yschen, csie, ccu1 chapter 7: mobile communication systems associate prof. yuh-shyan chen dept. of...

Yschen, CSIE, CCU 1

Chapter 7:Mobile Communication

Systems

Associate Prof. Yuh-Shyan Chen

Dept. of Computer Science and Information Engineering

National Chung-Cheng University

Yschen, CSIE, CCU 2

Introduction

A wireless systems implies support for subscriber mobility and the overall communication infrastructure needed to handle movements not only from one cell to another,in a cell’s mobile switching center area and in area controlled by other service providersHandoff among cells and mobile switching

centers (MSCs) that are serviced by the same service providers or different service providers needs to be supported

Yschen, CSIE, CCU 3

Outline

This chapterConsider handoff schemesAllocation of resourcesRouting in the backbone network

Yschen, CSIE, CCU 4

Cellular System Infrastructure

A generic block diagram of a cellular system is given in Fig. 9.1.Each BS consists of a base transceiver system

(BTS) and BS controller (BSC)Both tower and antenna are part of BTS, and all

associated electronics are contained in the BTSAuthentication Center (AUC) unit provides

authentication and encryption parameters that verify the user’s identify and ensure the confidentially

Yschen, CSIE, CCU 5

A detailed block diagram of a cellular system

Yschen, CSIE, CCU 6


Equipment Identity (EIR) is a database that contains information about the identify of mobile equipment that prevents calls from being stolenBoth AC and EIR could be implemented as

individual stand-alone nodes or as a combined AUC/EIR node

The home location register (HLR) and visitor location register (VLR) are two sets of pointers that support mobility and enable the use of the same telephone number over a wide range

Yschen, CSIE, CCU 7


The HLR is located at the MSC where the MS is initially registered and is the initial home location for billing and access information

Any incoming call, based on the calling number, is directed to the HLR of the home MS, which then points to the VLR of the MSC where the MS is currently located

The VLR contains information about all MSs visiting the particular MSC and hence points to the HLR of the visiting MSs for exchanging related information about the MS

Yschen, CSIE, CCU 8


Such two-way pointers allow calls to be routed or rerouted to the MS, wherever it is located

There are many control signals that traverse back and forth between the HLR and VLR (including billing and access permissions maintained at the home MSC), bidirectional HLR-VLR pointers help in carrying out various functionalities

Yschen, CSIE, CCU 9

Redirection of a call to MS at a visiting location

Yschen, CSIE, CCU 10

Redirection

This works very well if the destination MS has moved from one cell to another

If a call is initiated from a residential telephone, the call is forward through the backbone network to the wireless access point closet to the home MSC, where the MS being called is registered

Thereafter, a similar routing enables connection to the MS

The reverse path connection can be established


Redirection

The home MSC also maintains access information about all MSs registered, including ofState of the MS (active/nonactive)Type of allowed service (local and/or long-distance

calls)Billing information (past credit, current charges,

chronological ( 依年代順序 ) order of calls made, timings of each call)


Redirection

This work even if the home MSC is different from the visiting MSC

As long as two MSCs have information about how to forward message to each other

Mobility can also be supported is an unknown territory ( 領域 )


Registration

This is maintained not only for billing, but also for authentication, verification, as well as for access privileges ( 特權 )

The wireless systems needs to know whether the MS is currently located in its own home area or is visiting some other area

This enables incoming calls to be routed to an appropriate location and assures desirable support for outgoing calls


Registration

This is done by exchanging signals known as “beacons” between BS and the MS

BSs periodically broadcast beacon signals to determine and test nearby MSs

Each MS listens for new beacon signals, and information is the used by a MS to locate the nearest BS and establish an appropriate rapport to initiate dialogue with outside world through the BS as a gateway

Other information of beaconCellular network identifier, timestamp, gateway

address, ID (Identification) of the paging area


Beacon signals

Beacon signals help synchronize, coordinate, and manage electronic resource using miniscule ( 微不足道 ) bandwidth for a very short duration

Researchers continue to improve their functionality by increasing signal coverage while optimizing energy consumption


Using a mobile phone outside the subscription area


Registration Steps

1. A mobile phone listens for new beacons, and if it detects one, it adds it to the active beacon kernel table. If the device determines that it needs to communicate via a new BS, kernel modulation initiates the handoff process.


Registration Steps

2. The mobile phone locates the

nearest BS via user-level

processing.


Registration Steps

3. The visiting BS performs user-level processing and determines who the user (MS) is, the user’s registered home site for billing purposes, and what kind of access permission the user has.


Registration Steps

4. The home site sends an appropriate authentication response to the BS currently serving the users.


Registration Steps

5. The BS at the visited location approves or disapproves user access.


Handoff Parameters and Underlying Support

Handoff basically involves change of radio resource from one cell to another adjacent cell

From a handoff perspective, it is important that a free channel is available in a new cell whenever handoff occurs, so that undisrupted service is available


Handoff Underlying Support

Hard handoffAlso known as break before makeIs characterized by releasing current radio

resources from the prior BS before acquiring resource from the next BS

Both FDMA and TDMA employ hard handoffSoft handoff

It is possible for an MS to communicate simultaneously with the prior BS as well as the new BS, just for some short duration of time


Handoff Underlying Support

In CDMA, as the same channel is used in all the cells, if the code is not orthogonal to other codes being used in the next BS, the code could be changed

In fact, beacon signals and the use of the HLR-VLR pair allow MSs to roam anywhere as long as the same service provider, using particular frequency band in that area (Fig. 9.6)


