Table
of Contents
Mobile Station
Base Transceiver Station
Base Station Controller
Base Station Subsystem
Mobile Switching Center
Equipment Identity Register
Home Location Register
Authentication Center
Visitor Location Register
Network and Switching Subsystem
Base Transceiver Station
Base Station Controller
Base Station Subsystem
Mobile Switching Center
Equipment Identity Register
Home Location Register
Authentication Center
Visitor Location Register
Network and Switching Subsystem
GSM Interfaces
GSM Data Services
GSM Data Services
General Packet Radio Service
Benefits of GPRS
GPRS Applications
GPRS Applications
Communications
Value Added Services
Location-Based Services and Telematics
Vertical Applications
Advertising
Value Added Services
Location-Based Services and Telematics
Vertical Applications
Advertising
GPRS Architecture
GPRS Subscriber Terminals
GPRS Base Station Subsystem
GPRS Support Nodes
GPRS Base Station Subsystem
GPRS Support Nodes
GPRS Terminals
Class A Terminals
Class B Terminals
Class C Terminals
GPRS Device Types
Class B Terminals
Class C Terminals
GPRS Device Types
Data Routing
Data Packet Routing
Mobility Management
Mobility Management
GPRS Interfaces
GPRS Protocol Stacks
GPRS Tunneling Protocol
GPRS Access Modes
GPRS Protocol Stacks
GPRS Tunneling Protocol
GPRS Access Modes
Transparent Mode
Non-transparent Mode
Non-transparent Mode
GPRS Access Point Name
GPRS Processes
GPRS Processes
GPRS Attach Process
GPRS Authentication Process
PDP Context Activation Process
Detach Process Initiated by MS
Network Initiated PDP Request For A Static IP Address
Network Initiated PDP Request For A Dynamic IP Address
GPRS Authentication Process
PDP Context Activation Process
Detach Process Initiated by MS
Network Initiated PDP Request For A Static IP Address
Network Initiated PDP Request For A Dynamic IP Address
Universal Mobile Telecommunication System
UMTS Services
UMTS Architecture
UMTS Architecture
General Packet Radio System
UMTS Interfaces
UMTS Terrestrial Radio Access Network
Radio Network Controller
Node B
UMTS User Equipment
UMTS Interfaces
UMTS Terrestrial Radio Access Network
Radio Network Controller
Node B
UMTS User Equipment
The
Cisco Mobile Exchange (CMX) architecture provides mobile wireless solutions for
operators using General Packet Radio Service (GPRS) and Universal Mobile
Telecommunication System (UMTS) access technologies. This chapter provides an
overview of these technologies and their roles in the evolution from
second-generation (2G) to third-generation (3G) mobile wireless networks.
- Global System for Mobile Communications (GSM)—A
digital, mobile, radio standard developed for mobile, wireless, voice
communications
- GPRS—An extension of GSM networks that provides mobile,
wireless, data communications
- UMTS—An extension of GPRS networks that moves toward an
all-IP network by delivering broadband information, including commerce and
entertainment services, to mobile users via fixed, wireless, and satellite
networks
- "Global Systems for Mobile Communications"
section
- "GSM Technology Differentiator" section
- "GSM Network Elements" section
- "GSM Interfaces" section
- "GSM Data Services" section
- "General Packet Radio Service" section
- "GPRS Architecture" section
- "GPRS Terminals" section
- "Data Routing" section
- "GPRS Interfaces" section
- "GPRS Protocol Stacks" section
- "GPRS Tunneling Protocol" section
- "GPRS Access Modes" section
- "GPRS Access Point Name" section
- "GPRS Processes" section
- "Universal Mobile Telecommunication System"
section
In the early 1980s, many countries in Europe witnessed a
rapid expansion of analog cellular telephone systems. However, each country
developed its own system, and interoperability across borders became a limiting
factor.
In
1982, the Conference of European Post and Telecommunications (CEPT), an
association of telephone and telegraph operators in Europe, established a
working group to develop a new public land mobile system to span the continent.
Because their working language was French, the group was called the Groupe
Speciale Mobile (GSM).
- good speech quality
- low cost for terminals and service
- international roaming
- handheld terminals
- support for introduction new services
- spectral efficiency
- compatibility with Integrated Digital Services Network
(ISDN)
In
1989, the responsibility for GSM development was transferred to the European
Telecommunications Standards Institute (ETSI), and phase 1 of the GSM
specification was published in 1990. The first commercial service was launched
in 1991.
When
the official language of the GSM group changed from French to English, GSM was
changed from Groupe Speciale Mobile to Global System for Mobile Communications.
In
1994, phase 2 data/fax services were launched, and in 1995, the GSM phase 2
standard was completed. The first GSM services in the United States were
launched.
GSM
uses a combination of both the time division multiple access (TDMA) and
frequency division multiple access (FDMA) technologies. With this combination,
more channels of communications are available, and all channels are digital.
- 450-MHz—Upgrade of older analog cellular systems in
Scandinavia
- 900-MHz—Original band used everywhere except North
America and most of South America
- 1800-MHz—New band to increase capacity and competition
used everywhere except North America and most of South America
- 1900-MHz—Personal communications service band used in
North America and much of South America
One
of the unique benefits of GSM service is its capability for international
roaming because of the roaming agreements established between the various GSM
operators worldwide.
One
of the advantages of GSM is that it offers a subscriber identity module (SIM), also known as a smart card. The smart card contains a
computer chip and some non-volatile memory and is inserted into a slot in the
base of the mobile handset.
The
memory on the smart card holds information about the subscriber that enables a
wireless network to provide subscriber services. The information includes:
- The subscriber's identity number
- The telephone number
- The original network to which the subscriber is
subscribed
A
smart card can be moved from one handset to another. A handset reads the
information off the smart card and transmits it to the network.
