1.1 Cellular Network Architecture, 2G: Voice Connections to the Telephone Network

▣ The region covered by cellular-network is divided into no. of geographic coverage-areas called cells.

▣ Each cell contains a BTS (Base Transceiver Station) (Figure 1.1).

▣ BTS is responsible for delivering the signals to/from the mobile-stations in the cell.

▣ The coverage-area of a cell depends on following factors:

i. The transmitting power of the BTS.

ii. The transmitting power of the user devices.

iii. Obstructing buildings in the cell.

iv. The height of base-station antennas.

▣ The 2G systems use combined FDM/TDM for the air-interface.

▣ In combined FDM/TDM systems,

i. The channel is divided into a number of frequency sub-bands.

ii. Within each sub-band, time is partitioned into frames and slots.

Fig 1.1: Components of the GSM 2G cellular network architecture

▣ The GSM network contains many BSCs (Base Station Controllers).

▣ Main responsibilities of the BSC:

i. Providing service to many BTSs.

ii. Allocating radio-channels to mobile-users.

iii. Performing paging.

iv. Performing handoff of mobile-users.

▣ BSS (Base Station System) contains the BSC and its controlled BTSs.

▣ A MSC (Mobile Switching Center) contains upto 5 BSCs. This results in approx 200K subscribers/MSC.

▣ Main responsibilities of the MSC:

i. User authorization & accounting

ii. Call establishment & teardown and

iii. Handoff.

▣ A cellular-provider's network will have a number of special MSCs known as gateway MSCs.

▣ Gateway MSCs are used to connect the provider's cellular-network to the public telephone-network.

▣ Address must be registered with the home-agent. This can be done in 2 ways:

i. Via the foreign-agent who then registers the COA with the home-agent.

ii. By the mobile IP node itself.

Fig 2.1: Agent advertisement and mobile IP registration

Four steps are involved. Figure 2.1 illustrates the 4 steps.

I.

▩ The registration-request contains

i. COA advertised by the foreign-agent

ii. address of the home-agent (HA)

iii. permanent-address of the mobile (MA)

iv. registration identification and

v. requested lifetime of the registration.

▩ The requested registration lifetime indicates number of seconds the registration is valid.

▩ If registration is not renewed within the specified lifetime, the registration will become invalid.

II.

When the foreign-agent receives the registration-request, the foreign-agent records

▩ The foreign-agent then sends a registration-request to the home-agent.

III.

▩ The home-agent binds the mobile's permanent IP address with the COA.

▩ The home-agent sends a registration-reply.

IV.

▣ Mobile IP is the extension of IP protocol.

▣ Mobile IP allows laptops (or smartphones) to be connected to the Internet.

▣ Services of Mobile IP:

i. Support for many different modes of operation.

ii. Multiple ways for agents and mobile-nodes to discover each other.

iii. Use of single or multiple COAs.

iv. Multiple forms of encapsulation.

▣ Three main parts of mobile IP:

1) Agent Discovery

▩ Mobile IP defines the protocols used by a home or foreign-agent to advertise its services to mobile-nodes.

▩ It also defines the protocols for mobile-nodes to solicit the services of a foreign or home-agent.

2) Registration with the Home Agent

▩ Mobile IP defines the protocols used by the mobile-node to register COAs with the home- agent.

3) Indirect Routing of Datagrams

▩ Mobile IP defines the manner in which datagrams are forwarded to mobile-nodes by a home- agent.

▩ It also defines

i. rules for forwarding datagrams

ii. rules for handling error conditions and

iii. several forms of encapsulation

4.1 Handoffs in GSM

▣ A handoff occurs when a mobile-station moves from one base-station to another during a call.

▣ As shown in Figure 4.1,

i. Before handoff, a call is initially routed to the mobile through old base-station.

ii. After handoff, the call is routed to the mobile through another new base-station.

▣ Two reasons for handoff

:

1) The Call may be Dropped

▩ Because the signal between the current base-station and the mobile may have weakened.

2) To reduce Congestion

▩ Because a cell may be overloaded because of handling a large number of calls.

▩ This congestion may be reduced by handing off mobiles to less congested cells.

Fig 4.1: Handoff scenario between base stations with a common MSC

Eight steps are involved. Figure 4.2 illustrates the steps involved when a hand off occurs.

1. Old base-station (BS) informs both visited MSC & new BS that a handoff is about to happen.

2. The visited MSC performs following tasks

i. Initiates path setup to the new BS.

ii. Allocates the resources needed to carry the rerouted call.

iii. Signals the new BS that a handoff is about to occur.

3. The new BS allocates and activates a radio-channel for the mobile.

4. The new BS informs both visited MSC and old BS that the new path is set up.

5. The mobile is informed to perform a handoff.

6. The mobile & new BS exchange signaling messages to fully activate the new channel.

7. The mobile sends a handoff complete message to the new BS.

▩ This message is then forwarded to the visited MSC.

▩ The visited MSC then reroutes the ongoing-call to the mobile via the new BS.

8. The resources allocated along the path to the old BS are released.

Fig 4.2: A handoff between base stations with a common MSC

5.1 Direct Routing to a Mobile Node

Four steps are involved. Figure 5.1 illustrates the 4 steps.

Steps 1 & 2

▩ A correspondent-agent in the correspondent's n/w first learns the COA of the mobile-node.

▩ This can be done by having the correspondent-agent query the home-agent.

Steps 3 & 4

▩ Then, the correspondent-agent forwards datagrams directly to the mobile-node's COA.

Fig 5.1: Direct routing to a mobile user

5.2 Challenges in Direct Routing

▣ Two additional challenges:

i.The correspondent-agent needs a mobile location protocol to query the home-agent to obtain the mobile-node's COA (steps 1 & 2 in Figure 5.1).

ii. Problem: When the mobile-node moves from one foreign-network to another, how will data be forwarded to the new foreign-network?

Solution: Use anchor foreign-agent.

▩ An anchor foreign-agent refers to a foreign-agent in the foreign-network where the mobile- node was first found. (step 1 in Figure 5.2).

▩ When the mobile-node moves to a new foreign-network (step 2), the mobile-node registers with the new foreign-agent (step 3).

▩ The new foreign-agent provides the anchor foreign-agent with the mobile-node's new COA

(step 4).

▩ When the anchor foreign-agent receives an encapsulated-datagram, the anchor re- encapsulates and forwards the datagram to the mobile-node (step 5).