Alien Channel and Alien Spectrum ( The new weapon for the wholesale business)

The key to a sustainable business is the ability to understand its most simplest demands. 

-------- Gujarati Proverb

1. The scaling dilemma of wholesale

Wholesale bandwidth provider, although does not have the kind of growth and visibility of traffic like the mobile backhaul provider, has the ambition and vision to grow even beyond them. However, their business is always on a catch 22 scenario where they are always in a dilemma of growing their infrastructure or expanding their coverage in the area. Capacity and coverage have always dominated the scale of the telecom business and this is not new even today. While the methods have changed with respect to deploying the telecom services the fundamentals of growth have not changed at all. There is still this old dilemma of whether to increase the capacity of a small sector or increase the coverage of the area so that more of the geography could be covered. 

If we are considering a common DWDM oriented network in the telecom sector and especially for the wholesale bandwidth provider then the most important factor that dominates the allocation of CAPEX is the traffic matrix. In case we have 100 dollars for spending then 70 dollars are actually attributed to the traffic cards or the traffic section of the network. Let us understand that these traffic cards or the traffic elements can be IP, OTN-Transport or even the ODU-XC. 

Therefore, let us understand that for the entire capex that includes the infrastructure of layer-0 as well as the traffic there can be a limited coverage of the are, especially when the amount of traffic is not known. The planning department of a wholesale entity relies on the business forecasting group to tell them about the planned capacity and then the back-calculation of CAPEX is done. In this back-calculation many things are actually missed out and one of the most primary things out of them is the ability of the DWDM infrastructure (30% of the CAPEX) to act as a primary infrastructure provider that can accommodate the demands of the bandwidth should the end customer also have a tunable service device that can ride on the wavelength of the wholesale bandwidth provider. 

This particular thought process can save almost 50% to 60% of the CAPEX that the wholesale BW provider is trying to put in and can improve bottom-lines considerably. 

2. What is Alien Channel? 

We discussed above as to how the CAPEX can be optimized to the level of 50% to 60% based on the availability of the tunable interface in the end customer devices. Alien channel actually enables the same thing in a wholesale BW infrastructure. 

Basic concept of Alien channel

The figure above shows the basic concept of the transmission of Alien Channel. Here we see that the network and its cost entities are demarcated in two sections altogether. 

1. The OLS (Optical Line System)

2. The Service (Traffic) 

Generally when the wholesale entity plans the network it plans for both the components of traffic and for the OLS. The OLS is the infrastructure portion of the network, while the service part is the part of the network that is consisting of the traffic generation and distribution. The OLS part of the network is the most certain part of the network where the provider knows which are the sites that could be potential traffic sites and which ones are only there for amplification .

The entire coverage of the network is dependent on the planning of the OLS. Therefore, this can be called as the infrastructure layer or the skeleton of the network on which the traffic rides. 

Let us understand that a provider on day-1 only has the OLS and does not have the visibility of the traffic that it wants to carry. Now another operator comes and asks if it can lease capacity from the provider. The conventional answer is that the wholesale will provision OTN traffic across the two points and then provide the pricing for the bandwidth. However, if the end operator already has the relevant transceivers that can run traffic on a C-Band or L-Band network then it becomes technically easy to accommodate these traffic demands to the existing OLS that has been already created. 

So the bandwidth provider now needs to provision a OCH path in the OLS and then this can carry the signal of the operator as an "alien" channel, meaning a wavelength of a different vendor in the optical line system. In case the OLS is able to carry the signal of the other operator then it is called an alien channel enabled OLS. 

3. How is Alien Channel a benefit for wholesale? 

While one may come up with the fact that there are so many moving parts in this solution and so much of dependency involved, one has to understand the commercial benefit of this solution that is a driving factor for the wholesale business. When we talk about the commercial benefits we are talking about two basic things over here. Money and time. So let us cross analyze what are the benefits in each aspects. 

