Mobile Network Notes

Mr. Tushar Patil KCE COE&IT, Jalgaon

B. E. COMPUTER 2009-10

Introduction (1) PCS Architecture What is PCS • Personal Communication Services A wide variety of network services that includes wireless access and personal mobility services – Provided through a small terminal –Enables communication at any time, at any place, and in any terminal form. Several PCS systems: • High-tier Systems: – Widespread vehicular and pedestrian services • GSM: Global System for Mobile Communications – The mobile telephony system that we are using • IS-136 – USA digital cellular mobile telephony system – TDMA based multiple access • Personal Digital Cellular • IS-95 cdmaOne System – CDMA based multiple access • Low-tier systems: – Residential, business and public cordless access applications and systems • Cordless Telephone 2 (CT2) • Digital Enhanced Cordless Telephone (DECT) • Personal Access Communication Systems (PACS) • Personal Handy Telephone System (PHS) PCS Architecture:



Each PCS technology has similar architectures which consists two parts – Radio Network • MS (Mobile Station) • BS (Base Station) System – Wireline Transport Network • MSC (Mobile Switching Center) • The Mobility Database connected to MSC is used to track the locations of mobile station PCS (personal communications service) is a wireless phone service similar to cellular telephone service but emphasizing personal service and extended mobility. It's sometimes referred to as digital cellular (although cellular systems can also be digital). Like cellular, PCS is for mobile users and requires a number of antennas to blanket an area of coverage. As a user moves around, the user's phone signal is picked up by the nearest antenna and then forwarded to a base station that connects to the wired network. The phone itself is slightly smaller than a cellular phone. According to Sprint, PCS is now available to 230 million people.

The "personal" in PCS distinguishes this service from cellular by emphasizing that, unlike cellular, which was designed for car phone use and coverage of highways and roads, PCS is designed for greater user mobility. It generally requires more cell transmitters for coverage, but has the advantage of fewer blind spots. Technically, cellular systems in the United States operate in the 824-849 megahertz (MHz) frequency bands; PCS operates in the1850-1990 MHz bands.

Several technologies are used for PCS in the United States, including Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Global System for Mobile (GSM) communication. GSM is more commonly used in Europe and elsewhere.

(2) Mobility Management

A GSM or UMTS network, like all cellular networks, is a radio network of individual cells, known as base stations. Each base station covers a small geographical area which is part of a uniquely identified location area. By integrating the coverage of each of these base stations, a cellular network provides a radio coverage over a much wider area. A group of base stations is named a location area, or a routing area.

The location update procedure allows a mobile device to inform the cellular network, whenever it moves from one location area to the next. Mobiles are responsible for detecting location area codes. When a mobile finds that the location area code is different from its last update, it performs another update by sending to the network, a location update request, together with its previous location, and its Temporary Mobile Subscriber Identity (TMSI).

There are several reasons why a mobile may provide updated location information to the network. Whenever a mobile is switched on or off, the network may require it to perform



an IMSI attach or IMSI detach location update procedure. Also, each mobile is required to regularly report its location at a set time interval using a periodic location update procedure. Whenever a mobile moves from one location area to the next while not on a call, a random location update is required. This is also required of a stationary mobile that reselects coverage from a cell in a different location area, because of signal fade. Thus a subscriber has reliable access to the network and may be reached with a call, while enjoying the freedom of mobility within the whole coverage area.

When a subscriber is paged in an attempt to deliver a call or SMS and the subscriber does not reply to that page then the subscriber is marked as absent in both the MSC/VLR and the HLR (Mobile not reachable flag MNRF is set). The next time the mobile performs a location update the HLR is updated and the mobile not reachable flag is cleared.

