The problem with Layers
“Layers” were an analytical response to the breakdown of telecoms silos brought about by convergence (Werbach 2002, Whitt 2004). Since the “vertical” silos were merging, a “horizontal” approach seemed like a more useful abstraction, since it would remain relative constant even while technology and business model miscegenation raged between the silos.
Like any model, though, Layers has its limitations. First, layers are premised on a technological abstraction that was honored more in the breach than the observance: after learning about layers in Networking 101, engineers spend the rest of their career using cross-layer violations to improve system performance
Second, there isn’t a single “up direction” for stacking layers. The metaphor of layered stuff presumes gravity, which provides a direction for stacking. Even assuming one can discern an up direction in the network stack (e.g. by referring to successive encapsulation: the network stack is more like Russian Dolls than Legos), there are other important dimensions orthogonal to the network stack. For example, the supply chain from raw material to final product (from up-stream to down-stream, in the lingo) represents an important dimension. We all instinctively use vertical and horizontal as categories, but metaphors of the network stack and supply chains work at cross purposes: upstream in the network business is at the bottom of the stack, and downstream is at the top.
The upstream/downstream dimension (i.e. suppliers/customers) can be contrasted with entrants/substitutes in Michael Porter’s five forces analysis of industry dynamics. The interactions between these forces, as well as competition within an industry, providers of complementary products, and governments, obviate a simple layers model for an industry in general.
Geography and hierarchy are two more dimensions:
- geography: local, middle mile, long-distance; also related is IXPs where ISPs exchange traffic
- hierarchy: local, regional, national and international switches
The silo and layer models are attractive because they’re one-dimensional: a single parameter serves to distinguish between categories. In the silo model, the parameter is end-user service (broadcast TV, telephony, satellite communications, etc.); in the layer model, it’s the degree of network abstraction between the physical transmission of data and the end user experience.
Both these parameters (end user experience, and technical architecture) are relevant, so neither silos nor layers are sufficient on their own. Near to the physical layer, a packet is a packet is a packet, and end-user experiences can be more easily ignored; but from the user perspective, watching video is different from voice communications. Layers are helpful low in the stack, but the particular public interest mandates applied to TV are different from those on telephony, and so service silos can’t be ignored. The argument is at its most complex in the middle, where the two blur into each other.
Modules and Modularity
It’s easy to invoke modules and get knowing nods, particularly among techies, but coming up with a comprehensive defintion is tricky. To start concretely, here are some examples of things I count as modules:
- a network connection of whatever kind, e.g. Wi-Fi access to a base station, a wired thernet connection, a 3G cellular data service
- directories of all kinds, from the DNS to sites that organize links to other resources, like alluc.org’s pointers to videos
- a web browser, particularly one which runs on a variety of operating systems, in which an endless variety of services can be run
- an IM client, particularly on which runs against a variety of IM back-ends, e.g. Trillian plug-ins like web page hit counters, Netvibes widgets, and MySpace templates local and long-distance phone service provision
- voice over IP
- A Google AdSense plug-in on a web page
I don’t count these as modules:
- Cable or satellite TV service (I buy it in a take-it-or-leave-it lump)
- plain ol’ telephone service (before the AT&T break-up into local and long-distance, and before modems came along)
I’m unsure about some cases. For example, before the advent of plug-ins and RSS, portals like AOL, Yahoo and MSN allowed some on-site personalization, but were pretty much as you found them. You couldn’t mix in third party components to change them, which disqualifies them from modulehood in my book.
With these as a basis, I’d define a module as an interchangeable part of a larger collection of components that delivers an ICT user experience. “Modularity” is the design philosophy which builds functionality out of partial, separable and substitutable components, the modules. The key attributes of modules are:
- partial – the module is not sufficient on its own to provide a complete user experience; it’s a sub-set of the entire thing. The end user needs to assemble two or more modules to create the result they seek, like combining a local and long-distance phone service provider in the US
- separable – a module is self-contained and detachable, e.g. a web hit counter or other web plug-in can be removed without affecting functionality of the rest of the page
- substitutable – a module can replaced by another, equivalent one from another supplier, like replacing one web browser by another one.
Substitutability requires some public disclosure of the interface between modules. This leads to the Farrell & Weiser (2003) definition: “Modularity means organizing complements (products that work with one another) to interoperate through public, nondiscriminatory, and well-understood interfaces.” Note, though, that substitutability is not a sufficient condition; it presumes that an architecture of separable and partial pieces already exists.
Some user-facing modules have enough heft to qualify as applications, though in a modular world they build on some modules, and host others. MySpace or Netvibes are apps, but they plug into a browser, and their functions are extended by other plug-in modules.
Silos and layers provide straightforward ways to define markets, which can then be used to decide antitrust questions and figure out which groups of players should bear public interest mandates. They were designed to work this way. Modules don’t have this property.
Antitrust remedies are premised on well-defined markets within which companies compete. It’s hard to use modules to define markets, since players can mix and match the modules they use to offer a service, potentially working around a provider with market power. This is good news, of course; antitrust remedies wouldn’t be needed if it’s impossible to put together a platform that forms a bottleneck in the supply chain in a modular world. However, one should Never Say Never; the debate over the proposed Google/DoubleClick acquisition shows that bottlenecks could arise in the Web 2.0 world, too.
There’s also the question of public interest regulation. Universal Service Fund obligations is a well-worn topic, and won’t go away. However, arguably the harder questions revolve around public safety (CALEA, 911, pedophiles), content (obscenity, cultural protection), and access beyond connectivity (e.g. access for the disabled). Who should be responsible for delivering on these mandates, and how? Both silos or layers made it easy: pick a slice, and impose a mandate on companies in that segment. What does one do when not-quite-equivalent end user experiences can be assembled with widely different sets of modules?
My working hypothesis is that regulation should apply to the capabilities that are exposed, not the means by which they’re delivered (e.g. if something’s functionally equivalent to telephony, 911 applies regardless of how it’s done or by whom). Matters are complicated, though, because the context in which a capability is delivered makes a difference (e.g. a voice chat on X-Box Live during a game is different from voice communications module embedded into an employee’s work desktop).
Modules further complicate matters when an end-user builds up an experience by using modules from different providers. For example, imagine a visually disabled person builds a portal on Netvibes with newsfeeds from various web sites, an IM plug-in from one player, and a voice module from another – who’s responsible for delivering accessibility functions?
Odds 'n' Ends
If all modules are one-way pluggable, that is, they form chains without loops, then one can recover a layered categorization. For example, a CNET news feed plugs into my Netvibes page, which plugs into my browser; but CNET feed doesn’t itself host a Netvibes portal. (I feel in my bones that there must be module loops, but I haven’t come up with any yet.)
Modules relate to the interconnection, defined as connections between networks. Interconnect requires “horizontal pluggability” between modules, that is, pluggability among similar modules. The various network transport providers are at the same level of the network stack (i.e. the in- and out-connections use the same protocols) but may be at different geographical and hierarchical levels (e.g. local and long-distance). By contrast, the plug-ins for competing RSS viewers like Netvibes and Google Reader are neither interchangeable across platforms, nor do they directly connect to each other.