3.1 The money benefit:

The costliest part of the establishing a  multi terabit network is not the OLS it is actually the provisioning of the traffic card. The transponders, ODU-XC, IP routers etc are the larger part of the CAPEX pie. In the case of alien channel this huge portion is taken out from the wholesale bandwidth provider side. This means more room for savings and bring in the customer. The money benefit also translates to improving the margins to a great extent. 

3.2 The expansion benefit:

As discussed before, the wholesale provider should focus in the coverage of the territory. This is also CAPEX dependent in many terms. If the focus is shifted to Alien Channels then the provider, without locking its CAPEX to the service cards and other traffic generating devices can focus on expanding the Flex-Grid network across the territory and thus can cover more part of it. 

3.3 The delivery benefit: 

Today the wholesale industry is totally dependent on the time to delivery. The alien channel helps the provider to immediately activate the bandwidth required without any kind of additional hassles of integration with the grey interfaces and power requirements. The approach is direct and provides immediate activation of the bandwidth with respect to the ordered date. This provides a good level of comfort to the end customer for their services to be started asap. 

3.4 A Win-Win situation:

Frankly speaking, every telecom operator, big or small, would be keen to have a dark fiber to their own disposal so that they can have a better control over the traffic devices. Although this is ideal, it is not possible to achieve in the real world. Alien channel provides this comfort, especially to the end customers/operators who are desiring of controlling their own traffic but do not want to invest on optical infrastructure. The wholesale provider over here provides a slice of the fiber, which the end customer gladly accepts running their own network on it. This gives them better options of leveraging their bandwidth and capacity within the prescribed channel OSNR that they have got. On the other hand it provides the wholesale bandwidth provider a sort of relaxation of not requiring to maintain inventories for different kind of cards and having a liability of the same. 

4. Alien Spectrum

In continuation with the Alien-Channel where we are talking about just one wavelength that can be a carrier for any kind of throughput rates we are now progressing towards a concept that is called as Alien Spectrum also sometimes referred to as Shared Spectrum. 

Alien Spectrum is a new concept where it is possible to provide the end user a bigger slice of the spectrum on which based on the requirement the end user can provision multiple lines or throughput. In order to understand this concept let us take an example below. 

A Hyper-scaler named A wants to have its terrestrial bandwidth across a region. A CoC named X has already got its flex-grid network deployed in that region. The Hyper-scaler desires to have bandwidth from point -1 to point-2 in the region. However, the contention here is that the hyper-scaler has two data centers with different kind of traffic profiles from different end routers. Some routers are emanating the 400Gbps line with a need of 75GHz width while some are giving out line bandwidth with 100Gbps on 50GHz, some need 800Gbps with 150GHz spacing and so on. 

Here the common approach can be the Hyper-scaler coming to the CoC and asking for different channels each of say 50Ghz, 100GHz and so on. However, the CoC here proposes a smart plan for the hyper-scaler. The CoC tells the hyperscaler to take a common shared spectrum of 500Ghz on its network and provide the flexibility to the hyper-scaler to split it in the way it wants. 

Hyper-scalers asking for Alien Spectrum

The hyper-scaler sees a very good value over here as this provides the essential control of spectrum and channel width allocation that it wants in order to have complete flexibility with the bandwidth. On the other hand the CoC is also sanguine that most of the demands of the hyper-scaler can be met in a flexible manner while its main focus would be to control the shared spectrum from one point to another point. 

This particular arrangement is called Shared Spectrum or Alien Spectrum. Here as we can see the service is not limited to a wavelength or a channel but it is a collection of channel that can be provided to the end user where the end user has the control over the channel and the width allocation. 

5. And Finally: 

Alien channel and spectrum are very good tools for a CoC to sustain a healthy business while giving proper freedom to the end customer. This also removes a lot of liability from the head of the CoC and helps it to focus on the aspect of connectivity delivery that is a core objective of a CoC or a wholesale bandwidth provider. 

This concept provides the freedom to the wholesale provider to scale up and spread across the region so that the coverage of the region can be achieved in lower time-frame. 

If you are a part of a wholesale bandwidth provider and you are also a part of the planning department, do consider this option so that the spread of the network can be accelerated. 

Do give me your inputs in the comments. 

