Friday, July 26, 2013
Introduction
The rapidly growing demand for “anywhere and anytime” high speed
Internet access will be one of the major forthcoming challenges for mobile
network operators. As the need for mobility increases, the ability to connect
mobile terminals, from laptops and PDAs to future mobile videophones, to the
Internet and Intranet, and roam freely across geographical boundaries of
heterogeneous networks has become a business driver.
Next generation
mobile/wireless all-IP networks are expected to provide a substantially wider
and enhanced range of services, including global convergence, interoperability
and mobility management process under a world wide wireless communication
infrastructure. Terminal and personal mobility will enable users to access
services using their personal profile, independent of terminal type and the
point of attachment to the network. This capability, together with the inherent
IP support is a powerful combination to deliver personalized interactive
multimedia services to mobile users.
Deployment of a global all-IP
wireless/mobile network, however, is not a straightforward decision. First of
all the potential advantages and added value of such an evolution are not clear
to subscribers, while operators have to carry out significant investments to
enhance their infrastructure and obtain expensive frequency licenses. Moreover,
although IP is by far the most widely accepted protocol, it still has intrinsic
weakness, like limited address space, lack of inherent mobility and quality of
service (QOS) mechanisms and poor performance over wireless links.
As far as
mobility is concerned, current wireless networks may be organized in three
groups
·
Wireless LAN (WLANs) for
local area.
·
Cellular for wide area.
·
Satellite for worldwide
coverage.
WLANs are
supported by two international standards:
IEEE 802.11a and b .
ETSI HIPERLAN 1 and 2.
Bluetooth has
been proposed for shorter Distances.
Cellular
Networks: High Speed Circuit Switched Data(HSCSD).
General
Packet Switched Data(GPRS).
Satellite
Communications: medium/low Earth orbit (MEO/LEO) networks are considered.
Mobility Management Today
One of the major requirements of next generation mobile networks
will be personal and terminal mobility. Personal mobility provides the users
the ability for the user to access their personal services, independent of the
terminal type or point of attachment. Personal mobility is a primary concern of
service providers. Terminal mobility applies to the ability of the network to
locate a mobile terminal, route incoming or outgoing calls regardless of its
network point of attachment, and maintain connections while terminal roams in
the network.
The wireless network is an organized in
service region called cells. According to the type of network, the size of the
cell varies from a few square meters in wireless LANs to hundred of square
kilometers in satellite networks. In all cases, a central system handles the
frequencies or channels of each mobile terminal and supports location
management and handover. Location management tracks and locates a terminal for
delivering of incoming calls, while handover allows for an active connection to
remain alive while the terminal roams. Location management handles information
concerning the mobile terminal, its original cell, the cell where it is
currently located, and paths and routes towards the current location. The
information is updated either periodically or on demand when a specific event
occurs, and is stored and retrieved in location or paging databases,
independent of the specific network or location management protocol.
Handoff
management handles roaming in the same cell (Intracell handover) or between
cells (Intercell handover). In intracell handover, when the signal-to-noise
ratio (SNR) falls and the radio channel deteriorates, a dynamic channel
allocation (DCA) mechanism is activated that transfers the call to a new radio
channel or frequency with the appropriate strength within the cell. In
intercell handover connections are passed to a new central station that covers
a neighboring area. If the transfer is uninterrupted, it results in a soft
handover, while if the connections have to be re-established, we have a hard
handover. Handover may be initiated either the user terminal or the mobile
network. In network controlled or mobile assisted hand over, the network establishes
and handles the connection, searches for the new resources, and performs the
additional routing and updating functions. In the mobile controlled handover
the mobile initiates handover and seeks available resources while the network
authorizes the operation and handles the flow control and delay requirements.
Mobility in Wireless LANs
Mobility management in WLANs is primarily supported by Internet
Engineering Task Force (IETF) mobile IP and various extensions. The address
used by the traditional Ipv4 protocol, apart from identifying a specific
network node, contains the topological information. Under the current form of
Ipv4 , if the mobile terminal moves between different subnetworks without
changing its address ,the routing process will cease to be operational. On the
other hand, if the IP address is modified, all active connections will be
terminated .To overcome this problem and allow the mobile terminal to roam
freely around the network while still communicating and maintaining the same IP
address, the mobile IP protocol is utilized.
