lecture23.ppt

Hui Zhang, Fall 2012 1
15-441 Computer Networking
Mobility

Many Mobility Scenario’s
 Mobile hosts connecting to networked infrastructure
(focus of this lecture)
- Cell phones connecting to cell towers
- Laptops connecting to access points
 Ad hoc networks: a set of mobile nodes
communicating with each other with no additional
networked infrastructure
- Extra challenges: routing and possible mobility
 Sensor networks (ad hoc +)
- More resource constrained mobile nodes
 Vehicular networks
- Extreme mobility

Routing to Mobile Nodes
 Obvious solution: have mobile nodes advertise
route to mobile address/32
- Should work!!!
 Why is this bad?
- Consider forwarding tables on backbone routers
• Would have an entry for each mobile host
• Not very scalable
 What are some possible solutions?

How to Handle Mobile Nodes?
(Addressing)
 Dynamic Host Configuration (DHCP)
- Host gets new IP address in new locations
- Problems
• Host does not have constant name/address  how
do others contact host
• What happens to active transport connections?

(Naming)
 Naming
- Use DHCP and update name-address mapping
whenever host changes address
- Fixes contact problem but not broken transport
connections

(Transport)
 TCP currently uses 4 tuple to describe
connection
- <Src Addr, Src port, Dst addr, Dst port>
 Modify TCP to allow peer’s address to be
changed during connection
 Security issues
- Can someone easily hijack connection?
 Difficult deployment  both ends must support
mobility

(Link Layer)
 Link layer mobility
- Learning bridges can handle mobility  this is how it is
handled at CMU
- Encapsulated PPP (PPTP)  Have mobile host act like
he is connected to original LAN
• Works for IP AND other network protocols

(Routing)
 Allow mobile node to keep same address and name
 How do we deliver IP packets when the endpoint
moves?
- Can’t just have nodes advertise route to their
address
 What about packets from the mobile host?
- Routing not a problem
- What source address on packet?  this can
cause problems
 Key design considerations
- Scale
- Incremental deployment

Basic Solution to Mobile Routing
 Same as other problems in computer science
- Add a level of indirection
 Keep some part of the network informed about
current location
- Need technique to route packets through this location
(interception)
 Need to forward packets from this location to
mobile host (delivery)

Interception
 Somewhere along normal forwarding path
- At source
- Any router along path
- Router to home network
- Machine on home network (masquerading as
mobile host)
 Clever tricks to force packet to particular destination
- “Mobile subnet” – assign mobiles a special
address range and have special node advertise
route

Delivery
 Need to get packet to mobile’s current location
 Tunnels
- Tunnel endpoint = current location
- Tunnel contents = original packets
 Source routing
- Loose source route through mobile current location

Mobile IP (RFC 2290)
 Interception
- Typically home agent – a host on home network
 Delivery
- Typically IP-in-IP tunneling
- Endpoint – either temporary mobile address or
foreign agent
 Terminology
- Mobile host (MH), correspondent host (CH), home
agent (HA), foreign agent (FA)
- Care-of-address, home address

Lecture 21: 2006-11-16 13
Mobile IP (MH at Home)
Mobile Host (MH)
Visiting
Location
Home
Internet
Correspondent Host (CH)
Packet

Mobile IP (MH Moving)
Visiting
Location
Home
Internet
Packet
Home Agent (HA) Mobile Host (MH)
I am here

Mobile IP (MH Away – FA)
Visiting
Location
Home
Internet
Packet
Home Agent (HA) Foreign Agent (FA)
Encapsulated
Mobile Host (MH)

Mobile IP (MH Away - Collocated)
Visiting
Location
Home
Internet
Packet
Home Agent (HA) Mobile Host (MH)
Encapsulated

Other Mobile IP Issues
 Route optimality
- Resulting paths can be sub-optimal
- Can be improved with route optimization
• Unsolicited binding cache update to sender
 Authentication
- Registration messages
- Binding cache updates
 Must send updates across network
- Handoffs can be slow
 Problems with basic solution
- Triangle routing
- Reverse path check for security

