Mobile networks and mobility
management
Impacts of mobility on networks
A few mobility solutions
Raimo Kantola
raimo.kantola@hut.fi
SG210, 4512471
R.Kantola, (translation: A.Paju)/18.10.00/s38.118
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Mobility demands logical subscriber numbers that
must be mapped to the topology of the network
• Routing numbers describe the network topology.
• An example: 109 subscribers, the length of subscriber numbers is 13 digits
Let’s calculate an approximation of memory requirements :
The analysis tree consists of nodes of 64 octets. One digit is
analyzed in each node:
Usage of number positions:
m13 = 109
13 lg m = 9
m = 4.92
The number of nodes in the analysis tree is:
m13 - 1
= 305 million
1 + m + m2 + … m12 =
m-1
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The number analysis tree of the exchange
associates routing with signaling information
From signaling: ABC - destination (area)
ABCd - the shortest subscriber number
ABCdefgh - the longest subscriber
number
A
B
Buckets
C
We assume a treelike
data structure in
the analysis.
d
e
f
g
h
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Calculation continues ...
Memory required for the analysis tree: 64 * 305 * 106 = 19 Gb
•Copying this to many places is very expensive.
• Analysis of number in this database demands 13 reads, which
basically isn’t a problem
• The biggest technical problem is updating the database:
Let us assume that:
- one update demands a message
of 50b
- it takes 6h to update the entire DB
Note:
- For reliability, the update frequency
per subscriber may need to be
substantially higher for security.
Update traffic Mbit/s
1000
100
10
1
Partition of the problem required!
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0,01
0,1
1
10
updates/subscriber/day
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A solution is to partition by operators and
number prefixes
• GSM supports this solution:
– Every HLR knows locations of some hundreds of
thousands of subscribers with the accuracy of a VLR
– First digit positions of a subscriber number define
which HLR is asked for the location.
– The location area hierarchy also decreases the number
of necessary updates of HLR.
• All changes don’t need to be updated as far as the HLR.
– A subscriber has an MS-ISDN “subscriber phone book
number” and a separate roaming (i.e. routing) number
(MSRN).
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Location area hierarchy in GSM
MSC/VLR area
Sijaintialue
Location area
Solu
Cell
HLR knows the exchange/VLR
VLR knows
- in practice a set of cells
- updates once/6 min….24h and
on conditions of switching on/off phones
- an update demands also authentication
Final locations via paging:
- call is sent (paged) in the set of cells
known by the VLR
- MS responds in its own cell
- Result is that the best cell can be chosen
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Calculation of location update traffic in a
HLR with 200000 subscriber capacity
• 200 000 subscribers
• 1 update/5min/subscriber
• an assumption: 1 update = 100
octets
TRAFFIC = 200000 * 100 * 8/(5*60) = 0,53Mbit/s.
Could be transmitted using one PCM line! It feels like a viable
solution.
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It is interesting to study the number of
probable handovers during a call
Number of Hand-overs/call
Call duration 3 min
100
10
1
0
1
2
3
4
5
6
0,1
7
Speed 150 km/h
8Speed 100 km/h
Speed 50 km/h
Speed 15 km/h
Speed 5 km/h
0,01
The radius of the cell in km
The number < 1 preferably in a viable architecture!
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GSM architecture
BTS
HLR/AC/EIR
HLR - Home Location Register
MS
= ME+SIM
MSC
BSC
BSC
VLR
AC - Authentication Center
EIR - Equipment Identity Register
MSC - Mobile Switching Center
VLR - Visitor location Register
BSC - Base Station Controller
cells
BTS
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BTS - Base Transceiver Station
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MS is found in a mobile terminated call
using the “Routing information request”
PSTN
GMSC
MAP/C
MAP/D
HLR
VLR
ISUP - IAM
SendRoutingInformation
SendRoutingInformationACK
ProvideRoamingNumber
ProvideRoamingNumberACK
ISUP - IAM (a start-up message of normal signaling)
MSRN - Mobile Subscriber Roaming Number is the routing number
- supports the E.164 format (ordinary exchanges can handle)
- every MSC has restricted number of MSRNs
- MSRN has a period of validity
- MSRN can be allocated per call or for the duration of roaming
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MSC
More capacity can be built with multilayer cellular networks
GSM900 macro
GSM1800 macro
GSM1800 micro
GSM900 micro
Cell selection aims at
placing fast moving MSs
up into large cells.
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Would binary subscriber numbering
change the situation?
• Example: 109 subscribers, number length 128 bits (IPv6)
Approximate memory need for analysis: The analysis tree consists of
nodes of 64 octets. In each node 4 bits are analyzed.
Usage of hexa positions:
m8 = 109
8 lg m = 9
m = 13.34
The number of nodes
1 + m + m2 + … m7 =
m8 - 1
m-1
= 114 millions
No significant improvement!
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A “Brute Force” - solution to IP -mobility
Memory needed for the analysis tree (=RT) is: 64 * 114 * 106 = 7.3 Gb
• In the “Brute Force” - solution this is updated in all routers!