Handoff between MSCs


Handoff region


Hard handoff


Soft handoff


Parameter Influencing Handoff

Handoff depends on cell size, boundary length, signal strength, fading, reflection, and refraction of signals, and by man-made noise

The probability of a channel being available in a new cell area depends on the number of channels per unit area


Parameter Influencing Handoff

It can be easily observed that the number of channels/area increases if the number of channels allocated per cell is increased or if the area of each cell is decreased

This leads to a smaller cell size may be goodHowever, this would cause more frequent handoff,

especially for MSs with high mobility and speed

Handoff can be initiated either by the BS or the MS and it could be due to Radio linkNetwork ManagementService issues


Radio link-type factros

Number of MSs that are in the cellNumber of MSs that have left the cellNumber of calls generated in the cellNumber of calls transferred to the cell from

neighboring cells by the handoffNumber and duration of calls terminated in the

cellNumber of calls handed off to neighboring cellsCell dwell time


Network management

Network management may cause handoff if there is a drastic imbalance of traffic over adjacent cells and optimal balance of channels and other resources are required

Service-related handoff is due to degradation of quality of service (QoS), and handoff could be invoked when such a situation is detected

The factor that define the right time for handoff areSignal strength, signal phase, bit error rate, distance


The need for handoff is determined in two different ways

Signal strengthCarrier-to-interference ratio

A low value of CIR may force the BS to change the channel currently being used between the BS and the MS.


Roaming Support Using System Backbone

What happens when a channel and the radio contact is changed from one cell to another when handoff occursA number of cells are controlled by an MSC, and

depending on the destination, the signals go through the backbone network, interconnecting MSCs with the PSTN

The hardwired network is primarily supported by ultra-high-speed fiber optic cables


Roaming Support Using System Backbone

With mobility support, the real problem in routing becomes getting moving packets to appropriate endpoints of the backbone network (Fig. 9.7)


Handoff with different degree of mobility


Roaming

Assuming MSC1 to be the home of the MS for registration, billing, authentication, and all access informationWhen the handoff is from a to b, the routing of

message can be performed by MSC itself When the handoff occurs from b to c, the

bidirectional pointers are set up to link the HLR of MSC1 to VLR of MSC2 so that information can be routed to the cell where the MS is currently located


Information transmission path


Roaming

When handoff occurs at locations d and e, routing of information using simply the HLR-VLR pair of pointers may not be adequate

The paging area is the area covered by one or several MSC in order to find the current location of the MSThis concept is taken from the internet network

routing area


MSC connections to backbone


An example:

One involves determining the path along the shortest path

The second involves ascertaining the path according to the current location of the MS

The movement from a to c can be supported effectively by HLR-VLR, wherein MSC1 knows how to route the data to MSC2One option is to let all the messages reach MSC1

and forward the messages from there to the MS


An example:

Another option is to find a router along the original path, from where a new path needs to be used to reach the destination MSC along the shortest path

If this is done, then part of the message in the pruned tree could be lost if a hard handoff is performed, when breaks the connection before it makes

Therefore, after handoff, it may be desirable to forward message from an old location to a new one, for a short duration of time


Home Agents, Foreign Agents, and Mobile IP

In mobile internet protocol (Mobile IP), two important agents are associated with the routers: home agent (HA) and foreign agent (FA)A router closet to the home MSC can be selected to

serve as its HA (Table 9.2)Once MS moves from the home network (where it is

registered) to a foreign network, a software agent, FA, assists the MS by forwarding packets for the MS

The functionality of HA-FA is somewhat analogous to the HLR-VLR pair, as long as there is an agreement and understanding between the home network and the foreign network


Home MSC and Home Agent


The way it works as follows.

Whenever A MS moves into a new network, it retains its initial HA

The MS can detect the FA of the current domain by the periodic beacon signals that the FA transmitsOn the other hand, the MS can itself send agent

solicitation messagesWhen the FA detects that a new MS has moved into its

domain, it allocates a care-of-address (CoA) to the MSThe CoA can either be the address of the FA itself, or it

maybe a new address called colocated CoA (C-CoA) that the FA allocates to the MS using the dynamic host configuration protocol (DHCP, IETF RFC 2131, Marcg 1997)


Registration process


Registration process

Once the MS receives the CoA, it registers this CoA with its HA and the time limit for its binding validity

Such a registration is initiated either directly by the MS to its HA, or indirectly through the FA at the current location

The HA the confirms this binding through a reply to the MS


Message forwarding

A message sent from arbitrary source to the MS at the home address is received by the HA, binding for the MS is checked, and the HA encapsulates the packet with the CoA of the MS and forwards it to the FA areaIf the colocated CoA address is used, the MS

receives the packet directly and is decapsulated to interpret the infomration

If CoA for the FA is used, then the packet reaches the FA, which decapsulates the packet and passes on to the MS at the link layer


Message forwarding to the MS


Rerouting in Backbone Routers

A simplistic approachIs to have a global table at all routers of the

network so that the route from FA to HA can be foundedThis one-step global table is excessively large

Another oneOnly gateway routers that support routing within

the backbone are shown in Fig. 9.12 and Table 9.3


Paging areas (PAs)


Distributed Routing Table and Location of PAs

yschen, csie, ccu1 chapter 7: mobile communication systems associate prof. yuh-shyan chen dept. of...

Documents

visiting msc

particular msc

destination ms

bidirectional hlrvlr

initial home location

related information

visiting mss

locatedthe vlr