Cheers, 

Kalyan

Building the new wholesale network (Planning)

 

If an entity stays in this world beyond the fulfilment of its purpose it will only cause imbalance and anarchy.

 

-------Old Proverb

1. Introduction

In the last blog that I wrote about ODU-XC and how it is fading and giving way to OTN-Transport and IPoDWDM I mentioned the shortcomings and the reasons why ODU-XC in now a non-viable solution. This is especially to those who are in the bandwidth leasing business or in the CoC business. The wholesale business is marred with margins and ODU-XC does not provide a suitable option over economical options like OTN-Transport and IPoDWDM. In this blog we are going to talk about especially the nuances of planning of the wholesale network for the new telecom era.

The fact that we are trying to establish in this section of the blog is related to the planning part of the wholesale network. A wholesale network is very different from that of a mobile back-haul or an ISP backbone. A wholesale network is supposed to serve many functions and many kinds of network. This is why the planning of such a network that is business oriented is extremely complex and has a lot of challenges. We will explore the requirements, the challenges and the new aspects of planning that goes into building this wholesale telecom network.

2. The requirements of a wholesale bandwidth provider

It needs to be clear that a wholesale bandwidth provider is not the owner of the content or is not generating any revenue from the content delivery. They are just pipes  or media who carry the traffic from one point to another with a standardized SLA. They are not owners of QoS or QoE, they are channelizers or media enablers and they are also paid in the same way. Thus the requirements of a wholesale provider is extremely flat and bland (if I am excused to say so)

2.1 Optimized bandwidth infrastructure

The wholesale provider has to play between the CAPEX and the OPEX of the solution that is implemented. The biggest asset it has is the fiber and that it has to capitalize to great extent. This capitalization comes from being able to have an infrastructure layer that will be able to carry large amounts of channels across the distance by consuming least pairs of fiber. Here we are focussing on delivery of bandwidth rather than the QoS of it, which is generally controlled by the end user. Thus the infrastructure is more important. Flex Grid systems with the ability to carry more bandwidth in flexible channels is definitely the good way to go forward. Flex Grid infrastructure provides that quintessential platform on which the premise of a wholesale business in the modern era can be based on.

 2.2 Cost effective transport modules

After the flex-grid ecosystem is created there needs to be a focus on cost effective transport modules that need to be provided in order to carry the traffic from one point to another. In this regard the focus should be on having modules that can aggregate more number of high bandwidth clients in a higher rate line output. This will result into the optimization of the revenue. Most of the times due to the ODU-XC and its inherent separation of the client and the line modules the cost of the solution increases, however if we look at the efficient muxponders that can provide the same solution at a lesser cost then it becomes very cost effective. 

Muxponders serving as simple transport modules

The muxponder that we see in the picture is a clear depiction of how the entire functioning of the ODU-XC can be compressed to a single module that can fit as a service card in the optical photonic shelf. This is an ideal situation for the wholesale bandwidth provider. The muxponder provider the end to end services for the wholesale provider to an extent that the CAPEX is kept minimal and the photonic shelves, that are extremely efficient in power and space, can handle the services. 

2.3 OPEX Efficiency

Another very important need for the wholesale bandwidth provider is the OPEX efficiency of the solution. The wholesale provider makes a full account of the cost that includes CAPEX and OPEX and the latter is a very important aspect as it is recurring in nature. Costs such as power, space, maintenance etc is a major part of the OPEX of the solution. OPEX efficiency, thus, forms an integral part of offering for the wholesale provider. Studies reflect the fact that the classical ODU-XC is extremely OPEX hungry and does not justify the RoI for the wholesale provider. In some cases the OPEX runs out to be more than three times of what it ideally needs to be. For wholesale bandwidth providers this situation is no less than a nightmare where the bandwidth has to be delivered in a wafer-thin margin cost structure. So definitely there have to be alternatives for the ODU-XC for such purposes so that economical ways of bandwidth delivery can be achieved. 