- Mobile station (MS)
- Base transceiver station (BTS)
- Base station controller (BSC)
- Base station subsystem (BSS)
- Mobile switching center (MSC)
- Authentication center (AuC)
- Home location register (HLR)
- Visitor location register (VLR)
The
mobile station (MS) is the starting point of a mobile wireless network. The MS
can contain the following components:
- Mobile terminal (MT)—GSM cellular handset
- Terminal equipment (TE)—PC or personal digital
assistant (PDA)
The
MS can be two interconnected physical devices (MT and TE) with a point-to-point
interface or a single device with both functions integrated.
When
a subscriber uses the MS to make a call in the network, the MS transmits the
call request to the base transceiver station (BTS). The BTS includes all the
radio equipment (i.e., antennas, signal processing devices, and amplifiers)
necessary for radio transmission within a geographical area called a cell. The
BTS is responsible for establishing the link to the MS and for modulating and
demodulating radio signals between the MS and the BTS.
The
base station controller (BSC) is the controlling component of the radio
network, and it manages the BTSs. The BSC reserves radio frequencies for
communications and handles the handoff between BTSs when an MS roams from one
cell to another. The BSC is responsible for paging the MS for incoming calls.
A
GSM network is comprised of many base station subsystems (BSSs), each
controlled by a BSC. The BSS performs the necessary functions for monitoring
radio connections to the MS, coding and decoding voice, and rate adaptation to
and from the wireless network. A BSS can contain several BTSs.
The
mobile switching center (MSC) is a digital ISDN switch that sets up connections
to other MSCs and to the BSCs. The MSCs form the wired (fixed) backbone of a
GSM network and can switch calls to the public switched telecommunications
network (PSTN). An MSC can connect to a large number of BSCs.
The
equipment identity register (EIR) is a database that stores the international
mobile equipment identities (IMEIs) of all the mobile
stations in the network. The IMEI is an equipment identifier assigned by the
manufacturer of the mobile station. The EIR provides security features such as
blocking calls from handsets that have been stolen.
The
home location register (HLR) is the central database for all users to register
to the GSM network. It stores static information about the subscribers such as
the international mobile subscriber identity (IMSI), subscribed services, and a
key for authenticating the subscriber. The HLR also stores dynamic subscriber
information (i.e., the current location of the mobile subscriber).
Associated
with the HLR is the authentication center (AuC); this database contains the
algorithms for authenticating subscribers and the necessary keys for encryption
to safeguard the user input for authentication.
The
visitor location register (VLR) is a distributed database that temporarily
stores information about the mobile stations that are active in the geographic
area for which the VLR is responsible. A VLR is associated with each MSC in the
network. When a new subscriber roams into a location area, the VLR is
responsible for copying subscriber information from the HLR to its local
database. This relationship between the VLR and HLR avoids frequent HLR
database updates and long distance signaling of the user information, allowing
faster access to subscriber information.
The
HLR, VLR, and AuC comprise the management databases that support roaming
(including international roaming) in the GSM network. These databases
authenticate calls while GSM subscribers roam between the private network and
the public land mobile network (PLMN). The types of information they store
include subscriber identities, current location area, and subscription levels.
The
network and switching subsystem (NSS) is the heart of the GSM system. It
connects the wireless network to the standard wired network. It is responsible
for the handoff of calls from one BSS to another and performs services such as
charging, accounting, and roaming.
The
GSM uses various interfaces for communication among its network elements. Figure 2-2 shows
these interfaces.
Mobile
wireless communication occurs over the interfaces between the network elements
in a sequential manner. In Figure 2-2,
the MS transmits to the BTS, the BTS to the BSC, and the BSC to the MSC.
Communications also occur over the interfaces to the management databases (HLR,
VLR, AuC, and EIR). Communications might traverse multiple MSCs but ultimately
must reach the gateway MSC (GMSC). The GMSC provides
the gateway to the public switched telephone network (PSTN). A separate
interface exists between each pair of elements, and each interface requires its
own set of protocols.
In
the BSS block, mobile communication occurs over the air interface to the BTS
using the ISDN Link Access Procedure-D mobile (LAP-Dm). This traffic channel
carries speech and data. In this example, voice operates at full-rate 13 kbps
(supported by LAP-Dm), and data operates at full-rate 9.6 kbps.The BTS
communicates to the BSC over the Abis interface using the ISDN
LAP-D signaling protocol. The BSC communicates to the GMSC via the transcoder
rate adapter unit (TRAU), which translates between 16 kbps on the BTS side to
64 kbps on the GMSC side. This interface uses the signaling system 7 (SS7)
protocol, which defines call set-up and call services across the interface.
At
the NSS, the GMSC is the central node. Link-level traffic and signaling control
occurs over the interface between the GMSC and MSC and the interface to the
external network (PSTN, ISDN or PDN). Different signaling protocols are used on
the interfaces. Some NSS interfaces involve only control signaling protocols
with no traffic. For example, no traffic is generated on the interfaces between
the GMSC, HLR, and VLR. Instead, these interfaces carry only signaling using
the Mobile Application Part (MAP) of the SS7 protocol. The MAP is specified in
IS-41 and defines the application layer, signaling protocols, and procedures
for registering mobile users and handling handoffs between cellular systems.
The GMSC establishes call traffic (at 64 kbps) onto the PSTN via the ISDN user
part (ISUP), which is an SS7-based protocol. The GMSC and MSC exchange traffic
(over LAP-D at 64 kbps) and use SS7 (MAP and ISUP) control.
GSM
networks handle both voice and data traffic requirements of the mobile
communication by providing two modes of operation:
Circuit
switching provides the customer with a dedicated channel all the way to the
destination. The customer has exclusive use of the circuit for the duration of
the call, and is charged for the duration of the call.
With
packet switching, the operator assigns one or more dedicated channels
specifically for shared use. These channels are up and running 24 hours a day,
and when you need to transfer data, you access a channel and transmit your
data. Packet switching is more efficient than circuit switching.
The
general packet radio system (GPRS) provides packet radio access for mobile
Global System for Mobile Communications (GSM) and time-division multiple access
(TDMA) users. In addition to providing new services for today's mobile user,
GPRS is important as a migration step toward third-generation (3G) networks.
