FEATURE SOFTWARE-DEFINED NETWORKING
Today data services and packet traffic are
dominant, so you can say that packet has won. But it has been somewhat of a Pyrrhic victory because service providers are still independently planning, optimising, and operating two sets of networks – a packet network and a transport network. Returning to the public transportation
analogy, this status quo has been generally acceptable until now, since each network was able to build excess capacity so that end- customers received adequate service. But this situation is no longer sustainable for two reasons. Te first is budgetary, as service
The lack of knowledge that the packet and transport layers have of each other can clearly be improved upon
providers need to reduce their capital outlays. Te second is competition. Service providers need to streamline and speed up service delivery to maintain pace with the expectations being set by cloud-based services delivery. As we explore below, both these challenges can be met through multilayer optimisation and control of packet and optical transport networks.
How it works today Before discussing optimisation, let’s review how networks are designed today. To make things simpler we’ll just look at a Layer 3 router network for data services, and an underlying Layer 0–1 optical transport network. In the real world, other services like voice and Fibre Channel must also be considered, as well as use of Layer 2 packet switching. It starts with data services traffic. Tis traffic
originates from and terminates to many types of end users: enterprises, small and medium businesses, mobile base stations and mobile exchanges, residential traffic aggregation switches, all manner of large and small data centres, and traffic exchange points with other router networks. All these streams of data traffic get built up into a holistic geographic traffic matrix, which is the foundational requirement for a network design. Other key considerations are service-level agreements
26 FIBRE SYSTEMS Issue 15 • Spring 2017
(SLAs) specifying the mix between committed and excess information rates to any given user, service availability which governs network diversity, backup paths and cost targets. With these inputs we can design the packet
network layer, which is, essentially, deciding where to physically locate IP routers and how these should be connected with each other. Each router is specified in terms of capacity, the number of ports, and the speed of each port, typically 10, 25 or 100 Gigabit Ethernet (GbE). Internal routing tables are configured so that when packets arrive at one port, the router can decide which port the packets should exit. Routing rules will account for various traffic load conditions, and also for extreme circumstances such as when a link or port fails. Te actual connections among the routers
are performed by the optical transport layer. Tis has multiple degrees of freedom: mapping the GbE router port interfaces into fixed (ODUn) or flexible (ODUflex) Layer 1 OTN channels, employing OTN switching for grooming efficiency and flexibility on top of OTN transport, aggregating OTN channels onto Layer 0 WDM optical wavelengths, routing the wavelengths themselves between the optical nodes, and the use of ROADMs for handling wavelengths passing through a node to eliminate the need for expensive electrical conversion and provide optical layer flexibility. Tere are several noteworthy points with
the current approach. In the Layer 3 packet network, routers know with whom they are connected, but have no idea how they are connected, and whether this is being
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performed efficiently or not. Te Layer 0-1 optical transport network essentially just pairs up sets of interfaces across geographic distances, but has no knowledge of the services being carried. As a result, while the current approach
based on a ‘separation of concerns’ has a certain engineering elegance, the lack of knowledge that the packet and transport layers have of each other can clearly be improved upon by looking at them holistically.
Software-defined networking Packet and optical transport networks are inherently agile, equipped with points of flexibility that can be controlled remotely through soſtware. So, it would make sense that the major tool to harness this agility, to realise multilayer networking, is soſtware-defined networking. SDN provides intelligent, centralised, real-time control over networks with application-level awareness of the services they support. SDN for the WAN is based on a hierarchy. Domain controllers gather information and extend real-time control over different layers or geographical clusters of networking equipment. For instance there will be packet domain controllers and optical domain controllers. In turn, these domain controllers report into a parent controller, or SDN orchestrator (SDN-O), which is the mastermind ‘orchestrating’ all the underlying network resources. Te SDN-O is responsible for implementing service level requests coming from an operations support system, and continuously ensuring that all layers of the
Figure 1: Software-defined networking multilayer architecture
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