3. Why MNO requirements cannot be imposed on a wholesale network? 

Under normal circumstances it is always seen that the planning philosophy of MNOs often hijack the requirement scene of the wholesale bandwidth provider. This is the first step towards creating a loss-making network for the wholesale bandwidth provider. The wholesale bandwidth environment needs a network planning structure that is very different from the MNO planning structure.  There are certain factors which necessitate a separate design philosophy for the wholesale networks. 

3.1 Wholesale providers are not owners of the end-service

In the case of MNO the ownership of the end service and to an extent the content access lies with the MNO. The MNO is supposed to be the end to end provider of the service whether it is the wireless access or the back-haul. In the case of the wholesale bandwidth provider, this is not the case. The wholesale provider only owns the path and that too a part of the path than owning the entire service. The wholesale provider is like that of an active wire on which the service needs to pass. Since the ownership is limited the revenue expectancy is also of that category.  Therefore the technology that needs to be chosen under this case has to be extremely efficient in terms of cost whether it is CAPEX or OPEX. 

3.2 There are diverse routes in place of complex protection schemes

You agree or you do not agree the end customer will not solely depend on one wholesale network for the carriage of the bandwidth. The premium thus a customer will pay to you for a service that is protected by your network will be negligible in this respect. The focus should rather be on selling diverse routes that are unprotected while letting the protection mechanism work on the end customer. Diverse routes lead to a situation where the spectrum of the DWDM is more optimized and more static. Contrary to the cases of WSON or ASON where most of the resources are used up in protection. If we talk about WSON then there is a bit of economic sense involved as the resources are only blocked on the virtual channel level, however the ODU-XC ASON is of a really different level. Here we can see that the resources are pre-reserved in terms of hardware and interfaces. This increases the CAPEX dramatically.

The end customer on the other hand also ensures that there is enough diversity in the level of wholesale providers also. This is to ensure that if there is a major problem in the network of one wholesale provider the other wholesale provider can carry this bandwidth. This is a common phenomenon observed in the hyper-scaler sector where diversity is a key issue. 

Example of diverse route by the same provider

As seen in the figure above, we see that the diverse routes are provided here but the provider is the same for the wholesale bandwidth. The onus of the protection however is lying with the end service provider who does the switch between the main and the alternate path. Here the situation is a kind of a win-win.

For the wholesale bandwidth operator it is a sale of 2X bandwidth with low consumption of resources as there is no complexities of protection involved in the provisioning of the bandwidth. 

For the end customer there is a control of the main and the alternate path with full transparency. In addition to that the say 2X bandwidth can be used for passing best effort traffic when both the paths are available.

There is also a situation which is more ideal for the end service provider. Here the service provider does not rely on one wholesale entity but has two entities for the diverse carriage of the bandwidth. The diversity is more robust over here because here there is less reliance of the efficacy on one operator. 

Diverse route provided by diverse provider to the end customer

If we see the picture above then the situation is more pragmatic as now there are diverse routes provided by diverse providers. So for the end service provider instead of having an independent network of its own it can have a platter of bandwidths provided by different wholesale provider. The wholesale provider on the other side can be less robust and focus more on spectrum optimization rather than on the protection path.

Generally the end customer would seek such an arrangement with the wholesale provider(s) in order to maintain proper robustness in the network. This in a way lowers the load of onus on the wholesale network and thus can provide a more price optimized offer for the end customer from the wholesale side. 

3.3 Cost sensitivity is at its peak

As we can see from the above section that wholesale bandwidth is an extremely cost sensitive business, so the planning also needs to be cost sensitive. While the demands of technologies like WSON, ODU-Flex can be a sort of a wish-list, they do not have any kind of commercial leverage. So with the dwindling margins of bandwidth delivery there needs to be an innovative approach and cost effective approach in planning that is very much different from the planning that is involved to build a mobile network backhaul. A wholesale network will need to eliminate most of the features that are bulky in nature and are not required into the network. Focus has to be for delivering good quality bandwidth from point A to point B with all the SLA covered.

The planner has to realize that the margins in the wholesale network is less and it is more of a volume game. So, the mandatory aspect of a network needs to be covered for the wholesale network. More and more bandwidth is disposed off as a commercial entity in less and less cost incurred that will sustain the business to a greater level than creating space for more wishlists for the future. 