GPRS allows network operators to implement an IP-based core architecture for
data applications, which will continue to be used and expanded for 3G services
for integrated voice and data applications. The GPRS specifications are written
by the European Telecommunications Standard Institute (ETSI), the European
counterpart of the American National Standard Institute (ANSI).
- Open architecture
- Consistent IP services
- Same infrastructure for different air interfaces
- Integrated telephony and Internet infrastructure
- Leverage industry investment in IP
- Service innovation independent of infrastructure
- Overlays on the existing GSM network to provide
high-speed data service
- Always on, reducing the time spent setting up and
taking down connections
- Designed to support bursty applications such as e-mail,
traffic telematics, telemetry, broadcast services, and web browsing that
do not require detected connection.
By
implementing Cisco GPRS products and related solutions, mobile service
providers can optimize their networks to deploy high quality mobile voice and
data services. They can also benefit from new operating efficiencies,
peer-to-peer IP-based architecture for scalability, and IP standard interfaces
to billing and customer support.
GPRS
enables a variety of new and unique services to the mobile wireless subscriber.
These mobile services have unique characteristics that provide enhanced value
to customers. These characteristics include the following:
- Mobility—The ability to maintain constant voice and
data communications while on the move
- Immediacy—Allows subscribers to obtain connectivity
when needed, regardless of location and without a lengthy login session
- Localization—Allows subscribers to obtain information
relevant to their current location
The
combination of these characteristics provides a wide spectrum of possible
applications that can be offered to mobile subscribers. The core network
components offered by Cisco enable seamless access to these applications,
whether they reside in the service provider's network or the public Internet.
In
general, applications can be separated into two high-level categories:
corporate and consumer. These include:
- Communications—E-mail; fax; unified messaging;
intranet/Internet access
- Value-added services—Information services; games
- E-commerce—Retail; ticket purchasing; banking;
financial trading
- Location-based applications—Navigation; traffic
conditions; airline/rail schedules; location finder
- Vertical applications—Freight delivery; fleet
management; sales-force automation
- Advertising
Communications
applications include those in which it appears to users that they are using the
mobile communications network as a pipeline to access messages or information.
This differs from those applications in which users believe that they are accessing
a service provided or forwarded by the network operator.
The
first stage of enabling users to maintain contact with their offices is through
access to e-mail, fax, and voice mail using unified messaging systems.
Increasingly, files and data on corporate networks are becoming accessible
through corporate intranets. These intranets can be protected through firewalls
by enabling secure tunnels or virtual private networks (VPNs).
As
a critical mass of users is approached, more and more applications aimed at
general consumers are being placed on the Internet. The Internet is becoming an
effective tool for accessing corporate data and manipulating product and
service information. More recently, companies are using the Internet as an
environment for conducting business through e-commerce.
E-mail
on mobile networks may take one of two forms. E-mail can be sent to a mobile
user directly or the user can have an e-mail account maintained by the network
operator or their Internet service provider (ISP). In the latter case, a
notification is forwarded to the mobile terminal and includes the first few
lines of the e-mail, details of the sender, the date and time, and the subject.
Fax attachments can also accompany e-mails.
Unified
messaging provides a single mailbox for all messages, including voice mail,
faxes, e-mail, short message service (SMS), and pager messages. Unified
messaging systems allow for a variety of access methods to recover messages of different
types. Some use text-to-voice systems to read e-mail or send faxes over a
normal phone line. Most allow the user to query the contents of the various
mailboxes through data access such as the Internet. Others can be configured to
alert the user on the device of their choice when messages are received.
Value-added
services refer to the content provided by network operators to increase the
value of services to their subscribers. Two terms that are frequently used to
describe delivery of data applications are push and pull,
as defined below.
- Push describes the transmission of data at a
predetermined time or under predetermined conditions. It also refers to
the unsolicited supply of advertising (for example, delivery of news as it
occurs or stock values when they fall below a preset value).
- Pull describes the request for data in real time by
the user (for example, checking stock quotes or daily news headlines).
- Personalized information that is tailored to the user
(for example, a stock ticker that focusses on key quotes and news or an
e-commerce application that knows a user's profile)
- Localized content that is based on a user's current
location and includes maps, hotel finders, or restaurant reviews
- Menu screens that are intuitive and easy to navigate
- Security for e-commerce sites for the exchange of
financial or other personal information
E-commerce
is defined as business conducted on the Internet or data service. This includes
applications in which a contract is established for the purchase of goods and
services and online banking applications. These applications require user
authentication and secure transmission of sensitive data over the data
connection.
The
banking industry is interested in promoting electronic banking because
electronic transactions are less costly to conduct than personal transactions
in a bank. Specific banking functions that can be accomplished over a wireless
connection include balance checking, money transfers between accounts, bill
payment, and overdraft alert.
The
immediacy of transactions over the Internet and the requirement for
up-to-the-minute information has made the purchasing of stocks online a popular
application. By coupling push services with the ability to make secure
transactions from the mobile terminal, a service that is unique to the mobile
environment can be provided.
Location-based
services provide the ability to link push or pull information services with a
user's location. Examples include hotel and restaurant finders, roadside
assistance, and city-specific news and information. This technology also has
vertical applications. These allow, for example, tracking vehicles in a fleet
or managing the operations of a large workforce.
In
the mobile environment, vertical applications apply to systems using mobile
architectures to support the specific tasks within a company. Examples of
vertical applications include:
- Sales support—Configuring stock and product information
for sales staff, integrating appointment details, and placing orders
remotely
- Dispatching—Communicating job details such as location
and scheduling and permitting information queries to support the job
- Fleet management—Controlling a fleet of delivery or
service staff and vehicle, monitoring their locations, and scheduling
their work
- Parcel delivery—Tracking the locations of packages for
customers and monitoring the performance of the delivery system
Advertising
services are offered as a push information service. Advertising may be offered
to customers to subsidize the cost of voice or other information services.
