Network Slicing and its SDN/NFV Impact: A Real Issue or the Raising of Old Ones?

One of the 5G features that gets lots of attention is network slicing.  Most of the press on the topic has been at least positive if not gushing, but operators have some specific questions about the business case, and more about technology issues.  The big question on the technology side is how network slicing might play with SDN and NFV evolution.  It’s often said that it will promote both technologies, but it might actually limit them in the near term.

The basic idea of network slicing in 5G is to create service or even VNO partitions on a network to separate things that need either different resource policies or separation of service and tenant controls.  Each slice would operate (perhaps) like it was a totally independent network, with its own virtual elements (perhaps).  The reason for all the qualification here is that the specific nature of slicing and separation isn’t fully accepted at this point, and even some of the early notions have issues that might result in their being dropped down the line.

Slicing is a form of network virtualization, which is of course a form of virtualization.  All virtualization-based technologies consist of a real-resource layer, a mapping layer, and a set of abstractions that are the virtual equivalents of things in the real world below.  Virtualization is inherently “multi-tenant” or sliced, if it’s done right, so at one level the 5G slicing initiative is valuable because addressing network virtualization in wireless infrastructure in an explicit way would be a step toward getting it right.

The virtualization definition above points out both the benefits and the potential risks with network slicing.  You want to create a set of abstractions that represent what a service, an MVNO, or some other administration might see as their own services or resources, living independently.  You would then want to map the collection of those independent service/resource abstractions to a common real world.  Two fundamental questions emerge.  First, what exactly is in the “resource layer”?  Is it just connectivity or do we have higher-layer features, like IMS and mobility management?  Second, how does the mapping work?  Is it rather static partitioning, policy-based division, or what?

Let’s start with a basic question; how “independent” are the slices?  Does each slice represent a truly independent network, meaning that the slicing creates virtual-wire low-layer technology on which each slice-owner builds their own L2/L3?  Does each slice own its own hosting pools, or do they perhaps share some or all their hosting resources in some mediated way?  These two questions relate to the way that 5G and SDN/NFV would then relate.

Independent slices mean that the connectivity layers (L2 and L3) of the slice networks are created on totally independent elements, and that as far as each other are concerned they are totally separate networks.  This is analogous to networks that are built on separate (real or virtual) Level 1 elements.  You might also build slices on tunnel technology over IP or Ethernet, which is less “independent” since there are higher-layer devices shared among the slices.  Finally, you could build slices by partitioning the service-layer devices themselves.  This is the place where 5G evolution meets SDN and NFV.

If the slices are totally independent, then L2/L3 technology is duplicated for each slice, and each slice is able to manage its devices (real or virtual) in its own way.  Any traffic management or resource allocation is made below, by allocating L1 resources to the slices.  SDN and NFV adoption in this model would be little impacted by 5G; every slice owner could do their own thing in their own way, and any interconnection of slices or access by slices to common facilities like the Internet would be handled the same way as they would have been in an independent network.

If the slices share an underlying device network—IP/MPLS tunnels, Ethernet “Third Network” technology, or even SDN to create virtual wires—then the tenant slices are dependent on these common facilities, and they might compete for resources there.  This could make the behavior of the slices less deterministic and it might mean that management state and even some management/control processes would have to be coupled between the slices and the shared resources.  However, most SDN/NFV and L2/L3 processes at the service layer could still be based on independent real devices or virtual elements, and only minimal SDN/NFV impact would be likely.

If slices are based on partitioning at the service layer (L2/L3) or by a single SDN infrastructure complex, then we are dealing with slices as rather tightly connected tenants rather than as fully independent ones.  Service control within a slice would be a subset of service control overall, which means that isolation of tenants/slices and assurance of slice SLAs is now a service management function exercised not episodically (by allocating L1 resources) but continuously as connectivity and transport needs change.

The resource layer is one place where slices have to somehow converge, but another place is the device set, which in most cases mean (at least to a degree) the access part of the network.  Here, as with the resource layer, what happens will depend on just how we define a “slice”, and we have several options here as well.

The first option is to partition the access network itself.  5G wireline connections, or fixed 5G wireless tail connections off wireline fiber, could be considered hard partitions of access.  Thus, an access slice plus a resource slice equals a network slice, and all these slices are based on independent technology elements.  This is a simple approach, but for it to work, each slice owner would have to provide their own subscriber and mobility management elements.

The opposite model is one where the access infrastructure is not sliced per se, but rather is shared based on subscriber management principles applied in a common subscriber and mobility management framework.  Once a subscriber is “admitted” they’re assigned slice resources.  There is no replication of subscriber or mobility management here.

Where we have a “shared” element of service connectivity or subscriber/mobility management, we obviously have to design the element to support multi-slice/multi-tenant use.  In theory, at least in my view, different network slices are pretty much like different users/services for both SDN and NFV.  That means that service multi-tenancy processes for both technologies would probably serve slicing if they served their primary mission.  I don’t think they do, or at least they don’t do so provably.  We don’t have enough detail on SDN/NFV service lifecycle management to understand how strictly resources and service processes are partitioned.

Where there is no explicit need for sharing of elements—where we have “independent” slices in a true sense—there could still be a need to think about the SDN/NFV impact of 5G slicing.  First, it’s likely that an MVNO or “slice customer” would want to avail themselves of some sort of service structure within the slices, so they didn’t have to capitalize that part of their network any more than they wanted to capitalize the main part.  Too much “independence” defeats the purpose of slicing.  Second, even if they wanted to add technology to their slices, they can hardly haul traffic out of the 5G network owner’s infrastructure to route it between users, then stuff it back.  The virtual network of the slice would probably have to map to the topology of the 5G owner.  That would argue that the slice customer might want to lease resource capacity, and hosting virtual elements on the 5G owner resource pool would be a very logical strategy.

The most likely overall impact of network slicing is on resource multi-tenancy.  The next-most-likely is on “federation” or coordination of multiple service and resource domains to create a large-scale cohesive retail/wholesale offering.  Thus, what we need to be looking at in 5G in terms of SDN and NFV impact isn’t new at all, it’s something that’s been needed all along.  Hopefully, network slicing in 5G will make it harder to ignore.