State of Mobile Networks
 Getting a new name and address via DHCP is the most
commonly used solution for laptops, as long as
- it is a client machine (what is the difference between a server and
client machine?)
- Single consistent TCP connection during mobility is not important
 Cell network is the commonly used solution to support
continuous mobility
- Overview slides attached. For information only. Not required for
exam.
 Mobile IP not as widely used
- Why?
 Will cover ad hoc networks, sensor networks, vehicular
networks in two weeks
- Check out Professor Srini Seshan’s 15-446 class next semester

GSM Mobility Management
(not required for exam)
Originals by: Rashmi Nigalye, Mouloud Rahmani, Aruna Vegesana, Garima Mittal, Fall 2001
Prof. M. Veeraraghavan, Polytechnic University, New York
• GSM architecture overview
– Network layout
– Protocols
– Addresses & identifiers
• Location management
– Call delivery + location update
• Handover management

GSM network layout
GSM Network
(PLMN)
MSC region
MSC region
MSC region
Location
area
BSC
BSC
Location
area
BTS BTS
PLMN: Public Land Mobile Network
MSC: Mobile Switching Center
BTS: Base Transceiver Station
BSC: Base Station Controller

GSM network layout
BSC
MSC
BSC
BTS
EIR
HLR
AUC
VLR
BTS
BTS
Um
A
Abis
E
B,C
OMC
GMSC
PSTN
ISDN

GSM MAP protocol
 GSM MAP similar to IS41 MAP
 MAP uses Transactions Capabilities Part (TCAP)
of the SS7 stack
 MAP functions:
- Updating of location information in VLRs
- Storing routing information in HLRs
- Updating and supplementing user profiles in HLRs
- Handoff of connections between MSCs

What is a location area (LA)?
 A powered-on mobile is informed of an incoming
call by a paging message sent over the PAGCH
channel of a cell
 One extreme is to page every cell in the network
for each call - a waste of radio bandwidth
 Other extreme is to have a mobile send location
updates at the cell level. Paging cut to 1 cell, but
large number of location updating messages.
 Hence, in GSM, cells are grouped into Location
Areas – updates sent only when LA is changed;
paging message sent to all cells in last known LA

Addresses and Identifiers
 International Mobile Station Equipment Identity (IMEI)
- It is similar to a serial number. It is allocated by equipment
manufacturer, registered by network, and stored in EIR
 International Mobile Subscriber Identity (IMSI)
MCC MNC MSIN
MCC: Country Code
MNC: Mobile Network Code
MSIN: Mobile Subscriber Identification Number
When subscribing for service with a network, subscriber receives (IMSI)
and stores it in the SIM (Subscriber Identity Module) card.
The HLR can be identified by a VLR/MSC from the IMSI.

 Mobile Subscriber ISDN (MSISDN)
- The “real telephone number”: assigned to the SIM
- The SIM can have several MSISDN numbers for selection of
different services like voice, data, fax
CC NDC SN
NDC: National Destination Code (NDC identifies operator);
SN: Subscriber Number; CC: Country Code;
Digits following NDC identifies the HLR

 Mobile Station Roaming Number (MSRN)
- It is temporary location dependent ISDN number
- It is assigned by local VLR to each MS in its area.
CC NDC SN

Addresses and identifiers
 Temporary Mobile Subscriber Identity (TMSI)
- It is an alias of the IMSI and is used in its place for
privacy.
- It is used to avoid sending IMSI on the radio path.
- It is an temporary identity that is allocated to an MS by
the VLR at inter-VLR registration, and can be changed
by the VLR
- TMSI is stored in MS SIM card and in VLR.