Mobile IP introduces two new functional components the mobility
agent (MA) and mobile node (MN). An MA, which could be a home agent (HA) or
foreign agent (FA), is responsible for mobile terminals of a specific sub
network, while a MN is located inside a mobile terminal. The subnetwork to
which the IP address of a mobile terminal belongs is called the home network,
whereas any other subnetwork the mobile terminal might visit is called a
foreign network.
When a MN is located on the
home network, it operates without any mobility services. Whenever it detects
that it has moved to a foreign network, it obtains the care-of address, which
can be determined either from agent advertisement messages sent by the FA (a FA
care-of address) or using the Dynamic Host Configuration Protocol (DHCP).
Having obtained the care-of address, the MN registers that address through the
FA, with the HA. Form this point on, the HA intercepts all IP packets destined
to MN and tunnels them to FA, where after decapsulation, they are forwarded to
MN.
Mobility in 2G Networks
Mobility management in second-generation cellular network is
supported by two international standards:
1.
The Electronic
/Telecommunications Industry Associations Interim standard 41 (EIA/TIA IS-41).
2.
The GSM Mobile
Application part (MAP) for GSM.
The 2G networks are organized into cells, while the switching center
responsible for a specific geographical or logical area is known as the mobile
switching center (MSC). Location management is based on location databases,
called home location register (HLR) and visitor location register (VLR).
Visitor location register can be considered extra intelligence on each MSC, and
contains temporal information for a specific area. HLRs are hierarchical higher
databases that contain permanent information for each terminal. The entry of
each subscriber is registered in one HLR, including a link to the VLR, which is
responsible for the area the terminal is currently visiting.
When a mobile terminal changes the base station it may roam to a
cell that corresponds to a new serving VLR .In that case it has to update the
information stored in the HLR. Therefore the
terminal initiates an update message, which via the base station and MSC is
forwarded to current associated VLR. The VLR checks its local records
.If the terminal‘s mobile identification number
(MIN) is already stored there, no further action takes place, since the
terminal has not changed location area. Other wise, the terminal’s MIN is
stored locally and a new update message is forwarded to the HLR. The HLR in
turn authenticates the terminal and replies with a positive registration
acknowledgement to the new VLR. Additionally the HLR may send a registration
cancellation message to the old VLR, or a periodical mechanism may
automatically update the VLR database and remove out-of-date entries.
Whenever a new connection is initiated the VLR will check its local
records again for the called mobile. If both calling and called parties are in
the same servicing area, the call is directly routed to the terminal.
Otherwise, the VLR of the calling terminal initiates a location request to the
HLR .The HLR confirms that the terminal is located in this area and sends a
route request message. This message is forwarded via the VLR to the serving
MSC, which allocates a temporary local directory number (TLDN) for the specific
terminal. The TLDN is returned to the HLR and forwarded to the calling VLR .If
the terminal changes VLR while connections are established, all the steps have
to be repeated, increasing the signaling overhead, especially when the terminal
is far away from the HLR.
Mobility in satellite Networks
Telecommunication
satellite can be categorised in three groups : Geostationary earth orbit
satellite, MEO.The advantage of geo satellites is that they rotate at the same
angular velocity as the earth,always keeping a fixed position in reference to
the ground.in this way geo satellites appear at a fixed latitude and longitude
. moreover,rather long distance from the earth surface(roughly 36,000 km from
the ssurface),geo satellites have a very large servicing area of almost 1/3rd
of the eartrh surface.The combination of the fixed position along with very
large servicing area provides near-global coverage with a minimum of three
satellites in an orbit . Communications
geosatellites are very useful,especially for broadcasting services.
