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INTRODUCTION
The vision of the OpenRAN architecture is to design radio access network
architecture with the following characteristics:
Open,
Flexible,
Distributed,
Scalable.
Such architecture would be open because it defines open, standardized
interfaces at key points that in past architectures were closed and proprietary. It
would be flexible because it admits of several implementations, depending on the
wired network resources available in the deployment situation. It would be
distributed because monolithic network elements in architectures would have been
broken down into their respective functional entities, and the functional entities
would have been grouped into network elements that can be realized as a distributed
system. The architecture would define an interface with the core network that allows
the core network to be designed independently from the RAN, preserving access
network independence in the core. Finally, the architecture would not require
changes in radio link protocols; in particular, a radio link protocol based on IP would
not be necessary. This document presents the first steps in developing the OpenRAN
vision. In its first phase, the subject of this document, the OpenRAN architecture is
purely concerned with distributing RAN functions to facilitate achieving open
interfaces and flexible deployment. The transport substrate for implementing the
architecture is assumed to be IP but no attempts is made to optimize the use of IP
protocols, nor are specific interfaces designated as open. The architecture could as
well be implemented on top of existing functional architectures that maintain a strict
isolation between the transport layer and radio network layer, by splitting an existing
radio network layer into control and bearer parts.
In its second phase, consideration of protocols for the interfaces leads to
considering how IP can be used more efficiently in the radio access network. In
addition, the architecture currently has a CDMA focus, since CDMA radio access
networks tend to be the most demanding in terms of functionality
MOTIVATIONS
Open RAN work has been motivated by perceived shortcomings in current RAN
architectures, many of the issues raised in this section may be addressed by compatible
changes to existing architectures or even through particular implementations of existing
architectures. The intent of the OpenRAN work is to see whether all of these issues can
be addressed in a comprehensive fashion, by starting from scrach and redesigning the
RAN completely.
By deploying a radio access network based on the OpenRAN
architecture, public network operators could achieve independence of their core
networks from the access network technology. This is intended to allow public
network operators to leverage their core, service based network, including support for
mobility, across a variety of access technologies, achieving the potential of a larger
market for their services.
Because the OpenRAN architecture is designed to allow the coexistence
of multiple radio technologies within a single RAN infrastructure,
deployment of OpenRAN-based radio access networks is intended to allow an operator
to achieve cost-effective utilization of their expensive spectrum assets by selecting
the most appropriate radio protocol. Also, duplicate wire-line infrastructure for different
radio technologies is unnecessary.
The OpenRAN could also contribute to cost-efficiency in other
ways. By allowing the separation of the control and bearer plane functions onto
different servers the control plane functions could be implemented on all-purpose server
platforms while the specific real-time bearer plane functions could be implemented on
highly specialized hardware. When connected to an all-IP core, the application of
existing IP protocols, interfaces, and modules, e.g. IP mobility management and AAA
infrastructure, is intended to allow standard routers and servers to be used in both the
RAN and core, allowing for exploitation of their economies of scale. Load sharing
could reduce the cost of redundancy by avoiding duplication of each network entity.
The distributed nature of the OpenRAN could increase reliability
by removing any single points of failure. Functions that in past RAN architectures
were clustered into monolithic nodes are distributed in the OpenRAN. The result
improves the potential for redundancy, because the cost of deploying multiple instantiations for particular RAN network elements is reduced. In past architectures, the
cost of deploying redundant network elements was prohibitive and difficult because of
centralization, depending on implementation. Distribution also improves scalability.
Because new services are expected to become an important means for 3G operators to
win over customers from competitors, the unpredictable requirements of these services
on network resources and their typical introduction in hot spots call for an incremental
infrastructure growth capability. In the OpenRAN, the control plane, bearer plane and
transport plane infrastructure are intended to scale independently, increasing the
deployment flexibility.
