Transcript Registration Steps

Chapter 7:
Mobile Communication
Systems
Associate Prof. Yuh-Shyan Chen
Dept. of Computer Science and
Information Engineering
National Chung-Cheng University
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Introduction
A wireless systems implies support for
subscriber mobility and the overall
communication infrastructure needed to
handle movements not only from one cell
to another,
in a cell’s mobile switching center area and
in area controlled by other service providers
Handoff among cells and mobile switching
centers (MSCs) that are serviced by the same
service providers or different service providers
needs to be supported
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Outline
This chapter
Consider handoff schemes
Allocation of resources
Routing in the backbone network
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Cellular System Infrastructure
A generic block diagram of a cellular
system is given in Fig. 9.1.
Each BS consists of a base transceiver system
(BTS) and BS controller (BSC)
Both tower and antenna are part of BTS, and all
associated electronics are contained in the BTS
Authentication Center (AUC) unit provides
authentication and encryption parameters that
verify the user’s identify and ensure the
confidentially
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A detailed block diagram of a cellular
system
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Cellular System Infrastructure
Equipment Identity (EIR) is a database that
contains information about the identify of mobile
equipment that prevents calls from being stolen
Both AC and EIR could be implemented as individual
stand-alone nodes or as a combined AUC/EIR node
The home location register (HLR) and visitor
location register (VLR) are two sets of pointers
that support mobility and enable the use of the
same telephone number over a wide range
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Cellular System Infrastructure
The HLR is located at the MSC where the MS is
initially registered and is the initial home
location for billing and access information
Any incoming call, based on the calling number,
is directed to the HLR of the home MS, which
then points to the VLR of the MSC where the
MS is currently located
The VLR contains information about all MSs
visiting the particular MSC and hence points to
the HLR of the visiting MSs for exchanging
related information about the MS
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Cellular System Infrastructure
Such two-way pointers allow calls to be routed
or rerouted to the MS, wherever it is located
There are many control signals that traverse
back and forth between the HLR and VLR
(including billing and access permissions
maintained at the home MSC), bidirectional
HLR-VLR pointers help in carrying out various
functionalities
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Redirection of a call to MS at a visiting
location
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Redirection
This works very well if the destination MS has
moved from one cell to another
If a call is initiated from a residential telephone,
the call is forward through the backbone
network to the wireless access point closet to
the home MSC, where the MS being called is
registered
Thereafter, a similar routing enables connection
to the MS
The reverse path connection can be
established
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Redirection
The home MSC also maintains access
information about all MSs registered, including
of
State of the MS (active/nonactive)
Type of allowed service (local and/or long-distance
calls)
Billing information (past credit, current charges,
chronological (依年代順序) order of calls made,
timings of each call)
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Redirection
This work even if the home MSC is different
from the visiting MSC
As long as two MSCs have information about
how to forward message to each other
Mobility can also be supported is an unknown
territory (領域)
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Registration
This is maintained not only for billing, but also
for authentication, verification, as well as for
access privileges (特權)
The wireless systems needs to know whether
the MS is currently located in its own home area
or is visiting some other area
This enables incoming calls to be routed to an
appropriate location and assures desirable
support for outgoing calls
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Registration
This is done by exchanging signals known as
“beacons” between BS and the MS
BSs periodically broadcast beacon signals to
determine and test nearby MSs
Each MS listens for new beacon signals, and
information is the used by a MS to locate the
nearest BS and establish an appropriate rapport
to initiate dialogue with outside world through
the BS as a gateway
Other information of beacon
Cellular network identifier, timestamp, gateway
address, ID (Identification) of the paging area
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Beacon signals
Beacon signals help synchronize, coordinate,
and manage electronic resource using
miniscule (微不足道) bandwidth for a very short
duration
Researchers continue to improve their
functionality by increasing signal coverage while
optimizing energy consumption
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Using a mobile phone outside the
subscription area
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Registration Steps
1. A mobile phone listens for new
beacons, and if it detects one, it
adds it to the active beacon kernel
table. If the device determines that
it needs to communicate via a new
BS, kernel modulation initiates the
handoff process.
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Registration Steps
2. The mobile phone locates the
nearest BS via user-level
processing.
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Registration Steps
3. The visiting BS performs user-level
processing and determines who
the user (MS) is, the user’s
registered home site for billing
purposes, and what kind of access
permission the user has.
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Registration Steps
4. The home site sends an
appropriate authentication
response to the BS currently
serving the users.
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Registration Steps
5. The BS at the visited location
approves or disapproves user
access.