4. New aspects in the planning of the wholesale network

Now that we have understood the challenges that lure for the planning of the wholesale network let us delve into some points that consider the new aspects that are involved in the planning of the wholesale network. These new points will be extremely essential to build up wholesale networks rapidly and with extremely less cost. Before we venture into the building of the wholesale network we need to consider two fundamental principles on building it.

1. Faster network roll-out and faster delivery

2. Less CAPEX and OPEX involved in building the network. 

3. Prioritize service from the business point of view. 

Considering these fundamental principles we will explore some of the points that will make wholesale network building extremely efficient and cost effective. 

4.1 Build the infrastructure

The biggest requirement for a wholesale network is to secure the infrastructure. This forms almost 80-90 percent of the effort of building the wholesale network. More the delay in building the infrastructure more will be the delay in the delivery of the services. When we talk about the infrastructure we are talking about the following things. 

1. Fiber network readiness. 

2. Flex-Grid DWDM network with OTDR monitoring and fiber health monitoring. 

3. Proper channel/grid planning. 

4. Provision for the Alien wavelength/Spectrum on the network. 

This actually wins most of the battle. It enables the wholesale entity to start providing services on the day one with wavelength or spectrum basis. 

4.2 Identify high-bandwidth services

In a business of wholesale to sustain and to grow it is important to have an outlook and outreach towards cherry-picking the profitable kind of bandwidth deliveries that provide immediate revenue with minimal capex and opex. Our infrastructure is already done and ready. We can already provide spectrum and thus bandwidth on this logical infrastructure with minimal movement of CAPEX and OPEX. However, if there are operators who insist us to take ownership of the traffic segment also, the key guys would be the ones who are demanding bulk bandwidth. Let us take the example of an operator that is demanding bulk bandwidth across two points of 4X100G or more. Now for this the investment is quite small. 

We can assign muxponders that will aggregate this over a 400G or a 800G wavelength and provision across the two ports and be done with. This just is a module addition in the existing OLS not affecting the CAPEX and the OPEX as opposed to a full fledged system addition.

Thus, it becomes a very gradual addition to the infrastructure that we already have built without any additional hassle. Identifying the inventory is also easy as this mapped to the end user. Operations are not a hassle. 

4.3 Looking for those important low-rate subscribers

It is not rocket-science to understand that when you build an information superhighway then looking for a low-rate subscriber who wants the traffic also to be a part of your solution is a much expensive affair. For one such 10G there needs to be a line out towards 100G line or more and this built up can be a hassle for the network. Low rate customers can be the most expensive ones when it comes to the cost of delivery and the cost of maintenance. There cannot be a parallel line network for the 10G and for the 100G+. Therefore it is necessary to find the customers that are more suited to the network. A low-rate customer can be a very good strategic acquisition considering that there will be expansion in the future with growth of experience and the growth of the customers that the end customer has in this aspect. 

Generally low rate customers are coming from the enterprise sectors and more than the bandwidth what they require are. 

1. Reliability

2. Quality

3. Value addition

While the reliability and the quality part is already covered in the equipment and delivery of the wholesale provider the part of value addition is often missed out. Let us take some examples of the value addition that can be done for these enterprise customers. 

1. Customer network management (CNM)

2. Bundled services (Voice+Data Breakout)

3. SDWAN

4. Data monitoring and security. 

5. Encryption. 

Generally, when these value add services are provided with the wholesale bandwidth provider then there is a great deal of attraction for the enterprise services. 

4.4 Alien is very friendly these days

Today there are many service providers, MNOs, MVNOs, ISPs, MSOs etc who have the end device that can support the coherent but do not have either the DWDM infrastructure or the dark fiber. Under these circumstances they are looking for an infrastructure that can carry their traffic, which is already coherent in nature, on a specific wavelength or a spectrum. Hyperscalers, especially make a demand for such a service where they want just a chunk of the spectrum that is available in the DWDM infrastructure.

