Bionic Buffalo Tech Note #104: Summary of Pegasus/ISA versus DSM-CC Specification Coverage
The high-level abstraction seen by client and server applications is defined in Interface Definition
Language (IDL), part of the Common Object Request Broker Architecture (CORBA), defined by the
Object Management Group (OMG). IDL has mappings to various programming langugages and to an
over-the-wire protocol (GIOP, the General Inter-ORB Protocol, and its internet specialization, IIOP,
the Internet Inter-ORB Protocol).
What are Pegasus and ISA?
Pegasus is a program initiated by Time-Warner to support the deployment of the Time Warner Cable
Full Service Network
architecture. The Phase I RFP, which described the architecture, was published
in March 1996. Pegasus defines a client (the Pegasus Set-Top Terminal), a delivery network. network,
The Pegasus Set-Top Terminal includes certain specific software components (browsers and
applications), hardware interfaces, and protocol support. The applications rely heavily on HTML, the
basic document language of the internet. The RFP calls out a specific operating system API (the
OS v1.5 API), and specific hardware configuration.
The Pegasus specification also defines some spectral allocations, protocols, topologies, and headend
In September 2000, Time Warner published the first version of the Interactive Services Architecture
(ISA) specification. ISA is “intended to define the server side interfaces required for a service provider
(application developer) to implement a service for the Pegasus platform. It does not describe interfaces
on the client (set top or PC). It does not define any specific services...” However, ISA is supplemented
by various other specifications, for services (such as Movies on Demand) subsystems (such as Asset
The interfaces in ISA and related specifications are defined in IDL, as are the high-level client and
server interfaces of DSM-CC.
Contrasting DSM-CC against Pegasus
Object System. DSM-CC defines a distributed object system, so clients can invoke most object
methods on the server. By contrast, Pegasus objects are not visible on the client. Pegasus objects are
available only on the server side of the network. Both architectures define interfaces in terms of
CORBA and IDL. In DSM-CC, client applications can use CORBA interfaces for distributed objects.
In Pegasus, client applications do not have access to CORBA interfaces.
Network. DSM-CC anticipates a wide range of network topologies, technologies, and lower-level
protocols. Pegasus is specifically intended for the network architecture of Time Warner, although it
may be used in compatible environments.
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