Advertising may be location sensitive. For example, a user entering a mall can
receive advertisements specific to the stores in that mall.
GPRS
is a data network that overlays a second-generation GSM network. This data
overlay network provides packet data transport at rates from 9.6 to 171 kbps.
Additionally, multiple users can share the same air-interface resources
simultaneously.
GPRS
attempts to reuse the existing GSM network elements as much as possible, but to
effectively build a packet-based mobile cellular network, some new network
elements, interfaces, and protocols for handling packet traffic are required.
Therefore, GPRS requires modifications to numerous network elements as
summarized inTable 2-1 and
shown in Figure 2-3.
GSM
Network Element
|
Modification
or Upgrade Required for GPRS.
|
New
terminals are required because existing GSM phones do not handle the enhanced
air interface or packet data. A variety of terminals can exist, including a
high-speed version of current phones to support high-speed data access, a new
PDA device with an embedded GSM phone, and PC cards for laptop computers. These
terminals are backward compatible for making voice calls using GSM.
Each
BSC requires the installation of one or more PCUs and a software upgrade. The
PCU provides a physical and logical data interface to the base station
subsystem (BSS) for packet data traffic. The BTS can also require a software
upgrade but typically does not require hardware enhancements.
When
either voice or data traffic is originated at the subscriber terminal, it is
transported over the air interface to the BTS, and from the BTS to the BSC in
the same way as a standard GSM call. However, at the output of the BSC, the
traffic is separated; voice is sent to the mobile switching center (MSC) per
standard GSM, and data is sent to a new device called the SGSN via the PCU over
a Frame Relay interface.
In
the core network, the existing MSCs are based on circuit-switched
central-office technology and cannot handle packet traffic. Two new components,
called GPRS support nodes (GSNs), are added:
The
SGSN delivers packets to mobile stations (MSs) within its service area. SGSNs
send queries to home location registers (HLRs) to obtain profile data of GPRS
subscribers. SGSNs detect new GPRS MSs in a given service area, process
registration of new mobile subscribers, and keep records of their locations
inside a predefined area. The SGSN performs mobility management functions such
as handing off a roaming subscriber from the equipment in one cell to the
equipment in another. The SGSN is connected to the base station subsystem
through a Frame Relay connection to the PCU in the BSC.
GGSNs
are used as interfaces to external IP networks such as the public Internet,
other mobile service providers' GPRS services, or enterprise intranets. GGSNs
maintain routing information that is necessary to tunnel the protocol data
units (PDUs) to the SGSNs that service particular MSs. Other functions include
network and subscriber screening and address mapping. One or more GGSNs can be
provided to support multiple SGSNs. More detailed descriptions of the SGSN and
GGSN are provided in a later section.
The
term terminal equipment is generally used to refer to the
variety of mobile phones and mobile stations that can be used in a GPRS
environment. The equipment is defined by terminal classes and types. Cisco's
gateway GPRS serving node (GGSN) and data network components interoperate with
GPRS terminals that meet the GPRS standards.
Class
A terminals support GPRS and other GSM services (such as SMS and voice)
simultaneously. This support includes simultaneous attach, activation, monitor,
and traffic. Class A terminals can make or receive calls on two services
simultaneously. In the presence of circuit-switched services, GPRS virtual
circuits are held (i.e., placed on hold) instead of being cleared.
Class
B terminals can monitor GSM and GPRS channels simultaneously but can support
only one of these services at a time. Therefore, a Class B terminal can support
simultaneous attach, activation, and monitor, but not simultaneous traffic. As
with Class A, the GPRS virtual circuits are not disconnected when
circuit-switched traffic is present. Instead, they are switched to busy mode.
Users can make or receive calls on either a packet or a switched call type
sequentially, but not simultaneously.
Class
C terminals support only sequential attach. The user must select which service
to connect to. Therefore, a Class C terminal can make or receive calls from
only the manually selected (or default) service. The service that is not
selected is unreachable. The GPRS specifications state that support of SMS is
optional for Class C terminals.
In
addition to the three terminal classes, each handset has a unique form (housing
design). Some of the forms are similar to current mobile wireless devices,
while others will evolve to use the enhanced data capabilities of GPRS.
The
earliest available type is closely related to the current mobile phone. These
are available in the standard form with a numeric keypad and a relatively small
display.
PC
cards are credit card-sized hardware devices that connect through a serial
cable to the bottom of a mobile phone. Data cards for GPRS phones enable
laptops and other devices with PC card slots to be connected to mobile
GPRS-capable phones. Card phones provide functions similar to those offered by
PC cards without requiring a separate phone. These devices may require an ear
piece and microphone to support voice services.
Smart
phones are mobile phones with built-in voice, nonvoice, and Web-browsing
services. Smart phones integrate mobile computing and mobile communications
into a single terminal. They come in various form factors, which may include a
keyboard or an icon drive screen.
The
increase in machine-to-machine communications has led to the adoption of
application-specific devices. These black-box devices lack a
display, keypad, and voice accessories of a standard phone. Communication is
accomplished through a serial cable. Applications such as meter reading utilize
such black-box devices.
Personal
digital assistants (PDAs), such as the Palm Pilot series or Handspring Visor,
and handheld communications devices are data-centric devices that are adding
mobile wireless access. These devices can either connect with a GPRS-capable
mobile phone via a serial cable or integrate GPRS capability. Access can be
gained via a PC card or a serial cable to a GPRS-capable phone.
One
of the main requirements in the GPRS network is the routing of data packets to
and from a mobile user. The requirement can be divided into two areas: data
packet routing and mobility management.
The
main functions of the GGSN involve interaction with the external data network.
The GGSN updates the location directory using routing information supplied by
the SGSNs about the location of an MS. It routes the external data network
protocol packet encapsulated over the GPRS backbone to the SGSN currently
serving the MS. It also decapsulates and forwards external data network packets
to the appropriate data network and collects charging data that is forwarded to
a charging gateway (CG).