It is very important thus to plan for the alien spectrum and services like that because these are extremely quick and low maintenance businesses that a wholesale bandwidth provider can achieve. 

4.5 Automation is the key

Quick delivery, Express provisioning, Flexible BwoD and Self Healing networks. These are some of the important buzzwords that are in the market. The answer to all these buzzwords is “Automation”.  The automation has to be primarily in the network management system that the provider is choosing to manage the network. 

• Configuration automation to ease the delivery of the services. 
• Bandwidth on demand
• Fault automation with AI/ML to have a more self-healing network. 

All these aspects are to be seen when planning for a proper management system for a wholesale bandwidth ecosystem

5. Summary

Thus, we understand that the planning of the wholesale network cannot be congruent to the planning of the MNO network. There has to be expertise involved considering all the new aspects of network building.

To cut a long story short. The planner of a Mobile Back-haul is Network-centric but the planner of a wholesale network is network-centric and business-centric. So, in order to get into the planning of a profitable and revenue yielding wholesale network such attitudes have to be developed.

 

I will be adding more of wholesale networks in my next blogs. Keep watching. 

Cheers, 

Kalyan

Understanding UNI (User Network Interface)

Dear Friends, 

The year 2018 is coming to a close and this is the right time to write something on request of somebody. I had provided an article explaining Provider Bridge and Provider Edge concept where I touch-based upon the aspects of bridging and tunneling. Well I believe the article needed a prequel and that was to understand the different interface types that we have in the data networking. 

The networking that we do for data services are very dynamic and there can be several combinations of port-types and service types with domains realizing one important aspect and that is to carry the traffic from one point to another with reliability, security and prioritization. 

There are two kinds of interfaces that we have

UNI ----- User Network Interface

NNI ------ Network Network Interface. 

Today my post will concentrate only on the UNI aspect as I do not want to make this a very lengthy post. People may get grossly bored to read a lengthy post as for one I have noticed, the new generation lacks some kind of patience. 

What is UNI? 

In a very short and crisp definition we can say that UNI is the interface that connects a customer to the network. UNI port is an entity of the provider, however it interacts with a customer port or an interface. Please note that I have mentioned the terms port and interface separately. The reason being that a particular physical port may carry several logical interfaces that may be of UNI nature. 

Figure-1 Representation of a UNI

The figure above visualizes the definition of UNI. 

However, the concept of UNI is deep and needs more elaboration. For this we need to understand the concept of VLANs. A VLAN is a concept where we can break the broadcast domains to different smaller domains without involving the routing. You must be knowing that a switch breaks collision domains and the router breaks broadcast domains. However, what can be done in order to break a broadcast domain within a switching network without involving routers is to use VLAN. 

The VLAN gives a kind of identification to the packet or frame that is coming to the switching device as to which service that is should follow. VLANs are of 4 bytes and that are appended to a standard 802.3 frame. 

We see the structure of VLAN below. Disected. 

Figure-2 Dissection of the structure of VLAN

The VLAN is appended in the frame and it is providing the identification of the frame to follow a service. Here the main important thing is to see how the VLAN is appended in the frame. The frame can be untagged (without a Vlan) or it can be tagged (with a VLAN ID). Basis on this we have the two different types of interfaces of UNI. 

Figure-3: Types of UNI port

The types of UNI are the the basis of how the VLAN is treated in the service and the traffic is mapped. This is the thing that we will see in detail below. 

1. Access UNI port:

Imagine a situation where there are three customers who are all sending untagged frames. However the WAN which is carrying the traffic is a common WAN. How are we going to segregate this traffic. There is one way to do it by leaving it on the Mac Learning and bridging concept. The concept over here is that every device will have a different MAC and based on the MAC filter there will be communication and we will have one common LAN. However, this aspect has some disadvantages. 

Figure-4: Situation where three customers carry different traffic

> What happens in the unlearned state? 

> How will the traffic be treated? 

> What happens if one customer is sending a broadcast? This will affect the performance of all other customers. 

However, we cannot impose the condition on the customer to tag these frames. If we do that we are putting conditions on the customer and this way we will have to put similar conditions on all other customers, which is not a good idea. 

