OpenBTS and the Future of Cellular Networks
Overview
Range Networks is a new breed of network equipment provider. It is meeting the demand for
low cost, easy to install GSM cellular networks for remote rural service, rapid deployment and
private industrial networks on ships, oil rigs or in mines.
The Range Networks Approach
The defining feature of Range Networks systems is the replacement the conventional 2G/3G
network's SS7-MAP structure1 and all of its various components (MSCs, BSCs, TRAUs, SGSNs,
etc.) with a more modern SIP network, but without any changes to the handsets.
This approach offers the following advantages:
•
•
•
•
much lower deployment costs (CAPEX), especially for small operators
much lower operating costs (OPEX), for carriers of all sizes
plug-in compatibility with existing SIP-based core networks
plug-in compatibility with future IMS core networks
The key ingredient that makes this change possible is OpenBTS, a software defined radio
implementation of the GSM radio access network that presents normal GSM handsets as
1
Signaling System #7 is a protocol that has dominated telephone system core networks since
the 1980's. It is the basis for ISDN and Intelligent Network technologies. The Mobile Application
Part (MAP) is an extension added to SS7 in the 1990's to support functions required by cellular
networks.
1
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com
virtual SIP endpoints. In other words, through OpenBTS, any GSM handset appears as a SIP
device, without the need for any special software on the phone. Significant cost savings from
this approach are due to several factors:
•
For small networks, the core network hardware can be reduced to a single commodity
server, or core network applications can even be run on excess resources in the base
stations themselves.
•
For larger networks, the core network hardware is based on commodity servers and IP
routing equipment, making it possible for the provided to have one shared IP network
for both data and voice.
•
Because all of the cellular network software runs on Linux and connects with
commonly used TCP/IP and UDP/IP protocols, the core network can even be
virtualized and run in a “cloud” service, like Amazon's EC2.
•
All of the proprietary software found in a conventional GSM network can replaced with
open-source applications like open-source applications like SIP Express Router (SER),
Yate, Apache web server and MySQL database server, eliminating recurring licensing
fees.
•
Because the new core network is based on IETF internet-age protocols, newly
graduated engineers and software developers do not require additional training to
deal with archaic SS7 technologies.
Beyond cost savings, the conversion of the network from legacy telco protocols to internet
protocols gives the operator new opportunities to implement custom speech, text and USSD
applications, using web service technologies like Apache and Ruby, or through cloud-based
application platforms like Tropo or Twillio. These applications can even be installed in
individual cell sites to provide locally-tailored service.
Components of a Full Scale OpenBTS Network
The figure on the next page shows a full-scale OpenBTS network with complete integration
into the internet, PSTN and legacy PLMN. Not all components are necessary for all
deployments. In small deployments, all of these components can be run inside one of the
OpenBTS units to give a complete “network in a box”.
2
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com
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In this diagram, the outside networks are:
•
Public Switched Telephone Network (PSTN) This is the conventional global wireline
telephone network, based on SS7. Connections to this network are required to place
and receive telephone calls.
•
Public Land-Mobile Network (PLMN) This is the conventional global cellular network. The
PLMN is similar to the PSTN, but includes features to support handset mobility (the
“MAP”). Connections to this network are required to support cellular roaming.
•
Internet. This includes private, managed, bi-lateral connections between operators
using Internet technology. Connection to the Internet is required to support data
services like web browsing and email. It can also be used to route telephone calls and
text messaging to outside gateway operators with their own connections to the PSTN
and PLMN.
RAN Nodes
The subscriber radio interface of each RAN node is the GSM “Um” interface2. This is the radio
interface used by the cellular handsets and the same interface presented by conventional 2G
2
Named after the “U” interface of ISDN, on which GSM is modeled.
3
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com
and 2.5G networks. On its network interface, the RAN node uses SIP/RTP for call signaling and
SIP for mobility management and SMS. Each RAN node includes a local SIP switch for local call
connection and to support trunking between cell sites and the core network.
Subscriber Registry (SR)
The SR is the network's SIP registry, like a conventional SIP registry, but augmented to support
mobility and authentication functions associated with GSM. The SR is essentially a database
server with an interface for processing SIP REGISTER methods and, optionally, SS7-MAP and/
or DIAMETER interfaces into the PLMN (for roaming support). OpenBTS units communicate
with the SR via SIP. Other network elements access the SR directly with SQL. Pieces of the SR
are also cached locally in the RAN nodes to reduce network traffic and to allow the RAN nodes
to switch calls locally in the event of a backhaul failure.