- Mobile-originated message (path 1)—This path begins at
the GPRS mobile and ends at the Host
- Network-initiated message when the MS is in its home
network (path 2)—This path begins at the Host and ends at the GPRS mobile
- Network-initiated message when the MS roams to another
GPRS network (path 3)—This path is indicated by the dotted line
In
these examples, the operator's GPRS network consists of multiple GSNs (with a
gateway and serving functionality) and an intra-operator backbone network.
GPRS
operators allow roaming through an inter-operator backbone network. The GPRS
operators connect to the inter-operator network through a border gateway (BG),
which can provide the necessary interworking and routing protocols (for
example, border gateway protocol [BGP]). In the future, GPRS operators might
implement quality of service (QoS) mechanisms over the inter-operator network
to ensure service-level agreements (SLAs). The main benefits of the
architecture are its flexibility, scalability, interoperability, and roaming
attributes.
The
GPRS network encapsulates all data network protocols into its own encapsulation
protocol called the GPRS tunneling protocol (GTP). The GTP ensures security in
the backbone network and simplifies the routing mechanism and the delivery of
data over the GPRS network.
The
operation of the GPRS is partly independent of the GSM network. However, some
procedures share the network elements with current GSM functions to increase
efficiency and to make optimum use of free GSM resources (such as unallocated
time slots).
Data
is transmitted between an MS and the GPRS network only when the MS is in the
active state. In the active state, the SGSN knows the cell location of the MS.
Packet
transmission to an active MS is initiated by packet paging to notify the MS of
an incoming data packet. The data transmission proceeds immediately after
packet paging through the channel indicated by the paging message. The purpose
of the paging message is to simplify the process of receiving packets. The MS
listens to only the paging messages instead of to all the data packets in the
downlink channels. This reduces battery usage significantly.
When
an MS has a packet to transmit, it must access the uplink channel (i.e., the
channel to the packet data network where services reside). The uplink channel
is shared by a number of MSs, and its use is allocated by a BSS. The MS
requests use of the channel in a random access message. The BSS allocates an
unused channel to the MS and sends an access grant message in reply to the
random access message. The description of the channel (one or multiple time
slots) is included in the access grant message. The data is transmitted on the
reserved channels.
In
the standby state, only the routing area of the MS is known. (The routing area
can consist of one or more cells within a GSM location area).
When
the SGSN sends a packet to an MS that is in the standby state, the MS must be
paged. Because the SGSN knows the routing area of the MS, a packet paging
message is sent to the routing area. On receiving the packet paging message,
the MS relays its cell location to the SGSN to establish the active state.
The
main reason for the standby state is to reduce the load in the GPRS network
caused by cell-based routing update messages and to conserve the MS battery.
When an MS is in the standby state, the SGSN is informed of only routing area
changes. By defining the size of the routing area, the operator can control the
number of routing update messages.
In
the idle state, the MS does not have a logical GPRS context activated or any
packet-switched public data network (PSPDN) addresses allocated. In this state,
the MS can receive only those multicast messages that can be received by any
GPRS MS. Because the GPRS network infrastructure does not know the location of
the MS, it is not possible to send messages to the MS from external data
networks.
When
an MS that is in an active or a standby state moves from one routing area to
another within the service area of one SGSN, it must perform a routing update.
The routing area information in the SGSN is updated, and the success of the
procedure is indicated in the response message.
A
cell-based routing update procedure is invoked when an active
MS enters a new cell. The MS sends a short message containing the identity of
the MS and its new location through GPRS channels to its current SGSN. This
procedure is used only when the MS is in the active state.
The
inter-SGSN routing update is the most complicated routing update. The MS changes
from one SGSN area to another, and it must establish a new connection to a new
SGSN. This means creating a new logical link context between the MS and the new
SGSN and informing the GGSN about the new location of the MS.
The
GPRS architecture consists of signaling interfaces with various protocols that
control and support the transmission of packets across the networks and to the
mobile stations. The interfaces in a GPRS network are:
- Ga—Interface between GSN nodes (GGSN, SGSN) and charging
gateway (CG)
- Gb—Interface between SGSN and BSS (PCU); normally uses
Frame Relay
- Gc—Interface between GGSN and HLR
- Gi—Interface between GPRS (GGSN) and an external packet
data network (PDN)
- Gn—Interface between two GSN nodes, i.e., GGSN and
SGSN; this connects into the intra-network backbone, for example, an
Ethernet network
- Gp—Interface between two GSN nodes in different PLMNs;
this is via border gateways and is an inter-PLMN network backbone
- Gr—Interface between SGSN and HLR
- Gs—Interface between SGSN and the MSC/VLR
- Gf—Interface between SGSN and EIR
Figure 2-6 shows these interfaces.
Figure 2-7 shows the GPRS protocol stack and
end-to-end message flows from the MS to the GGSN. The protocol between the SGSN
and GGSN using the Gn interface is GTP. This is a Layer 3 tunneling protocol
similar to L2TP.
Although Figure 2-7 defines
the Gn and Gi interface as IP, the underlying protocols are not specified,
providing flexibility with the physical medium. The GGSN software runs on a
Cisco 7206VXR hardware platform, which provides a wide range of supported
physical interfaces and a high port density. The GGSN software uses a virtual
template interface, which is a logical interface within the router and does not
depend on the physical medium directly. A list of supported physical interfaces
for the 7206VXR can be found at this
The
most common physical interface used with GPRS is Fast Ethernet. This interface
provides high bandwidth, low cost, and universal connectivity to other vendor
equipment. For the Gi interface, common interfaces are Serial, E1/T1 or
Ethernet. Running over the physical WAN interfaces can be a wide range of
protocols including Frame Relay, ISDN, and HDLC.
The
GTP tunneling protocol is a Layer 3 tunneling protocol. The IP header
identifies a session flow between the GGSN and SGSN. The UDP header identifies
the GTP application protocol (Port 3386). The GTP header identifies the GTP
tunnel session. The payload identifies the session flow between the mobile
station and the remote host. See Figure 2-8.