TMSI, IMSI, MSRN and MSISDN
 Unlike MSISDN, IMSI is not known to the GSM user. The
CC of MSISDN translates to an MCC of IMSI as follows,
e.g, Denmark CC: 45 MCC: 238
 TMSI is used instead of IMSI during location update to
protect privacy. As user moves, TMSI is used to send
location update. Thus a third party snooping on the wireless
link cannot track a user as he/she moves.
 MSRN is the routing number that identifies the current
location of the called MS.
- MSRN is temporary network identity assigned to a
mobile subscriber.
- MSRN identifies the serving MSC/VLR.
- MSRN is used for call delivery (calls incoming to an
MS).
 MSISDN is the dialed number to reach a GSM user

 Location Area ID (LAI)
- CC: Country Code, MNC:Mobile Network Code, LAC:
Location Area Code
- LAI is broadcast regularly by Base Station on BCCH
- Each cell is identified uniquely as belonging to an LA by its
LAI
CC MNC LAC

Location management
 Set of procedures to:
- track a mobile user
- find the mobile user to deliver it calls
 Current location of MS maintained by 2-level
hierarchical strategy with HLRs and VLRs.

Ways to obtain MSRN
1. Obtaining at location update – MSRN for the MS is
assigned at the time of each location update, and is
stored in the HLR. This way the HLR is in a position
to immediately supply the routing info (MSRN)
needed to switch a call through to the local MSC.
2. Obtaining on a per call basis – This case requires
that the HLR has at least an identification for the
currently responsible VLR. When routing info is
requested from the HLR, it first has to obtain the
MSRN from the VLR. This MSRN is assigned on a
per call basis, i.e. each call involves a new MSRN
assignment

Routing information: case when MSRN
is selected per call by VLR/MSC
 If MSRN is allocated to each subscriber visiting at an MSC, then the
number of MSRNs required is large. If instead, an MSRN is allocated
only when a call is to be established, then the number of MSRNs is
roughly equal to number of circuits at MSC – a much smaller number –
hence MSRNs typically allocated per call by VLR/MSC
MSISDN
GMSC
HLR

MSC/VLR



MSISDNIMSI, VLR number
MSRN

Call routing to a mobile station:
case when HLR returns MSRN
GMSC
BSC
BSC
EIR
HLR
AUC
VLR
MSC
BTS
BTS
BTS
LA 1
LA 2
ISDN
1
MS
1
MSISDN
6
TMSI
4
MSRN
3
MSRN
2
MSISDN
7
TMSI
7
TMSI
7
TMSI
8
TMSI
5
MSRN
MSC

Messages exchanged: call delivery
PSTN
GMSC
HLR
VLR
Target
MSC
Originating
Switch
GMSC HLR VLR
Target
MSC
1. ISUP IAM
2. MAP_SEND_ROUTING_INFO
3. MAP_PROVIDE_ROAMING_NUMBER
4. MAP_PROVIDE_ROAMING_NUMBER_ack
5. MAP_SEND_ROUTING_INFO_ack
6. ISUP IAM
1
2 3
4
5
6

Find operation in GSM
 ISDN switch recognizes from the MSISDN that the call
subscriber is a mobile subscriber. Therefore, forward
the call to the GMSC of the home PLMN (Public Land
Mobile Network)
 GMSC requests the current routing address (MSRN)
from the HLR using MAP
 By way of MSRN the call is forwarded to the local MSC
 Local MSC determines the TMSI of the MS (by
querying VLR) and initiates the paging procedure in the
relevant LA
 After MS responds to the page the connection can be
switched through.

36
Abbreviations
 ISC: International switching center
 OMC: Operations and maintenance center
 GMSC: Gateway switching center
 MSC: Mobile switching center
 VLR: Visitor location register
 HLR: Home Location register
 EIR: Equipment Identification register
 AUC: Authentication center
 BSC: Base station controller
 BTS: Base transceiver station
 MS: Mobile subscriber
 TMSI: Temporary Mobile Subscriber Identity
 IMSI: International Mobile Subscriber Identity

lecture23.ppt

More Related Content

Similar to lecture23.ppt (20)

More from HazemElabed2 (8)

Recently uploaded (20)

lecture23.ppt