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Handoff Parameters and Underlying
Support
Handoff basically involves change of radio
resource from one cell to another adjacent
cell
From a handoff perspective, it is important
that a free channel is available in a new
cell whenever handoff occurs, so that
undisrupted service is available
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Handoff Underlying Support
Hard handoff
Also known as break before make
Is characterized by releasing current radio
resources from the prior BS before acquiring
resource from the next BS
Both FDMA and TDMA employ hard handoff
Soft handoff
It is possible for an MS to communicate
simultaneously with the prior BS as well as the
new BS, just for some short duration of time
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Handoff Underlying Support
In CDMA, as the same channel is used in all
the cells, if the code is not orthogonal to other
codes being used in the next BS, the code
could be changed
In fact, beacon signals and the use of the
HLR-VLR pair allow MSs to roam
anywhere as long as the same service
provider, using particular frequency band
in that area (Fig. 9.6)
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Handoff between MSCs
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Handoff region
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Hard handoff
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Soft handoff
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Parameter Influencing Handoff
Handoff depends on cell size, boundary
length, signal strength, fading, reflection,
and refraction of signals, and by manmade noise
The probability of a channel being
available in a new cell area depends on
the number of channels per unit area

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Parameter Influencing Handoff
It can be easily observed that the number of
channels/area increases if the number of channels
allocated per cell is increased or if the area of each cell
is decreased
This leads to a smaller cell size may be good
 However, this would cause more frequent handoff,
especially for MSs with high mobility and speed
 Handoff can be initiated either by the BS or the
MS and it could be due to
Radio link
Network Management
Service issues
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Radio link-type factros
Number of MSs that are in the cell
Number of MSs that have left the cell
Number of calls generated in the cell
Number of calls transferred to the cell from
neighboring cells by the handoff
Number and duration of calls terminated in the
cell
Number of calls handed off to neighboring cells
Cell dwell time
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Network management
Network management may cause handoff if
there is a drastic imbalance of traffic over
adjacent cells and optimal balance of channels
and other resources are required
Service-related handoff is due to degradation of
quality of service (QoS), and handoff could be
invoked when such a situation is detected
The factor that define the right time for handoff
are
Signal strength, signal phase, bit error rate, distance
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The need for handoff is determined in
two different ways
Signal strength
Carrier-to-interference ratio
A low value of CIR may force the BS to change
the channel currently being used between the
BS and the MS.
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Roaming Support Using System
Backbone
What happens when a channel and the radio
contact is changed from one cell to another
when handoff occurs
A number of cells are controlled by an MSC, and
depending on the destination, the signals go through
the backbone network, interconnecting MSCs with
the PSTN
The hardwired network is primarily supported by
ultra-high-speed fiber optic cables
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Roaming Support Using System
Backbone
With mobility support, the real problem in
routing becomes getting moving packets to
appropriate endpoints of the backbone network
(Fig. 9.7)
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Handoff with different degree of
mobility
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Roaming
Assuming MSC1 to be the home of the MS for
registration, billing, authentication, and all
access information
When the handoff is from a to b, the routing of
message can be performed by MSC itself
 When the handoff occurs from b to c, the
bidirectional pointers are set up to link the HLR of
MSC1 to VLR of MSC2 so that information can be
routed to the cell where the MS is currently located
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Information transmission path
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Roaming
When handoff occurs at locations d and e,
routing of information using simply the HLRVLR pair of pointers may not be adequate
The paging area is the area covered by one or
several MSC in order to find the current location
of the MS
This concept is taken from the internet network
routing area
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MSC connections to backbone
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An example:
One involves determining the path along the
shortest path
The second involves ascertaining the path
according to the current location of the MS
The movement from a to c can be supported
effectively by HLR-VLR, wherein MSC1 knows
how to route the data to MSC2
One option is to let all the messages reach MSC1
and forward the messages from there to the MS
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An example:
Another option is to find a router along the original
path, from where a new path needs to be used to
reach the destination MSC along the shortest path
If this is done, then part of the message in the
pruned tree could be lost if a hard handoff is
performed, when breaks the connection before
it makes
Therefore, after handoff, it may be desirable to
forward message from an old location to a new
one, for a short duration of time
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Home Agents, Foreign Agents, and
Mobile IP
In mobile internet protocol (Mobile IP), two
important agents are associated with the
routers: home agent (HA) and foreign agent (FA)
A router closet to the home MSC can be selected to
serve as its HA (Table 9.2)
Once MS moves from the home network (where it is
registered) to a foreign network, a software agent, FA,
assists the MS by forwarding packets for the MS
The functionality of HA-FA is somewhat analogous to
the HLR-VLR pair, as long as there is an agreement
and understanding between the home network and
the foreign network
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Home MSC and Home Agent
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The way it works as follows.
Whenever A MS moves into a new network, it retains its
initial HA
The MS can detect the FA of the current domain by the
periodic beacon signals that the FA transmits
 On the other hand, the MS can itself send agent
solicitation messages
When the FA detects that a new MS has moved into its
domain, it allocates a care-of-address (CoA) to the MS
 The CoA can either be the address of the FA itself, or it
maybe a new address called colocated CoA (C-CoA) that
the FA allocates to the MS using the dynamic host
configuration protocol (DHCP, IETF RFC 2131, Marcg
1997)
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Registration process
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Registration process
Once the MS receives the CoA, it registers this
CoA with its HA and the time limit for its binding
validity
Such a registration is initiated either directly by
the MS to its HA, or indirectly through the FA at
the current location
The HA the confirms this binding through a
reply to the MS
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Message forwarding
A message sent from arbitrary source to the MS
at the home address is received by the HA,
binding for the MS is checked, and the HA
encapsulates the packet with the CoA of the MS
and forwards it to the FA area
If the colocated CoA address is used, the MS
receives the packet directly and is decapsulated to
interpret the infomration
If CoA for the FA is used, then the packet reaches the
FA, which decapsulates the packet and passes on to
the MS at the link layer
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Message forwarding to the MS
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Rerouting in Backbone Routers
A simplistic approach
Is to have a global table at all routers of the
network so that the route from FA to HA can be
founded
This one-step global table is excessively large
Another one
Only gateway routers that support routing within
the backbone are shown in Fig. 9.12 and Table
9.3
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Paging areas (PAs)
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Distributed Routing Table and
Location of PAs
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