Meo
satelllites rotate at an altitude of around 10,000 km. In contrast to geo, meo
satellites donot have a fixed position over the earth. They rotate at different
angular velocities compared to the earth; thus, they move in reference to the
ground. The global communication system using meo orbits requires a reasonable
number of satellites in two to three orbotal planes to achieve global coverage.
Finally, leo
satellites rotate in orbits much closer to the earth at a height of 500 to 2000
km above the surface of the earth. Like meo, neo satellites donot have a fixed position over the earth
and move in reference to the ground. This may be acceptable for a store and
forward type of communication system, but not for interactive commununication,
Howeverleo satellites, due to their smaller distance from the surface, may achieve comparable very
good end to end delay and have low power consumption requirements for both the
mobile terminal and satellite. In order to increase accessibility and make
global coverage possible, more than one satellite and multiple orbital planes
are utilised.
Location
management in geo networks may be considered similar to 2G networks. However,
in meo and leo it is not only the termilal movement, but the satellite movement as well that must be taken into
account. In order to better locate a terminal, the coverage area of a single
satellite is dividsed into small cells, called spot beams.
Handovering
in geo networks is not a very common issue. On the contrary, handover in leo
networks is much more important and demanding . As terminals and satellites
change position, spotbeam and satellite handovers are defined. The size of the
spotbeam is rather small; therefore satellite handover typically happens every
10 min, spotbeam handover every 38 s. Spotbeam handover occurs in most cases
due to satellite rotation, not to terminal movement; thus, all connections may
be transfered as a group to a neighbouring satellite. The strightforward
approach to satellite is to establish a new connection each time a handover
happens.
Next
Generation Network Architecture
Next
generation mobile/wireless networks are expected to provide a substancially
wider and enhanced range of services. Global convergence, interoperability, and
mobility are some of the differentiating factors form current networks.
Moreover, the inherent IP suppory will
encourage new personalised interactive multimedia services
as well as new bradband applications, such as video telephony,
videoconfrrencing, and mobile Internet.
The wireless
network infrastructure may be
organised in cell hierarchy, based on technology either deployed or still
under development. Starting from the home cell, coverage in private
buildings(e.g., house, office) or in public "hot-spot"
locations(e.g., airport, train station, conference center) may be provide by
access points(AP). IEEE 802.11, HIPERLAN, Bluetooth, and Home-RF are
alternative technologies that may be deployed. The APs may also provide
connectivity in pinocells, while a combination with pico- GCM or DECT can also
be considered. Moreover, fixed wireless access via central stations(CSs) and
remote stations(RSs) may provide wireless access upto macrocells in suburban
areas. Horizontal mobility to mobile terminals that move with different speeds
in micro or macrocells may be provided utilizing 2G and 2G+
networks(GSM,HSCSD,GPRS,EDGE,CDPD,1S-95,CDMA). Connectivity and mobility in
satellite cells are provided via GEO,MEO, or LEO satellites and Fixed
EarthStations (FESs) or mobile satellite terminals(STs).
In order to
support both horizontal and vertical roaming in such a complex environment, the
first step is to gain connectivity at the physical layer. In this respect,
either multimode or adaptive terminals are considered. For example, terminals
equipped with commercial wireless LAN(e.g., IEEE 802.11b), cellular soft radio
techniques have also been proposed. Global roaming, however, requires integrates and interoperation of the mobility management processes of each
independent network. IP is the most widely accepted protocol; and thus.
mobility based on IP will be leverged.
A detailed architecture of an all-IP wire
less/mobile network architecture is shown in below Fig., WLAN,2G, and 3G,
cellular and satellite networks are selected as alternative radio access
network. Due to different physical protocol Starting characteristics, each
radio access network consists of
different base stations and radio control nodes, connected to the common core
network via a service support node(SSN). This may be an MSC+ for cellular
networks, an IP L1/L2 switch for the WLAN, or an FES. The SSN also provides the VLR or FA
functionality, respectivel, in cooperation with an extended HLR+ or home
subscriber server authentication, authorisatin dial-in user server(RADIUS)
and/or an authenticatian, authorization, and accounting (AAA) server for
user authentication and authorization.
