Mobile networks and mobility management

1
Mobile networks and mobility management

• Impacts of mobility on networks
• A few mobility solutions
• Raimo Kantola
• raimo.kantola_at_hut.fi
• SG210, 4512471

2
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

Lets calculate an approximation of memory
requirements The analysis tree consists of
nodes of 64 octets. One digit is analyzed in
each node
3
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
Buckets
C
We assume a treelike data structure in the
analysis.
Nodes d,e,f,g are necessary depending both on the
number length
and the father node.
h
4
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 isnt 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.
5
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 dont 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).

6
Location area hierarchy in GSM
HLR knows the exchange/VLR
MSC/VLR area
Sijaintialue
Location area
Solu
Cell
7
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/(560) 0,53Mbit/s.
8
It is interesting to study the number of probable
handovers during a call
9
GSM architecture
HLR/AC/EIR
BTS
HLR - Home Location Register AC -
Authentication Center EIR - Equipment
Identity Register MSC - Mobile
Switching Center VLR - Visitor
location Register BSC - Base Station
Controller BTS - Base Transceiver
Station
BSC
MSC
BSC
MS MESIM
VLR
cells
BTS
10
MS is found in a mobile terminated call using the
Routing information request
MAP/C
MAP/D
GMSC
PSTN
ISUP - IAM
SendRoutingInformation
ProvideRoamingNumber
ProvideRoamingNumberACK
SendRoutingInformationACK
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
11
More capacity can be built with multi-layer
cellular networks
GSM900 macro
GSM1800 macro
GSM1800 micro
GSM900 micro
Cell selection aims at placing fast moving MSs
up into large cells.
12
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.
m - 1
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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.

14
Mobility in packet networks/background

• Routing is based on routing tables that are read
  per packet.
• Routers maintain routing tablesusing routing
  protocols.
• Size of feasible tables is today under 100 000
  entries. A search based on destination addresses
  demands many references (reads) in the memory
  (lt32).

- 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)
15
In Mobile-IP user has a home agent and foreign
agent.
1 - normal IP -routing 2 - tunnel HA -gtFA 3 -
normal IP - routing
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In Mobile-IP triangle routing could be avoided
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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.

18
Packet traffic extension to GSM is GPRS
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.
Trx 1
Circuit switched time slots
Circuit switched time slots
max
Circuit switched time slots
Additional GPRS
Default GPRS
max
Dedicated GPRS time slots
Trx n
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
Company Xyzs Intranet
HLR/AC/EIR
GGSN
BSC
BSC
MS MESIM
SGSN
Company ABcs Intranet
GGSN
SGSN - Serving GPRS Support
Node GGSN - Gateway GPRS Support
Node A context is supported from MS to GGSN
GGSN
Public Internet
cells
BTS
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Mobility management in GPRS
IDLE
Attach
Detach
READY
Standby timer
PDUforwarding
Ready timer
MS is in the online standby state, but isnot
forwarding packets right now. The network
monitors the subscriber with the accuracy ofa
routing area cell lt RA lt location area. MS is
found by paging in the set of cells.
STANDBY
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 networkownership and
  responsibilities.
• Big header overhead (gt100 octets)!

22
GSN to GSN -networks have a big header overhead
E.g. 20ms voice bits
Header overhead on different layers
RTP
RTP
12
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?
UDP
UDP
8
20
IP
IP
GTP
GTP
UDP
UDP
8
Gn interface
IP
IP
20
L2
L2
L1
L1
GSN1
GSN2
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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 cant 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.