in overhead and stuffing, which can also create delays in the transmission, leading to problems for the end application, especially if it is designed for asynchronous, bursty communications behavior. This over-complexity is probably one of the reasons why the evolution of SDH has stopped at STM 256 (40 Gbps).

References: OTN and NG-OTN: Overview by GEANT

How to calculate OTN bit rates?

Standards 1 Min Read

We know that in SDH frame rate is fixed i.e. 125us.

But in case of OTN, it is variable rather frame size is fixed.

So, frame rate calculation for OTN could be done by following method:-

Frame Rate (us) =ODUk frame size(bits)/ODUk bit rate(bits/s)…………………………………….(1)

Also, we know that

STM16=OPU1==16*9*270*8*8000=2488320000 bits/s

Now assume that multiplicative factor (M_k)** for rate calculation of various rates

Mk=

OPUk=(238/239-k)

ODUk=239/(239-k)

OTUk=255/(239-k)

Now, Master Formula to calculate bit rate for different O(P/D/T)U_k will be

Bit Rate for O(P/D/T)U_{k b/s} =M_k*X*STM16=M_k*X*2488320000 b/s………..(2)

Where X=granularity in order of STM16 for OTN bit rates(x=1,4,40,160)

Putting values of equation(2) in equation(1) we will get OTN frame rates.

Eg:-

For further queries revert:)

**Multiplicative factor is just a simple math :eg. for ODU1/OPU1=3824/3808={(239*16)/(238*16)}

Here value of multiplication factor will give the number of times for rise in the frame size after adding header/overhead.

As we are using Reed Soloman(255,239) i.e we are dividing 4080bytes in sixteen frames (The forward error correction for the OTU-k uses 16-byte interleaved codecs using a Reed- Solomon S(255,239) code. The RS(255,239) code operates on byte symbols.).

Hence 4080/16=255…I have understood it you need to do simpler maths to understand..)