The
GTP packet structure, like any other packet, typically has a fixed-size header
and other information called payload or information elements. Currently, bits
1-5 of Octet 1 and Octets 7-12 are not in use. TID is the tunnel ID that
identifies a tunnel session. The length field of GTP is different from the
length field of IP. In IP, the length includes the header; in GTP, length
indicates only the GTP payload. See Figure 2-9.
The
GPRS access modes specify whether or not the GGSN requests user authentication
at the access point to a PDN (Public Data Network). The available options are:
- Transparent—No security authorization/authentication is
requested by the GGSN
- Non-transparent—GGSN acts as a proxy for authenticating
Transparent
access pertains to a GPRS PLMN that is not involved in subscriber access
authorization and authentication. Access to PDN-related security procedures aretransparent to
GSNs.
In
transparent access mode, the MS is given an address belonging to the operator
or any other domain's addressing space. The address is given either at
subscription as a static address or at PDP context activation as a dynamic
address. The dynamic address is allocated from a Dynamic Host Configuration Protocol
(DHCP) server in the GPRS network. Any user authentication is done within the
GPRS network. No RADIUS authentication is performed; only IMSI-based
authentication (from the subscriber identity module in the handset) is done.
Non-transparent
access to an intranet/ISP means that the PLMN plays a role in the intranet/ISP
authentication of the MS. Non-transparent access uses the Password
Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol
(CHAP) message issued by the mobile terminal and piggy-backed in the GTP PDP
context activation message. This message is used to build a RADIUS request
toward the RADIUS server associated with the access point name (APN).
The
GPRS standards define a network identity called an access point name (APN). An
APN identifies a PDN that is accessible from a GGSN node in a GPRS network
(e.g., www.Cisco.com). To configure an APN, the operator configures three
elements on the GSN node:
- Access point—Defines an APN and its associated access
characteristics, including security (RADIUS), dynamic address allocation
(DHCP), and DNS services
- Access point list—Defines a logical interface that is
associated with the virtual template
- Access group—Defines whether access is permitted
between the PDN and the MS
The
Cisco GGSN is based on the routing technology, Cisco IOS. It integrates GPRS
with already deployed IP services, like virtual private data networks (VPDNs)
and voice over IP (VoIP).
The
mobile VPN application is the first service targetted for business subscribers
that mobile operators are offering when launching GPRS networks. In GPRS, the
selection of the VPN can be based on the same parameters that are used in VPDN
applications:
- Dialed number identification service (DNIS), i.e., the
called number
- Domain, e.g., user@domain
- Mobile station IDSN (MSISDN) number, i.e, the calling
number
In
GPRS, only the APN is used to select the target network.The Cisco GGSN supports
VPN selection based on the APN.
- Attach process—Process by which the MS attaches (i.e,
connects) to the SGSN in a GPRS network
- Authentication process—Process by which the SGSN
authenticates the mobile subscriber
- PDP activation process—Process by which a user session
is established between the MS and the destination network
- Detach process—Process by which the MS detaches (i.e.,
disconnects) from the SGSN in the GPRS network
- Network-initiated PDP request for static IP address—Process
by which a call from the packet data network reaches the MS using a static
IP address
- Network-initiated PDP request for dynamic IP
address—Process by which a call from the packet data network reaches the
MS using a dynamic IP address
2.
The new SGSN queries the old SGSN for the identity of this handset. The old
SGSN responds with the identity of the handset.
3.
The new SGSN requests more information from the MS. This information is used to
authenticate the MS to the new SGSN.
4.
The authentication process continues to the HLR. The HLR acts like a RADIUS
server using a handset-level authentication based on IMSI and similar to the
CHAP authentication process in PPP.
6.
If the equipment ID is valid, the new SGSN sends a location update to the HLR
indicating the change of location to a new SGSN. The HLR notifies the old SGSN
to cancel the location process for this MS. The HLR sends an insert subscriber
data request and other information associated with this mobile system and
notifies the new SGSN that the update location has been performed.
7.
The new SGSN initiates a location update request to the VLR. The VLR acts like
a proxy RADIUS that queries the home HLR.
Figure 2-10 and Figure 2-11 show
the GPRS attach process (the numbers in the figures correspond to the numbered
steps above).
The
GPRS authentication process is very similar to the CHAP with a RADIUS server.
The authentication process follows these steps:
1.
The SGSN sends the authentication information to the HLR. The HLR sends information
back to the SGSN based on the user profile that was part of the user's initial
setup.
2.
The SGSN sends a request for authentication and ciphering (using a random key
to encrypt information) to the MS. The MS uses an algorithm to send the user ID
and password to the SGSN. Simultaneously, the SGSN uses the same algorithm and
compares the result. If a match occurs, the SGSN authenticates the user.
Figure 2-12 describes the GRPS
authentication process that the MS uses to gain access to the network (the
numbers in the figure correspond to the numbered steps above).
1.
The SGSN receives the activation request from the MS; for example, the MS
requests access to the APN Cisco.com.
3.
The SGSN initiates a DNS query to learn which GGSN node has access to the Cisco.com APN.
The DNS query is sent to the DNS server within the mobile operator's network.
The DNS is configured to map to one or more GGSN nodes. Based on the APN, the
mapped GGSN can access the requested network.
4.
The SGSN sends a Create PDP Context Request to the GGSN. This message contains
the PAP information, CHAP information, PDP request, APN, and quality of service
information.
5.
If operating in the non-transparent mode, the PAP and CHAP information in the
PDP request packet is sent to the RADIUS server for authentication.
6.
If the RADIUS server is to provide a dynamic IP address to the client, it sends
a DHCP address request to the DHCP server. In transparent mode, the RADIUS
server is bypassed.
7.
If IPSec functionality is required, security functions occur between the GGSN
and network access server (NAS).
Figure 2-13 shows the PDP context activation
procedure. The red arrows indicate the communication between the SGSN and GGSN.
