1. MITP 413: Wireless Technologies Week 9
Michael L. Honig
Department of EECS
Northwestern University
March 2008

2. Wireline Call Setup Switch Helen’s phone Bob’s phone Information Flow
wire pair
PSTN
Helen’s phone
network
address
Bob’s phone
Information Flow
off hook

3. Wireline Call Setup Switch Helen’s phone Bob’s phone Information Flow
wire pair
PSTN
Helen’s phone
network
address
Bob’s phone
Information Flow
off hook
dial tone

4. Wireline Call Setup Switch Helen’s phone Bob’s phone Information Flow
wire pair
PSTN
Helen’s phone
network
address
Bob’s phone
Information Flow
off hook
dial tone
keystrokes
alert signal

5. Wireline Call Setup Switch Helen’s phone Bob’s phone Information Flow
wire pair
PSTN
Helen’s phone
network
address
Bob’s phone
Information Flow
off hook
dial tone
keystrokes
alert signal
ring indication
off hook
remove ring indication

6. Wireline Call Setup Switch Helen’s phone Bob’s phone Information Flow
wire pair
PSTN
Helen’s phone
network
address
Bob’s phone
Information Flow
off hook
dial tone
keystrokes
alert signal
ring indication
off hook
remove ring indication
conversation

7. Cellular Call Setup
1. Call Request

8. Cellular Call Setup 1. Call Request 2. Send numbers to switch
3. Page Receiver
4. Request Channel

9. Cellular Call Setup (cont.)
5. Switch assigns channels
6. Cellular conversation is set up

10. Information Flow for Cellular Call

11. Mobility Management Location management How to track mobile users?
Handoff management
How to change the wireless access point?
Uses wireline resources:
Uses wireless resources:

12. Mobility Management Location management How to track mobile users?
Handoff management
How to change the wireless access point?
Uses wireline resources: database and signaling link capacity
Uses wireless resources: paging, registration

13. Location Areas (LAs) Mobiles announce changes in LAs LA-1 LA-2
LOCATION UPDATE
NO LOCATION UPDATE
Mobiles announce changes in LAs
Requires database updates

14. Paging Identifies cell of mobile user.
LA-1
LA-2
LOCATION UPDATE
How should the size of the location area be chosen?
NO LOCATION UPDATE
Identifies cell of mobile user.
Mobile is paged within reported LA.

15. Location Area Tradeoff
Small location areas:
Mobile users update frequently
Fewer wireless resources spent on paging
Large location areas:
Mobile users update infrequently
More wireless resources spent on paging
Can also implement dynamic update rules (e.g., depending on time since last update, distance traveled, mobility and call patterns)

16. Common Channel Signaling
Provides control and management functions in the PSTN
Supervisory functions, addressing, and call information provisioning
CCS channel:
Conveys messages to initiate and terminate calls
Determines the status of some part of the network
Controls the amount of traffic allowed
Separated from data traffic (out-of-band signaling)
Signaling System (SS)7: CCS for the PSTN

17. SS7 Network Components Service Switching Point (SSP)
Telephone switch interconnected by SS7 links.
Can be a Central Office in the PSTN, or an MSC in a PCS/cellular network
Signal Transfer Point (STP)
A switch that relays SS7 messages between network switches and databases.
Relays message to appropriate signaling links (e.g., for database query)
Provided in matched pairs to reduce chances of failure.
Service Control Point (SCP)
Contains databases for providing enhanced services (e.g., an HLR or VLR)
Accepts and answers queries from an SSP
Connected to an SSP through an STP

18. Interconnection Between a PCS Network and the PSTN
SCP
SCP
STP
STP
( HLR )
SCP: Service Control Point
STP: Signal Transfer Point
SSP: Service Switching Point
SSP
MSC
Trunk
PSTN
PCN (PCS network)

19. Location Databases
HLR
Local service area
VLR
Service provider footprint
Home Location Register:
Stores and manages all mobile
Subscriptions for a specific operator
Contains directory number, profile
Information, current location, and validation period
Visitor Location Register: Stores and manages subscription information for visiting subscribers. Directs calls to and from visiting subscribers.
Both are accessed through the Mobile Switching Center (MSC).

20. MS Registration (IS-41)
NEW
VLR
HLR
OLD
VLR 2 4 1 3
Morristown, NJ
New York City, NY
Los Angeles, CA
1. MS turns on, registers with local (new) VLR.

21. MS Registration (IS-41) 1. MS turns on, registers with local (new) VLR
HLR
OLD
VLR 2 4 1 3
Morristown, NJ
New York City, NY
Los Angeles, CA
1. MS turns on, registers with local (new) VLR
2. New VLR informs user’s HLR of new location. HLR sends an ack, which includes the MS’s profile, to the new VLR.

