Making sense of Broadband networks - Part 1
Broadband is a hot topic in the telecommunications industry nowadays; it is becoming the bread and butter for so many service providers and mobile carriers with the big growth and penetration rates in these technologies in the recent years. ( Check this report about the EOCD countries). Although it is that important, broadband topics probably are not covered in any certification/education track and the reason might be that there is no "one shop suits all" in this field as in routing and switching technologies. In this post and others my aim is to provide some engineers with a brief but useful information about some of the deployed models today according to my own experience.
First of all let's set the scope for this series by starting with a basic question like what is broadband networks as I deal with them?
Technically broadband is any internet connection with higher bandwidth than dial-up connections. Broadband connections can be DSL, cable, satellite or fibre connections. In broadband typically the subscriber has a residential gateway at home/office which might be connected to a MSAN and probably an aggregation network and finally the BNG which will provide the L3 termination point of the connection and is connected to the rest of the service provider network and the Internet. Many devices are used in broadband networks and cover all the OSI model layers and they typically come from different vendors and they all interact together to provide the service. There is also a decent amount of servers used to provide AAA, billing, Multicast streams, etc services to subscribers. This creates some challenges for engineers working with these networks.
With such number of devices and interactions, I have seen service providers doing a lot of fancy stuff when it comes to service delivery models. Various platforms are used and some providers have fully customized platforms to provide the required flexibility that allows them to design new services anytime, but regardless of the differences, there is always common key points to look for in any network and this is where you can start. I am summing up the story from the networking perspective in the following 5 basic questions, that you might need to ask and answer to make sense of a network you are dealing with. Here they are:
- Single or Multi-edge topology? In single edge topologies you would find all traffic flows (services) going through the same BNG/BSR, on the other hand in Multi-edge ones you might find that some services are assigned dedicated router, typically this is video services and the dedicated router might be called a VSR or a Video service router.
- Centralized, distributed model or hybrid? In a distributed model typcially BNGs are distributed in different locations and L3 termination is done as close to the subscriber as possible, however in centralized models you would find that BNGs are collocated in few strategic locations for L3 terminations and subscribers are being aggregated in these locations. Each model has it's own advantages which I will try to touch from my experience later on also it is good to know that the centralized/distributed model might refer to the service intelligence in which service delivery functions might be centralized on the BNG or distributed on multiple devices along the path and then some protocols are used for intercommunication between devices like ANCP which I have seen once so far.
- Traffic Aggregation? This highly depends on the previous two factors and also the next two. It also depends on number of subscribers in each POP, bandwidth requirements and some other considerations and constraints. I have mostly seen aggregation networks with one or more layers of aggregation in big networks.
- Which Access protocols? PPPOE, DHCP or both? Firstly my experince is that ATM still exists in large old networks but is being migrated or at least not growing anymore. PPPOE and DHCP both are commonly deployed together and each protocol has its own advantages as we will discuss later.
- Which VLAN model? Customer VLAN (know as C-VLAN) or service VLAN (S-VLAN) or hybrid which combines both models.
Now we will discuss each of these questions in more details in a separate post and There will still be a lot other things you might need to know about a broadband network your are dealing with, as an example if L2TP or whole sale model is used, also QOS models which I am ignoring because they are usually the most complicated and the most customizable to each service provider, I also sometimes find it hard to understand if I didn't do it myself. But I think the previous five questions is a good starter to dig deeper your design or to get familiar with an existing network.