Strategies to detect the need for Handoff: Mobile-controlled handoff (MCHO) –MS continuously monitors the signals of the surrounding BSsand initiates handoff process when some criteria are met –Used in DECT and PACS Network-controlled handoff (NCHO) –The surrounding BSsmeasure the signal from the MS, and the network initiates the handoff process when some criteria are met –Used in CT-2 Plus and AMPS Mobile-assisted handoff (MAHO) –The network asks the MS to measure the signal from the surrounding BSs. The network makes the handoff decision based on reports from the MS –Used in GSM and IS-95 CDMA I) Inter-BS Handoff : •New and Old BSsare connected to the same MSC •Need for Handoff is detected by the MS •Steps of Actions: –MS momentarily suspends conversation and initiates the Handoff procedure by signaling on an idle channel in the new BS. Then itresumes conversation on the old BS –MSC transfers the encrypted information to the selected channel of the new BS and setup the new conversation path. The switch bridges the new path with the old path and informs the MS to transfer from the old channel to the new channel –After the MS has been transferred to the new BS, it signals the network, and resumes conversation using the new channel –Upon receipt of the Handoff completion signal, the network removes the bridge from the path and releases resources associated with the old channel



Intersystem Handoff •New and Old BSsare connected to two different MSCs •Network-controlled Handoff •Steps of Actions: –MSC A requests MSC B to perform handoff measurements on the callin progress. MSC B selects a candidate BS and interrogates it for signal quality parameters on the call in progress. MSC B returns the signal quality parameter values to MSC A. –MSC A checks if the MS has made too many handoffs recently or Intersystem trunks are not available. If so MSC A exits the procedure. Otherwise MSC A asks MSC B to setup a voice channel, and then MSC B instructs MSC A to start the radio link transfer. –MSC A sends the MS a handoff order. The MS synchronizes to new BS. After the MS is connected to new BS, MSC BinformsMSC A that handoff is successful. MSC A then



What happens if the mobile moves again ?



(3) Roaming Management: Roaming is one of the fundamental mobility management procedures of all cellular networks. Roaming is defined as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when travelling outside the geographical coverage area of the home network, by means of using a visited network. This can be done by using a communication terminal or else just by using the subscriber identity in the visited



network. Roaming is technically supported by mobility management, authentication, authorization and billing procedures. Basic Operations •Two basic operations in roaming management. –Registration (location update): an MS informs theb system of its current location –Location Tracking: the system locates the MS Location Tracking and Update •When a mobile user moves from one PCS system to another, the current location of the user should be updated. •How to get the current location of the user ? •Where to store the current location of the

Two-level Hierarchical Strategy •In IS-41 and GSM MAP, the two-level strategies are proposed.•A two-tier system of home and visited databases–Home Location Register (HLR)–Visited Location Register (VLR) Home Location Register (HLR)



•HLR is a network database that stores and manages all subscriptions of a specific operator. •The information in HLR: –MS Identity, directory number, profile information, current location, validation period

Visitor Location Register (VLR) •The VLR has temporary information for the visiting mobile users. •The information in VLR: –MS Identity, directory number, current location

MS Registration Process



1.When the mobile user moves from one visited system to another, it must register in the VLR of the new visited system 2.The new VLR informs the mobile user’s HLR of the person’s current location –address of the new VLR. The HLR sends an acknowledgement, which includes 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 of the MS in the old VLR. The old VLR acknowledges the deregistration. Call Origination Procedure :

•MS contacts the MSC in the visited PCS network •The call request is forwarded to the VLR for approval •If the call is accepted, the MSC sets up the call to the called party following the standard PSTN call setup procedure Call Delivery :



1.Call attempted by a wirelinephone is forwarded to a switch in PSTN, 3 which queries the HLR to find the current VLR of the MS. 2.The VLR returns the routable address to the originating switch through the HLR 3.Based on the routable address, the trunk is setup from the originating switch to the MS through the visited MSC Roaming Management under SS7 : •How mobile roaming is managed by the PSTN signaling? •Common Channel Signaling (CCS) is a signaling method that provides control and management functions in the telephone network. •CCS channel conveys messages –to initiate and terminate calls –determines the status of some part of the network –controls the amount of traffic allowed •CCS uses a separate out-of-band signaling network to carry signaling messages •Signaling System No. 7 (SS7) is a CCS system •Signaling between a PCS network and the PSTN network are achieved by the SS7 network Interconnection between a PCS network and the PSTN

Components of SS7 •Service Switching Point (SSP) –Telephone switch interconnected by SS7 link