PSTN Gateway
The PSTN Gateway is a gateway device used to originate and terminate PSTN/PLMN calls. It
can also be used as a central switch for inter-BTS calls. The PSTN Gateway communicates with
OpenBTS units using SIP/RTP and communicates with outside networks using SIP, SIP-I or SS7TCAP as required. The PSTN Gateway is implemented using an existing SIP switch with an
ISDN/SS7 gateway function3 . Some networks might contain multiple PSTN Gateway units, for
added capacity, for redundancy or to provide special-case support for legally-required
services like emergency calls or lawful intercept.
Messaging Server (MS)
The MS serves the same role as the SMSC in a GSM network. It has no equivalent in most VoIP
networks. The MS communicates with OpenBTS using SIP/SIMPLE and communicates with
conventional SMSCs using SS7-MAP or SMPP. Other internet messaging protocols, like XMPP,
can also be supported. Like the SR, the MS is essentially a database server wrapped with a set
of protocol interfaces.
3
Examples of such products include Asterisk, FreeSWITCH, Yate and Cisco 5400 series
gateways.
4
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com
Internet Gateway
The purpose of the internet gateway is to bind IP addresses to GPRS data sessions on the
handsets. Its function is the same as that of a GGSN, but it is implemented using much simpler
Linux IP routing features.
Network Integration
Connecting to the PSTN
Integration into the PSTN is the same as for any VoIP network via an SS7 or ISDN gateway
function in the PSTN Gateway. For small carriers, this service can be purchased from a
commercial VoIP carrier, in which case the PSTN Gateway is just a SIP switch.
Connecting to the Legacy PLMN (SMS and Roaming Support)
Integration into the PLMN is similar to integration into the PSTN, but with the addition of the
mobile application part (MAP) in the SS7 interface on the PSTN Gateway. With this interface,
the OpenBTS network appears as a conventional roaming partner to other cellular carriers.
Connecting to 4G IMS Networks
Because OpenBTS is SIP-based, it is a natural fit into next-generation IMS core networks. A SR
is required to translate between 2G SIM and 3G/4G USIM authentication procedures, but
speech and SMS transactions are performed directly with the IMS servers. This compatibility
allows network operators to operate Range Networks 2G (and future 3G) cell sites in the same
core network as their 4G LTE equipment. This mixed-mode operation allows operators to
abandon their legacy 2G/3G core networks (and associated licensing and maintenance OPEX)
while still running 2G and 3G RANs in those areas where 4G service is not economically
justified.
Small Networks and Rapid Deployment
Because OpenBTS allows GSM handsets to be used directly with Asterisk, a SIP PBX that is
reasonably easy to configure and operate, it can provide network-in-a-box solutions that are
much less complex and much more user-friendly than solutions based on more conventional
cellular technology. In these deployments, PSTN speech calling can be provided through
nearly any SIP-based VoIP carrier, or combined speech and SMS service is available through
5
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com
Voxeo's Tropo application servers. Range Networks can provide preconfigured accounts and
preprogrammed SIMs to simplify the use of these services.
Satellite-Backed Small Sites
For sites backhauled by satellite, operators can achieve considerable bandwidth savings
through “trunking”, in which audio channels are combined to reduce per-packet overhead in
the UDP/IP link. This trunking can be combined with low-rate speech codecs to achieve
additional bandwidth savings. Additionally, OpenBTS consumes little/ no bandwidth when
calls are inactive (traditional BTS’s consumer 20+ kbps in idle mode). Range Networks can
provide products, services and expertise to configure and operate these trunks and interface
them to VoIP services.
Mixed 2G/3G/4G Networks
Range Networks supports 2.5G today and will soon support 3G UMTS, 4G LTE (E-UTRA) and
2.75G (EDGE). Although these technologies have very different air interfaces, in their Range
Networks realizations, the core network is always the same. This means that a greenfield
carrier can start with a simple 2G network and over time develop a mixed 2G-3G-4G system,
using whatever technology is best adapted to particular sites. Core network upgrades are just
capacity upgrades, replacing existing servers with more or faster processors as the traffic
volume increases, or incremental upgrades to provide new features, like MMS, as they
become available.
6
Published by Range Networks. Content may not be repurposed or reproduced
without the express permission of Range Networks.
Copyright (c) 2012-2013 Range Networks
www.rangenetworks.com