Here the thing is to have an access UNI port. 

The access UNI port accepts an untagged port on the LAN and then appends the VLAN on the frame of customer and then forwards it to the WAN. 

The flow is explained in the figure below. 

Figure 5: How the access VLAN UNI works

Here you see that the customer 1 sends the untagged frame and then it reaches the access port. Here the access port adds a VLAN ID to the frame and this travels through the WAN. On the other side when the frame is coming out the VLAN is matched and stripped and the original frame is received by the end. 

Points to note: 

1. Access VLAN ports in the UNI append VLAN in the ingress and match and strip the VLAN in teh egress. 

2. This action is like attaching a unique envelope to the untagged frame and taking it out on the other end without distorting any information. 

In our case we can have the following scenario for our three customers. 

Figure-6: How the three customers will be segregated

Here we see that every customer is connected to a different access port and so we put the access VLAN accordingly to identify the customer. Also note that the customer 2 may have a different drop point than customer 1 so this has to be organized in that way. 

2. Trunk Port: 

Once we understand the Access VLAN port functioning understanding trunk port is very easy. The trunk port in this case is our WAN. The WAN port is not adding or stripping any VLAN however it does a validation of VLAN that it has to pass. In our case we will put a validation that the trunk link should only pass VLAN 100, 200 and 300. 

The algorithm is the trunk port accepts frames that are already tagged and then matches them with the validation. Based on that it forwards. 

So friends, this is about the UNI port. We shall see the functioning of the NNI port in a later post. Understanding the UNI interface is very important in order to do selection of interfaces in data traffic planning. Therefore, be careful in deciding the same. 

Till then have a fantastic New Years' Eve.... 

Cheers and a Happy New Year, 

Kalyan 

RSTP/MSTP Part –V How does switching take Place in RSTP and MSTP?

 Dear friends of the Telecom Fraternity,

I was writing a series of RSTP in 2013 and possibly, there are many things to catch up. I would like to divert your attention to this blog that was the fourth part of my RSTP series.

RSTP-MSTP part IV

It is from this blog we continue our journey ahead to this vast topic called RSTP. Generally, people believe that RSTP is for switching, but I had clarified before that this is a Loop avoidance Mechanism. RSTP makes the switching of services in case of Failure more salient because there is a transition of paths.

In this section, we will concentrate on a simple architecture and that is RSTP Ring. As explained before we optimize the ring with a selection of optimal blocking port in the service.

Figure-1: RSTP ring topology example

Now in this ring let us understand that there is a service from the root Bridge to N-2 with SVLAN 200 and another service from Root Bridge to N-3 with SVLAN 300. This can be shown in the figure below.

Figure-2 Service configuration in the RSTP domain

To understand the concept of service switching let us understand a failure scenario. So in our case we imagine that the link between N-1 and N-2 has failed. Definitely, the service will be routed from another direction. However, in our case we will see this in a step-by-step basis.  Please remember that a service switching in RSTP is not as simple as it looks. Because there are no predefined main and protection paths like you have in TDM or MPLS. Here the entire switching of the service from one direction to another is working broadly on two principles.

1. RSTP Re-convergence
2. Mac Learning renewal at all switching points. 

We will see all these happening but you have to remember that all these happen very instantly. Typically the re-routing time of the services in the event of failure in RSTP is 200ms. Note this is not 50ms and that is why it is not recommended to run voice services or any real time services involving voice in the RSTP network. This is the reason why RSTP is regarded to be a Non-Carrier grade method. However, for a normal http service or a https service it does not matter as there is a TCP retransmission always happening and so RSTP works very well.

So we see the figure below to understand the failure scenario. 

Figure -3 Failure Occurrence in the link

 After this failure has occurred the first thing that happens is that N-2 does not get the BPDU packets from the N-1 (its designated bridge). So a Root port transition takes place and the link that is between N-2 and N-3 becomes a forwarding link. One special thing to remember over here is that in the N-2 to N-3 link, which is the blocking one of the ports between N-2 and N-3 will be the discarding port. RSTP in this case will not have two discarding port. So we have two cases over here. 