22. MS Registration (IS-41) 1. MS turns on, registers with local (new) VLR
HLR
OLD
VLR 2 4 1 3
Morristown, NJ
New York City, NY
Los Angeles, CA
1. MS turns on, registers with local (new) VLR
2. New VLR informs user’s HLR of new location. HLR sends an ack, which includes the MS’s profile, to the new VLR.
3. The new VLR informs the MS of the successful registration.

23. MS Registration (IS-41) NEW VLR HLR OLD VLR 2 4 1 3 Morristown, NJ
New York City, NY
Los Angeles, CA
1. MS turns on, registers with local (new) VLR
2. New VLR informs user’s HLR of new location. HLR sends an ack, which
includes the MS’s profile, to the new VLR.
3. The new VLR informs the MS of the successful registration
4. After step 2, the HLR also sends a deregistration message to cancel the
obsolete location record in the old VLR.

24. Call Delivery (IS-41)
MSC
HLR
OLD
VLR 1
PSTN 2 3

25. Call Delivery (IS-41) 3 OLD 1 HLR VLR PSTN 2 MSC
1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS.

26. Call Delivery (IS-41) 3 OLD 1 HLR VLR PSTN 2 MSC
1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address.

27. Call Delivery (IS-41) 3 OLD 1 HLR VLR PSTN 2 MSC
1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address.
2. The VLR returns the address to the SSP through the HLR.

28. Call Delivery (IS-41) 3 OLD 1 HLR VLR PSTN 2 MSC
1. Call is forwarded to a switch (SSP), which queries the HLR to find the current VLR of the MS. The HLR queries the VLR associated with the MS to get an address.
2. The VLR returns the address to the SSP through the HLR.
3. A trunk (voice circuit) is set up from the originating switch to the MS through the visited MSC.

29. Pointer Fowarding Move Operation Find Operation
MSC
( HLR )
Move Operation
SCP
( VLR )
Find Operation
Move operation (registration): Pointer is created from the old VLR to the new VLR. No HLR registration is required.
Find operation (call delivery): The pointer chain is traced to locate MS. Pointer from HLR moved to destination VLR.

30. Mobile IP
Enables computers to maintain internet connectivity (TCP connections) while moving among internet attachments.
Applies to both mobile use and to nomadic use (e.g., with wired connections)
All connections are automatically maintained.
Nomadic use does not require mobile IP.
Forwards packets to new IP address via tunneling:
IP datagram is encapsulated inside a new IP datagram with a care-of-address (COA).
Some analogies with GSM mobility management:
HLR  “Home agent”
VLR  “Foreign agent”

31. Mobile IP Scenario
Mobile
node A 3
Home network
for A
Foreign
network 4
Internet or other
topology of routers
and links 2
Home
agent
Foreign
agent 1 5
Server X
1. Server X transmits IP datagram to mobile node A’s home address.
2. IP datagram is intercepted by home agent, and tunneled to A’s COA.
3. Foreign agent strips off outer IP header, encapsulates the original IP datagram in a network-level packet, and delivers the original datagram to A across the foreign network.
4. A sends IP traffic to X through router on foreign network (e.g., foreign agent).
5. IP datagram is delivered to X over internet.

32. Mobile IP: Basic Operations
Discovery: Identifies prospective Home Agents and Foreign Agents for mobile node (MN).
Registration: Authenticated procedure to inform the home agent of its COA.
Tunneling: Forwards IP datagrams from a home address to a COA.

33. Discovery
Determines whether or not attachment point has changed (e.g., due to handoff).
MN continually listens for “advertisements” from foreign and home agents.
Eligible routers issue periodic broadcast messages.
Advertisements include:
IP address of router
If registration is required
The maximum lifetime of registration request
If router is busy
Nature of agent (home and/or foreign )
Maximum allowable time of registration request
COAs supported
MN compares the network portion of IP address with its own home address. Mismatch implies that the MN is on a foreign network.
MN can solicit an agent advertisement (e.g., if timer has expired).
If no foreign agents are available, then MN may act as its own foreign agent by using a “co-located” COA.

34. Registration Message is sent to Home Agent to set up COA.
MN sends registration request to Foreign Agent.
Foreign agent relays request to MN’s Home Agent.
The HA accepts or denies the request and sends a registration reply to the FA.
The FA relays this reply to the MN.
The HA creates a mobility binding between the MN’s home address and the current COA.
Registration request message includes:
Request to retain old bindings (e.g., for handoff)
Request to receive broadcast datagrams in home network
If the mobile node is using a co-located COA
Lifetime of binding
IP addresses of the HA and FA (i.e., the COA), and the home address of the MN
Authentication extension for security.