The numbers in the figure correspond to the numbered steps above.
When
a mobile subscriber turns off their handset, the detach process initiates. The
detach process is described below.
3.
The SGSN sends an IMSI Detach Indication message to the MSC/VLR indicating the
MS request to disconnect.
 |
Note The GSN nodes must always respond to the
detach request with a positive delete response to the MS and accept the
detach request requested by the client. The positive delete response is
required even if the SGSN does not have a connection pending for that client.
|
Figure 2-14 describes the detach process
initiated by the MS. The numbers in the figure correspond to the numbered steps
above.
The
PDP protocol data unit (PDU) initiated from the network side is not fully
specified by ETSI standards. A connection request generated from the
Internet/intranet site specifies only the IP address of the client in the IP
packets destined for the MS. The requesting host provides no indication of the
mobile device IMSI (i.e., the MAC address of the MS). In mobile communications,
all communications are based on the MS MAC address called the IMSI. The IP
address must be mapped to an IMSI to identify a valid GTP tunnel. Cisco's GGSN
implementation provides a mapping table via command line interface (CLI) that
allows the operator to key in the MS IMSI and the associated static IP address.
The
following steps describe a PDP request initiated from the network side when the
client has been assigned a static IP address.
1.
When the GGSN receives a packet, it checks its mapping table for an established
GTP tunnel for this packet.
2.
When the GGSN locates the IMSI associated with this IP address, it sends a Send
Routing Information message to HLR through an intermediate SGSN. The
intermediate SGSN notifies the GGSN of the actual SGSN currently serving this
client.
3.
On locating the appropriate SGSN, the GGSN sends a PDU Notification Request
message to the serving SGSN.
4.
The SGSN sends a Request PDP Context Activation message to the MS and notifies
it of the pending connection request.
5.
If the MS agrees to accept the call, it enters the PDP Context Activation
procedure with the requesting GGSN.
Figure 2-15 shows a PDP request initiated
from the network side when the client has been assigned a static IP address.
The numbers in the figure correspond to the numbered steps above.
The
ETSI standards do not fully specify requirements for a network-generated PDP
request when the client is dynamically assigned a temporary IP by a DHCP server.
The following message sequence is Cisco's implementation for this scenario.
This method uses Cisco's Network Registrar (CNR), which includes a DHCP, DNS,
and an LDAP server.
1.
The host initiates a DNS query to obtain the IP address of the MS from a DNS
server. The DNS server resolves the client's name to an IP address previously
assigned to the client by the DHCP server.
3.
The GGSN queries the LDAP server to obtain the MS IMSI. The LDAP server stores
a record for the MS with the client IMSI, name, and IP address.
5.
The SGSN sends a Request PDP Context Activation message to the MS and notifies
it of the pending connection request.
6.
If the MS agrees to accept the call, it enters the PDP Context Activation
procedure with the requesting GGSN.
Figure 2-16 describes a PDP request
initiated from the network side when the client has been assigned a dynamic IP
address. The numbers in the figure correspond to the numbered steps above.
The
Universal Mobile Telecommunication System (UMTS) is a third generation (3G)
mobile communications system that provides a range of broadband services to the
world of wireless and mobile communications. The UMTS delivers low-cost, mobile
communications at data rates of up to 2 Mbps. It preserves the global roaming
capability of second generation GSM/GPRS networks and provides new enhanced
capabilities. The UMTS is designed to deliver pictures, graphics, video communications,
and other multimedia information, as well as voice and data, to mobile wireless
subscribers.
The
UMTS takes a phased approach toward an all-IP network by extending second
generation (2G) GSM/GPRS networks and using Wide-band Code Division Multiple
Access (CDMA) technology. Handover capability between the UMTS and GSM is
supported. The GPRS is the convergence point between the 2G technologies and
the packet-switched domain of the 3G UMTS.
The
UMTS provides support for both voice and data services. The following data
rates are targets for UMTS:
Data
services provide different quality-of-service (QoS) parameters for data
transfer. UMTS network services accommodate QoS classes for four types of
traffic:
- Conversational class—Voice, video telephony, video
gaming
- Streaming class—Multimedia, video on demand, webcast
- Interactive class—Web browsing, network gaming,
database access
- Background class—E-mail, short message service (SMS),
file downloading
- Internet access—Messaging, video/music download,
voice/video over IP, mobile commerce (e.g., banking, trading), travel and
information services
- Intranet/extranet access—Enterprise application such as
e-mail/messaging, travel assistance, mobile sales, technical services,
corporate database access, fleet/warehouse management, conferencing and
video telephony
- Customized information/entertainment—Information
(photo/video/music download), travel assistance, distance education,
mobile messaging, gaming, voice portal services
- Multimedia messaging—SMS extensions for images, video,
and music; unified messaging; document transfer
- Location-based services—Yellow pages, mobile commerce,
navigational service, trading
The
public land mobile network (PLMN) described in UMTS Rel. '99 incorporates three
major categories of network elements:
- GSM phase 1/2 core network elements—Mobile services
switching center (MSC), visitor location register (VLR), home location
register (HLR), authentication center (AuC), and equipment identity
register (EIR)
- GPRS network elements—Serving GPRS support node (SGSN)
and gateway GPRS support node (GGSN)
- UMTS-specific network elements—User equipment (UE) and
UMTS terrestrial radio access network (UTRAN) elements
The
UMTS core network is based on the GSM/GPRS network topology. It provides the
switching, routing, transport, and database functions for user traffic. The
core network contains circuit-switched elements such as the MSC, VLR, and
gateway MSC (GMSC). It also contains the packet-switched elements SGSN and
GGSN. The EIR, HLR, and AuC support both circuit- and packet-switched data.
The
Asynchronous Transfer Mode (ATM) is the data transmission method used within
the UMTS core network. ATM Adaptation Layer type 2 (AAL2) handles
circuit-switched connections. Packet connection protocol AAL5 is used for data
delivery.