Because the OpenRAN admits flexible deployment scenarios,
operators could select a deployment that matches the available backhaul network
resources. Past RAN architectures, based on a star topology, were optimized for cases
where rich, high –bandwidth backhaul network resources were not available. As growth
in Internet connectivity has occurred, many metropolitan areas now support a rich
variety of wire-line and fixed wireless backhaul options. Increasing backhaul bandwidth
is generally coupled with lower costs for backhaul. Operators could deploy with a star
topology where bandwidth is limited, or a mesh topology where richer bandwidth
resources are available, the OpenRAN is compatible with both.
The ability to handle multiple radio link protocols with a single
radio network layer protocol facilitates interoperability between radio link protocols.
In its more advanced version, some functions (mobility management, wire-line QoS,
security) that today are handled at the radio network layer are planned to move into the
IP transport layer. As a result, these functions could be shared between radio link
protocols. This has the potential to allow load balancing between different radio link
protocols. For example, an operator could arrange to hand off a data call from GSM to
WCDMA if the over-the-air bandwidth requirements became large enough.
Finally, the OpenRAN design could allow more flexible
business models to evolve around provision of wireless Internet access. The separation
between core and RAN allows separation between the business entities providing core
services and wireless access. For example, a public network operator could decide to
organize their service-based core network and wireless access networks into separate
business units. This would allow the core unit to solicit business from other access
businesses, and the access business to solicit business from other service suppliers. The potential is also available for new wireless ISPs to arise. A wireless ISP would supply
wireless access only, and depend on existing service-based core suppliers for services.
If the RAN is based on Internet protocols and mechanisms, ISPs are more likely to
become potential customers because they already understand Internet protocols and
already have existing equipment based in Internet protocols, and so they do not have as
steep a learning curve or a difficult integration operation if they want to provide access.
3. OBJECTIVES
The long term vision of the OpenRAN architecture is to extend the peer-to-peer
and distributed Internet architecture to radio access networks, so a radio access network
becomes just another access network, like cable, DSL, Ethernet, etc.. The first step
toward this goal is to gather requirements. The second step is to determine the basic
functionality of a radio access network as a collection of functional entities in a
functional architecture, identifying the interfaces between the functional entities. The
third step is to describe which interfaces are open and how they can be implemented
using IETF protocols, or protocols based on IP but designed specifically for the radio
access network functions. The fourth step is to trace back the architecture to validate
that it does, in fact, meet the requirements. The objectives of the first version of this
report are to address steps one and two in the above process; future versions of this
report will address the remaining steps.
4. REQUIREMENTS
The proposed architecture shall satisfy all the applicable architectural
requirements for basic RAN functionality.
4.1. ARCHITECTURAL REQUIREMENTS
Wireless Access Technology Independent
The OpenRAN architecture shall at least support the current 2G and 3G radio
technologies such as WCDMA, CDMA2000, EDGE, IS -95. Also wireless LAN.
Minimal Functionality The Open IP RAN architectures and protocol set shall provide at least
equivalent functionality to existing RAN architectures[10] . 3 end to end in this context
means from terminal to the termination point of the call or session Mobile Wireless
IP Based Transport Network
The OpenRAN architecture shall transport bearer and control traffic based on IP
technology. Detailed requirements of IP in the RAN as transport option are documented
in MTR-006 [5]. Layer 1 and layer 2 can be any technology.
Separated Control and Bearer Function
The OpenRAN architecture shall be defined in terms of separate functions such
that it is possible to separate transport/bearer from control [8] to the extent allowed by
the radio protocols.
Distributed Control and Bearer Function
The OpenRAN architecture shall support distributed control and bearer function,
in order to facilitate load balancing and reliability.
Radio Resource Management
The OpenRAN architecture shall allow optimized use of different wireless
access technologies that are supported.
Availability and Reliability
Platform, link and system (or sub-system) availability and reliability is driven by
a combination of operator, subscriber and regulatory needs (e.g. emergency services).
The OpenRAN architecture shall support the necessary functions to ensure that the
necessary availability and reliability can be achieved.
Scalability
The OpenRAN architecture shall be able to support deployment within LAN,
MAN, WAN environment. The architecture shall also support increase in capacity
without architectural impact. Mobile Wireless Internet Forum Technical Report MTR-
007 Release v1.0.0 September 4, 2001 MWIF Page 23 of 64