–SSPsperform call processing on calls that originate or terminate at that node –A local SSP in the PSTN can be central officeor end office –An SSP in a PCS network is MSC •Signal Transfer Point (STP) –Switch that relays SS7 messages between network switches and databases –Based on the address fields of the SS7 messages, the STPsroute the messages to the correct outgoing signaling links •Service Control Point (SCP) –Contains databases (HLR or VLR) for providing enhanced services, and accepts queries from SSP Registration through SS7

1.MSC2 launches a registration query to its VLR through STP2, assuming that

VLR2 and MSC2 are not colocated

2.VLR2 sends a registration message to the MS’s HLR (HLR4). VLR2 may not

know the actual address of HLR. Instead, VLR2 sends the message containing



Mobile Identification Number (MIN) to an STP (STP3) that can translate the MIN

into the HLR address

3.MIN-to-HLR address translation is performed at STP3 by a GTT. STP3

then forwards the registration message to HLR.

4.HLR sends an acknowledgement back to VLR2

5.HLR sends a deregistration message to VLR1, and then VLR1 acknowledges the

cancellation Obsolete VLR records are not deleted until the database is full.

•If the database is full when an MS arrives, a record is deleted, freeing storage space to

accommodate the newly arrived MS.

•A replacement policy is required to select a record for replacement.

•Advantage: No deregistration messages are sent among the SS7 network elements.

Periodic re-registration •The MS periodically re-registers to the VLR.

•If the VLR does not receive the reregistrationmessage within a timeout period, the

record is deleted

•Creates local message traffic between MSC and VLR

•No SS7 signaling messages are generated if VLR is collocated with the MSC

Pointer Forwarding Scheme



•Move Operation (Registration): –When MS moves from one VLR to another, a pointer is created from the old VLR to the new VLR. –No registration to HLR is required •Find operation (Call delivery): –When the HLR attempts to locate the MS for call delivery, the pointer chain is traced. –After find operation, the HLR points directly to the destinationVLR. •Number of pointers visited in the find operation is limited by k •Pointer forwarding scheme should not be considered when the net cost of pointer creation and pointer traversal is higher than the cost of accessing the HLR Call Delivery through SS7

(4)Handoff Management



Handoff In a cellular telephone network, handoff is the transition for any given user of signal transmission from one base station to a geographically adjacent base station as the user moves around. In an ideal cellular telephone network, each end user's telephone set or modem (thesubscriber's hardware) is always within range of a base station. The region coveredby each base station is known as its cell. The size and shape of each cell in anetwork depends on the nature of the terrain in the region, the number of base stations,and the transmit/receive range of each base station. In theory, the cells in anetwork overlap; for much of the time, a subscriber's hardware is within range of morethan one base station. The network must decide, from moment to moment, which basestation will handle the signals to and from each and every subscriber's hardware.

Each time a mobile or portable cellular subscriber passes from one cellinto another, the network automatically switches coverage responsibility from one basestation to another. Each base-station transition, as well as the switching processor sequence itself, is called handoff. In a properly functioning network, handoffoccurs smoothly, without gaps in communications and without confusion about which basestation should be dealing with the subscriber. Subscribers to a network need not doanything to make handoff take place, nor should they have to think about the process orabout which base station is dealing with the signals at any given moment.

BS Coverage Area •BS coverage area: irregular •In the cell boundary–Signal from a neighboring BS –Signal from the serving BS •Otherwise: Forced termination



