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

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

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

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

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

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

Cellular Call Setup 1. Call Request

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

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

Information Flow for Cellular Call

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

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

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

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.

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)

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

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

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)

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).

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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”

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.

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.

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.

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.

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

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

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”

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

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

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.

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

Handoff in 802.11

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.

Handoff Information Flow MS BSS1 MSC BSS2 Measurement report Handoff required Handoff command Handoff request Handoff request ACK Handoff Handoff complete Clear command Clear complete

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.

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

Handoff Issues Intersystem handoffs cellular (e.g., GPRS)  802.11 WLAN 802.11  802.16 (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