The
General Packet Radio System (GPRS) facilitates the transition from phase1/2 GSM
networks to 3G UMTS networks. The GPRS supplements GSM networks by enabling
packet switching and allowing direct access to external packet data networks
(PDNs). Data transmission rates above the 64 kbps limit of integrated services
digital network (ISDN) are a requirement for the enhanced services supported by
UMTS networks. The GPRS optimizes the core network for the transition to higher
data rates. Therefore, the GPRS is a prerequisite for the introduction of the
UMTS.
- Uu interface—User
equipment to Node B (the UMTS WCDMA air interface)
- Iu interface—RNC
to GSM/GPRS (MSC/VLR or SGSN)
- Iub interface—RNC
to Node B interface
- Iur interface—RNC
to RNC interface (no equivalent in GSM)
The Iu, Iub,
and Iur interfaces are based on the transmission principles of
aynchronous transfer mode (ATM).
The
major difference between GSM/GPRS networks and UMTS networks is in the air
interface transmission. Time division multiple access (TDMA) and freqency
division multiple access (FDMA) are used in GSM/GPRS networks. The air
interface access method for UMTS networks is wide-band code division multiple
access (WCDMA), which has two basic modes of operation: frequency division
duplex (FDD) and time division duplex (TDD). This new air interface access
method requires a new radio access network (RAN) called the UTMS terrestrial
RAN (UTRAN). The core network requires minor modifications to accommodate the
UTRAN.
Two
new network elements are introduced in the UTRAN: the radio network controller
(RNC) and Node B. The UTRAN contains multiple radio network systems (RNSs), and
each RNS is controlled by an RNC. The RNC connects to one or more Node B
elements. Each Node B can provide service to multiple cells.
The
RNC in UMTS networks provides functions equivalent to the base station
controller (BSC) functions in GSM/GPRS networks. Node B in UMTS networks is
equivalent to the base transceiver station (BTS) in GSM/GPRS networks. In this
way, the UMTS extends existing GSM and GPRS networks, protecting the investment
of mobile wireless operators. It enables new services over existing interfaces
such as A, Gb, and Abis, and new interfaces
that include the UTRAN interface between Node B and the RNC (Iub) and
the UTRAN interface between two RNCs (Iur).
The
radio network controller (RNC) performs functions that are equivalent to the
base station controller (BSC) functions in GSM/GPRS networks. The RNC provides
centralized control of the Node B elements in its covering area. It handles protocol
exchanges between UTRAN interfaces (Iu, Iur, and Iub).
Because the interfaces are ATM-based, the RNC performs switching of ATM cells
between the interfaces. Circuit-switched and packet-switched data from
the Iu-CS and Iu-PS interfaces are multiplexed
together for transmission over the Iur, Iub, and Uu interfaces to and from the
user equipment (UE). The RNC provides centralized operation and maintenance of
the radio network system (RNS) including access to an operations support system
(OSS).
The
RNC uses the Iur interface. There is no equivalent to manage
radio resources in GSM/GPRS networks. In GSM/GPRS networks, radio resource
management is performed in the core network. In UMTS networks, this function is
distributed to the RNC, freeing the core network for other functions. A single
serving RNC manages serving control functions such as connection to the UE,
congestion control, and handover procedures. The functions of the RNC include:
- Radio resource control
- Admission control
- Channel allocation
- Power control settings
- Handover control
- Macro diversity
- Ciphering
- Segmentation and reassembly
- Broadcast signalling
- Open loop power control
Node
B is the radio transmission/reception unit for communication between radio
cells. Each Node B unit can provide service for one or more cells. A Node B
unit can be physically located with an existing GSM base transceiver station
(BTS) to reduce costs of UMTS implementation. Node B connects to the user
equipment (UE) over the Uu radio interface using wide-band
code division multiple access (WCDMA). A single Node B unit can support both
frequency division duplex (FDD) and time division duplex (TDD) modes. The Iub interface
provides the connection between Node B and the RNC using asynchronous transfer
mode (ATM). Node B is the ATM termination point.
The
main function of Node B is conversion of data on the Uu radio
interface. This function includes error correction and rate adaptation on the
air interface. Node B monitors the quality and strength of the connection and
calculates the frame error rate, transmitting this information to the RNC for
processing. The functions of Node B include:
- Air interface transmission and reception
- Modulation and demodulation
- CDMA physical channel coding
- Micro diversity
- Error handling
- Closed loop power control
Node
B also enables the UE to adjust its power using a technique called downlink
transmission power control. Predefined values for power control are derived
from RNC power control parameters.
The
UMTS user equipment (UE) is the combination of the subscriber's mobile
equipment and the UMTS subscriber identity module (USIM). Similar to the SIM in
GSM/GPRS networks, the USIM is a card that inserts into the mobile equipment
and identifies the subscriber to the core network.
The
USIM card has the same physical characteristics as the GSM/GPRS SIM card and
provides the following functions:
- Supports multiple user profiles on the USIM
- Updates USIM information over the air
- Provides security functions
- Provides user authentication
- Supports inclusion of payment methods
- Supports secure downloading of new applications
The
UMTS standard places no restrictions on the functions that the UE can provide.
Many of the identity types for UE devices are taken directly from GSM
specifications. These identity types include:
- International Mobile Subscriber Identity (IMSI)
- Temporary Mobile Subscriber Identity (TMSI)
- Packet Temporary Mobile Subscriber Identity (P-TMSI)
- Temporary Logical Link Identity (TLLI)
- Mobile station ISDN (MSISDN)
- International Mobile Station Equipment Identity (IMEI)
- International Mobile Station Equipment Identity and
Software Number (IMEISV)
- PS/CS mode—The UE is attached to both the packet-switched
(PS) and circuit-switched (CS) domain, and the UE can simultaneously use
PS and CS services.
- PS mode—The MS is attached to the PS domain and uses
only PS services (but allows CS-like services such as voice over IP
[VoIP]).
No comments:
Post a Comment