35. Tunneling Home Agent “steals” identity of MN
Example: R3 is the HA for a host H attached to a foreign network
R3 informs IP layer in LAN Z that datagrams destined for H’s address should be sent to R3
R2 and D (connected to LAN Z) insert address of R3 at the MAC-level for all packets transmitted to H.
LAN X A B R1
INTERNET
LAN Y C R2 R3
LAN Z D

36. IP-within-IP Encapsulation
IP version number, and other header fields
Tunnel source IP address (HA)
COA (for FA)
__________________________________
IP version number and other header fields
Original source IP address
Home IP address of MN
TCP and rest of packet

37. Issues “Triangle routing” may be inefficient.
Handoff during registration.
Old data is dropped by old FA, retransmitted, and re-tunneled.
Other possibilities:
Smooth handoff: old FA tunnels to new FA
Old FA may tunnel back to HA
Packets from MN may have to be tunneled through the HA.
Foreign network may have a firewall
Called “Reverse tunneling”

38. Handoff Decision
Depends on RSS, time to execute handoff, hysteresis, and dwell (duration of RSS)
Proprietary methods
Handoff may also be initiated for balancing traffic.
1G (AMPS): Network Controlled Handoff (NCHO)
Handoff is based on measurements at BS, supervised by MSC.
2G, GPRS: Mobile Assisted Handoff (MAHO)
Handoff relies on measurements at mobile
Enables faster handoff
Mobile data, WLANs (802.11): Mobile Controlled Handoff (MCHO)
Handoff controlled by mobile

39. Soft Handoff (CDMA) ”Make before break”
DURING
AFTER
MSC
MSC
MSC
BSC
BSC
BSC
BSC
BSC
BSC
Hard Handoff (TDMA)
MSC
MSC
MSC
BSC
BSC
BSC
BSC
BSC
BSC

40. Generic Handoff Procedure
(3)
Home
database
(4)
Anchor
point
(5)
Old visiting
database
(6)
Old
New
New visiting
database
(1)
(2)
1. Decision is made to handoff.
2. MS registers with the “new” visiting database.
3. New visited database requests subscriber profile from home database.
4. Home database responds with authentication of mobile.
New up/downlink channels are assigned (circuit-switched).
The two databases are updated with new location.
5. Home database sends a message to the old visited database to flush or redirect packets sent to or associated with MS.
6. Old database flushes/redirects packets, removes MS from its list.

41. Handoff in 802.11: BSS Transition
Distribution System (DS)
BSS
BSS Transition
Basic Service Area 1
Distribution System is a wired network (L2/L3 devices) connecting BSS
Mobile station (MS) moves to a different BSS within the same Extended Service Set (ESS)
MS sends dissociate message to AP 1.
Need not be received.
MS sends re-associate message to AP 2.
AP 1 notified about change of location via wired network.
Basic Service Area 2

42. Handoff in

43. Handoff in GSM
Mobile Station
(MS)
Base Station
Subsystem (BSS)
Network and Switching
Subsystem (NSS) MS
VLR
BSC
AuC
BTS
HLR MS
OMC
BTS
BSC MS
MSC
EIR A Um
Abis
Interface to other
networks
PSTN etc.
BTS
Radio interface
Internal handoff: between BTSs controlled by the same BSS.
External handoff: between BSSs controlled by the same MSC.
Mobile monitors the RSS for channels in adjacent cell, reports to MSC.
BTS also monitors RSS from mobile.

44. Handoff Information FlowMS
BSS1
MSC
BSS2
Measurement report
Handoff required
Handoff command
Handoff request
Handoff request ACK
Handoff
Handoff complete
Clear command
Clear complete

45. Intersystem Handoff Before the handoff MSC A MSC B PSTN
Trunk
BS1
BS2
Base Stations
PSTN
After the handoff
MSC A
MSC B
Trunk
BS1
BS2
Base Stations
PSTN
Anchor MSC
1. MSC A requests MSC B to set up voice channel with BS2.
2. MS synchronizes to BS2.
3. MSC A connects the call path (trunk) to MSC B.

46. Forward/Backward Handoffs
MSC A
MSC B X
Handoff forward
MSC A X
MSC B X
Handoff backward X
Handoff through 3rd switch
MSC C
MSC A
MSC B
MSC B X X
MSC A
MSC C X
Path minimization

47. Handoff Issues Intersystem handoffs
cellular (e.g., GPRS)  WLAN
 (WMAN)
Appropriate metrics?
Performance in mobile data networks
Different metrics than for voice (e.g., outage, average number of handoffs)
Throughput, maintaining QoS
Latency generally not an issue
Can retransmit