In practice this is not feasible!
• An analysis of 128 bits in this DB requires 8 reads which
basically is not a problem.
• The biggest technical problem is updates!
• Mobility architectures must decrease mobility update traffic
to lower than the user traffic.
• Updates to places that have no reads must be eliminated
or at least minimized.
Two solution models: Mobile-IP and GPRS.
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Mobility in packet networks/background
RT
• Routing is based on routing tables
that are read per packet.
• Routers maintain routing tables Destination IP-address
using routing protocols.
Outgoing port/
Next. R IP-ad.
• Size of feasible tables is today
under 100 000 entries. A search
based on destination addresses
demands many references (reads) in
the memory (<32).
- In networks of 100m users a feasible RT-size is reached by using provider addressing
and searching from the table with address prefixes. (So, IP-addresses of the full length
of 32 bits are hardly ever used)
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In Mobile-IP user has a home
agent and foreign agent.
Care-of-Address
Mobile
Foreign
Agent
3
2
1
Correspondent
Host
1 - normal IP -routing
2 - tunnel HA ->FA
3 - normal IP - routing
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Home Network of the Mobile
Tunnel =
IP over IP
Home
Agent
Home-IP-Address
of the Mobile
A mobile has to update its
location from time to time.
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In Mobile-IP triangle routing
could be avoided
a mobile
3
4
5
Foreign
Agent
2a
1
Correspondent
Host MAY have
a binding cache
Mobile’s Home Network
2b
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Home
Agent
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Features of the Mobile-IP
• Care-of-address changes are authenticated.
• Routing Optimization is a draft proposal, not a
part of the basic Mobile-IP.
– Optimization of routing can also try to retrieve
messages going to a moving mobile with help of a
negotiation between the old and the new FAs.
• Matters of radio technology and other issues of the
data link layer are ignored.
• Not considered, who owns the networks and who
will pay for the operations.
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Packet traffic extension to GSM is GPRS
Trx 1
Circuit switched time slots
Circuit switched time slots
max
Additional GPRS
Trx n
Default GPRS
Dedicated GPRS time slots
max
Circuit switched time slots
Every trx has 8 time slots, that are
classified:
- dedicated to circuit switched service
- dedicated to packet service (optional)
- packet service as a default (is kept
free from calls even using
hand-overs
- packet service allowed if no
circuit switched traffic.
Selecting the size of areas appropriately for circuit switched and packet
switched traffic generates an elastic boundary so that the quality of service, traffic
revenue and usage of the network are optimized. Definition of areas is an
additional step brought by GPRS into cellular network design.
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SGSN takes care of the mobility and GGSN takes care
of connectivity to other networks in GPRS
An interface to a circuit switched network
BTS
HLR/AC/EIR
Company Xyz’s
Intranet
GGSN
MS
= ME+SIM
cells
BTS
BSC
BSC
SGSN
Company ABc’s
GGSN Intranet
SGSN - Serving GPRS
Support Node
GGSN
Public
GGSN - Gateway GPRS
Internet
Support Node
A context is supported from MS to GGSN”
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Mobility management in GPRS
The network doesn’t monitor the location of
the MS
Attach
Detach
IDLE
READY
A subscriber is active. The network monitors
the subscriber with the accuracy of a cell.
STANDBY
MS is in the “online” standby state, but is
not forwarding packets right now. The network
monitors the subscriber with the accuracy of
a routing area:
cell < RA < location area. MS is found by
paging in the set of cells.
GPRS mobility model
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Features of GPRS
• Two different mobility management systems in the network:
for circuit switched services and packet switched services.
• The GGSN owns the externally visible IP address of the
Mobile. Normal IP routing outside the GGSN.
• BSC-SGSN-GGSN (+HLR) network takes care of the
mobility and uses internal IP addresses aligned with the
topology. ( cmp. SGSN/FA, GGSN/HA.)
• Two IP networks on top of each other in the tunnel MS GGSN : an IP transmission network and an IP network seen
by the applications.
– This solution is a result of the need to manage the issues of network
ownership and responsibilities.
– Big header overhead (>100 octets)!
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“GSN to GSN” -networks have a big
header overhead
E.g. 20ms voice bits
Header overhead on different layers
RTP
RTP
12
UDP
UDP
IP
IP
8
20
GTP
GTP
UDP
IP
Gn
interface
UDP
IP
L2
L2
L1
GSN1
L1
GSN2
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A voice packet 6...12kbit/s
takes 15 …30 octets.
If there is an ATM network
beneath (48 octet payload +
5 octets header/cell),
How much is the overhead?
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Summary
• Mobility solutions in packet switched networks and circuit
switched networks are different from each other.
• GMSC asks per call a HLR for routing information: the
centralized architecture is feasible.
• External nodes can’t be asked about destination of a packet
in packet switched networks. A mobility solution is either
adaptive or distributed.
• GPRS and mobile-IP architectures are quite similar.
– GPRS has been carefully adapted to GSM.
– GPRS takes care of who owns the equipment and who is allowed
to use it and where and with what kind of rights.
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