Mobile networks and mobility management

R.Kantola, (translation: A.Paju)/18.10.00/s38.118 1

Mobile networks and mobility management

Impacts of mobility on networks

A few mobility solutionsRaimo Kantola

[email protected]

SG210, 4512471


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:

The number of nodes in the analysis tree is:

1 + m + m2 + … m12 = m13 - 1

m - 1= 305 million

m13 = 109 13 lg m = 9 m = 4.92

Usage of number positions:


The number analysis tree of the exchange associates routing with signaling information

ABC - destination (area)ABCd - the shortest subscriber number ABCdefgh - the longest subscriber number

A

B

C

d

e

f

g

h

Nodes d,e,f,g are necessary depending both on the number length

and the father node.

Buckets

From signaling:

We assume a treelike data structure in the analysis.


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

1

10

100

1000

0,01 0,1 1 10

updates/subscriber/dayPartition of the problem required!


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).


MSC/VLR area

Sijaintialue

Location area hierarchy in GSM

Location area

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

SoluCell

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


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.


It is interesting to study the number of probable handovers during a call

Number of Hand-overs/call

0,01

0,1

1

10

100

0 1 2 3 4 5 6 7 8

The radius of the cell in km

Speed 5 km/h

Speed 15 km/h

Speed 50 km/hSpeed 100 km/hSpeed 150 km/h

The number < 1 preferably in a viable architecture!

Call duration 3 min


GSM architecture

HLR/AC/EIR

MSC

VLRBSC

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

BTS

cells

BSCMS= ME+SIM

BTS


MS is found in a mobile terminated call using the “Routing information request”

PSTN GMSC HLR MSCVLR

ISUP - IAMSendRoutingInformation ProvideRoamingNumber

ProvideRoamingNumberACKSendRoutingInformationACK

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

MAP/C MAP/D


More capacity can be built with multi-layer cellular networks

GSM900 macro

GSM1800 macro

GSM1800 micro

GSM900 micro

Cell selection aims atplacing fast moving MSs up into large cells.


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

The number of nodes

1 + m + m2 + … m7 = m8 - 1

= 114 millions

No significant improvement!

m8 = 109 8 lg m = 9 m = 13.34

Usage of hexa positions:


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.


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 (<32).

Outgoing port/Next. R IP-ad.

Destination IP-address

RT

- 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)


In Mobile-IP user has a home agent and foreign agent.

CorrespondentHost

1

3 ForeignAgent

ForeignAgent

Mobile

Care-of-Address

HomeAgentHomeAgent

Home Network of the Mobile

Home-IP-Addressof the Mobile

A mobile has to update itslocation from time to time.

1 - normal IP -routing2 - tunnel HA ->FA3 - normal IP - routing

tunn

el

Tunnel =IP over IP

2


In Mobile-IP triangle routing could be avoided

HomeAgentHomeAgent

Mobile’s Home Network

ForeignAgent

ForeignAgent

a mobile

CorrespondentHost MAY have a binding cache

1

2a

2b

Binding Update

Bin

ding

war

ning

3

54


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.


Packet traffic extension to GSM is GPRS

Circuit switched time slots



Additional GPRS

Default GPRS

Dedicated GPRS time slotsm

ax

max

Trx 1

Trx n

Every trx has 8 time slots, that areclassified:- 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 packetswitched 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.


SGSN takes care of the mobility and GGSN takes care of connectivity to other networks in GPRS

BSC

BTS

cells

BSCMS= ME+SIM

BTS

An interface to a circuit switched network

SGSN

GGSN

Company Xyz’sIntranet

GGSN

GGSN

HLR/AC/EIR

PublicInternet

Company ABc’sIntranet

SGSN - Serving GPRS Support NodeGGSN - Gateway GPRS Support NodeA context is supported from MS to GGSN”

A tunnel/context


Mobility management in GPRS

IDLE

STANDBY

READY

GPRS mobility model

Atta

ch

Det

ach

Rea

dy ti

mer

Sta

ndby

t im

er

PD

Ufo

rwar

ding

The network doesn’t monitor the location of the MS

A subscriber is active. The network monitors the subscriber with the accuracy of a cell.

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 < RA < location area. MS is found bypaging in the set of cells.


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)!


“GSN to GSN” -networks have a big header overhead

RTP

UDP

IP

GTP

UDP

IP

L1

L2

E.g. 20ms voice bits

RTP

UDP

IP

GTP

UDP

IP

L1

L2

GSN1 GSN2

Gninterface

12

8

20

8

20

A voice packet 6...12kbit/s takes 15 …30 octets.

If there is an ATM networkbeneath (48 octet payload +5 octets header/cell), How much is the overhead?

Header overhead on different layers


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.

Mobile networks and mobility management

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