(continued)
Figure 6: Multilevel Enterprise Core--Distributed Organization
This model worked acceptably for centrally controlled enterprises,
but did not scale well for inter-enterprise networks such as credit
card authorization. Large banks, for example, needed to optimize
their own cores for internal use, but needed to connect to the credit
authorization network. The logical characteristics of such networks
fit best into the distribution tier, which becomes the place of
interconnection.
Interconnecting at the distribution tier allowed the core to return
to its original simple-and-fast role of interconnecting sites inside
one organization. The requirement for a distribution layer function
between access and core, however, did not disappear. Increasingly,
network architects defined two distinct sets of function at the
distribution tier: the traditional one between core and access, and
a border function concerned with inter-organizational connectivity
(Figure 7). Border functions could deal both with controlled
cooperative relationships (e.g., a bank to the Visa or MasterCard
service networks, or to the Federal Reserve), and to the Internet via
firewalls.
Figure 7: Distribution Tier Evolution
This model has its limitations in dealing with provider environments.
Figure 8 shows some of the ambiguity with which many providers
approached the model. The provider called their own POP entry point
access. There are a variety of names for interprovider connection
devices, but border router is gaining popularity.
Figure 8: Data Carrier Interconnection Evolution
Matters become especially confusing when referring to "the
thing at the customer site that connects to the provider." This
"thing" is sometimes called a subscriber access device, but certainly
that makes the term "access" rather ambiguous. To complicate matters
even further, the "subscriber access device," with respect to the
enterprise network, is probably a device in the (enterprise
network's) distribution tier.
Entangling the terminology to yet another level, there is
usually a device at the customer location that establishes the
demarcation of responsibility between subscriber and provider. It
may be either a simple interface converter and diagnostic box, or a
full-functioned router or switch.
The general terms customer premises equipment (CPE) and customer
location equipment (CLE) have emerged, but still may have some
ambiguity. The basic assumption is that the customer owns the CPE
and the provider owns the CLE, but operational responsibility may
vary from that. For example, I own my DSL access router, but I don't
have the configuration password to it; my ISP does.
CPE, not CPE
A telephony tradition resulted in a good deal of confusion, due to
acronym collision. Traditionally, "CPE" meant customer premises
equipment. In the traditional telco environment, CPE was, of course,
owned and operated by the carrier.
As more and more deregulation affected the industry, customer
premises equipment variously could be owned and operated by the
customer, leased to the customer by the provider, owned by the
customer but operated by the provider, or owned and operated by the
subscriber. Redefining the former CPE into CPE and CLE at least
identified operational responsibilities.
The customer, of course, may have a complex enterprise network. What
we think of as CLE or CPE, however, is an increasingly intelligent
interface between customer and provider. The interface may contain a
firewall functionality, which can be either at the customer site or
at the POP. As seen in Figure 9, the customer edge function may
contain equipment to multiplex outgoing Internet traffic, VPNs, and
VoIP onto a broadband access facility.
Figure 9: Intelligent Edge--Subscriber Side
Any of the edge devices may be managed by the provider; at least one
device normally will. If the provider allows the subscriber to manage
their own device, the provider will have ironclad configuration
settings, which are not negotiable.
Service Provider Models
The hierarchical enterprise model was useful, but did not quite fit
modern service provider networks, independently of whether the
provider was data or voice oriented. Particular problems came from
increased competition, with competition both in the internetwork core
and in the local access system.
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