1. In case the discarding port is N-2 and there is a Root port failure on the N-2 then the Topology Change request of RSTP will immediately come into action and N-2 discarding port will turn to forwarding. 

2. In case the discarding port is N-3 then the TC message is communicated from N-2 to N-3  and N-3 changes the port from discarding to forwarding. 

Here the critical part is the topology change notification message that is carried by the BPDU and this always happen after a minimum hold-off time which is 200ms. The difference between STP and RSTP is over here. In STP there is a wait of three hello intervals which makes the initiation of TCN happens delayed. This results to a switching time that is more than 3 seconds. However, when we talk about RSTP (Rapid Spanning Tree Protocol) the TCN notification are subject to port transitions in any switch. Therefore N-2 and N-1 will both have transition changes and will initiate TCN immediately after the expiry of the hold-off timer. 

Now after the TCN is communicated the new state of RSTP will be as the figure below. Please note we have not yet considered how the service is being rerouted, we are still seeing the first part of the switching and that is RSTP re-convergence. 

Figure-4: RSTP topology change

Now the topology change has occurred. But what is remaining still is the re-routing of the service. I told this earlier that RSTP does not have a pre-defined protection and main path so the service re-routing is happening plainly on the basis of Mac Learning. RSTP is a scheme that is used in the case of Provider Bridge networks. To understand what is a provider Bridge network please refer to my earlier blog post in the permalink given below. 

Provider Bridge and Provider Edge Networks

In this blog post you will find clearly how the traffic moves in the provider bridge networks. So as this is a provider bridge we see that for the service affected, which is the service with SVLAN = 200, the mac learning has been done in the following manner of (Root Bridge - N1-N2). Now the path between N1-N2 has failed and there has to be a sort of notification to the root bridge to send the traffic via the other path. 

The self healing way of such a scenario is that the traffic stops and we wait for the expiry of the aging time of the mac table. The aging time of the mac table is a user configurable parameter, however the minimum value is 10 seconds. So technically if such a failure has occurred the service rerouting should take place after 10 seconds (aging time). 

Phew!!!!!! This is long. So the developers of RSTP thought of another approach and this was to flush the mac-table of every bridge that is involved in the RSTP domain. Therefore, the TCN also sends a command to flush the mac-table of all the bridges involved in a particular RSTP domain. 

Something like the figure below. 

Figure-5 Mac-Flush happening n all the nodes involved in RSTP

Here we see that all the points of the RSTP domain are flushed. 

Now it is anybody's guess what will happen after the flush of the FDB occurs. There will be relearning of mac address for the services. In this case the service with SVLAN 300 will have the same path of mac learning but the service with SVLAN-200 will not have the same path of mac learning. Now N-2 which is the destination point will learn the mac via N-3 and not N-1 and this will make N-3 the Designated Bridge for N-2 and the service will now be re-routed. 

Figure-6 Final Re-routing of the service

So here we see a complete step-by-step process of re-routing of the services. Tough but not so tough to understand. 

In this case please note, now the bandwidth distribution in the ring is not optimized and there can be a scene of congestion between the link of RB to N-4 and N-4 to N-3. Under such scenario the QoS will come to play and the RSTP domain has to be properly traffic engineered. 

What happens when the link restores?

Now we saw about the failure the restoration of the link is also treated like a seperate failure in this case. RSTP recognizes only topology changes and now with the link repaired there is another topology change. A similar TCN will pass through the ring and there will be re-convergence and the block port will now be as per before. The TCN will flush the Mac tables of all the bridges and this will lead to service re-routing again. 

So friends, pretty long blog post, but cannot help. In order to understand the switching part there has to be more description, which I have tried to bring in. But, we have just touched the tip of the ice-berg. There are lot many things happening beneath the skin of the water and to dissect it threadbare it would need another 50 blog posts. We will see the operational aspect of RSTP in multiple topology scenarios as well and dual homing cases. 

Till then 

See you.

Regards, 

Kalyan 

Keep thinking!!!! Keep Reading!!!! Keep Evolving!!!!