Issues for Handoff Management I) Handoff detection –Who and how II) Channel assignment III) Radio link transfer I) Handoff Detection •Handoff detection: –Who initiates the handoff process? –How is the need for handoff detected? •Handoffs are expensive. •Overlap of adjacent coverage area is desired •Handoff criteria –If not chosen appropriately, then the call might be handed back and forth several times between two adjacent BSs –If too conservative, then the call may be lost before the handoff •Unreliable and inefficient handoff procedures will reduce the quality and reliability of the system Link Measurement •Handoff detection is based on link measurement. •Signal measurements used to determine the quality of a channel: –WEI (Word Error Indicator) •Metric that indicates whether the current burst was demodulated properly in the MS –RSSI (Received Signal Strength Indication) –QI (Quality Indicator) •Signal to interference and noise (S/I) ratio •To make the handoff decision accurately and quickly, it is desirable to use both WEI (over a period of time) and RSSI (instantaneously) •RSSI measurements are affected by Fading Fading •Distance-dependant Fading or Path Loss –Occurs when the received signal becomes weaker due to increasing distance between MS and BS •Lognormal Fading or Shadow Fading –Occurs when there are physical obstacles (e.g. hills, towers, and buildings) between the BS and MS, which can decrease the received signal strength •RayleighFading or MultipathFading –Occurs when two or more transmission paths exist between MS and BS –Two types of MultipathFading i)RayleighFading: when obstacles are close to the receiving antenna



ii)Time Dispersion: when the object is far away from the receiving antenna •Ideally, the Handoff decision should be based on distance-dependent fading and, to some extent, on shadow fading •Handoff decision is independent of RayleighFading •This can be accomplished by averaging the received signal strength for a sufficient time period Channel Comparison •MS must also measure or sample all frequencies in the band of interest to find suitable handoff candidate •Channel comparisons for handoff are based on RSSI and QI metrics •Since multipathenvironment tends to make the RSSI and QI metric vary widely in the short term •Since it is preferable not to perform handoff to mitigate brief multipathfades •Such handoff could cause unnecessary load on the network •Hence the MS should average or filter these measurements before using them to make decision •Handoff should be initiated whenever the channel has the best filtered RSSI exceeding that of the current channel. •Filtering process applied to the RSSI and QI metrics will reduce their usefullnessin mitigating sudden “shadow” fades, such as when rounding a corner or closing a door. •The downlink WEI can be used to detect and correct these trouble situations on an “override” basis. •Cdown: number of downlink word errors that is reset by every complete measurement cycle •If Cdownexceeds some threshold, the MS should initiate a handoff when an appropriate channel can be found Dwell Timer •To reduce the potential tendency of an MS to request a large number of handoffs in quick succession •This timer prevents the MS from requesting another handoff until some reasonable period of time after a successful handoff. •Adaptive measurement interval for handoffs: –Uses Doplerfrequency to estimate the velocity of the vehicle –Then averaging measurement interval As the MS moves away from one BS toward another, the signals received from the first BS become weaker, and from the second BS become stronger •This slow effect is often masked by the multipathRayleighfading and the lognormal shadow fading •Short-term Rayleigh fading is handled in mobile system designs by techniques: –Diversity techniques such as Frequency hopping, multiple receivers –Signal Processing techniques such as bit interleaving, equalizers •Rayleighfading is frequency dependent



•Longer-term shadow fading is compensated by increasing transmitter power and the co-channel reuse distance Strategies for Handoff detection •Who makes a decision for handoff? •Three handoff detection schemes: 1>Mobile-controlled handoff (MCHO) •MS continuously monitors the signals of the surrounding BS sand initiates handoff process when some criteria are met 2>Network-controlled handoff (NCHO) •The surrounding BSsmeasure the signal from the MS, and the network initiates the handoff process when some criteria are met 3>Mobile-assisted handoff (MAHO) •The network asks the MS to measure the signal from the surrounding BSs. The network makes the handoff decision based on reports from the MS •Advanced mobile systems follow MAHO 1>Mobile-Controlled Handoff (MCHO) •Popular for low-tier radio systems •MS continuously monitors the signal strength and quality from the accessed BS and several handoff candidate BSs •When some handoff criteria are met, the MS checks the best candidate BS for an available traffic channel and launches a handoff request •Two common handoffs: –automatic link transfer (ALT) -transfer between two base stations –time slot transfer (TST) -transfer between channels of a single Automatic link transfer control requires the MS to make quality measurements of the current and candidate channels in the surrounding BSs. •The MS’s handoff control between channels on the same BS is made possible by passing uplink-quality information, in the form of word-error indicator, back to the MS on the downlink •As a part of the demodulation process, the MS obtains two pieces of information: RSSI and QI



2>Network-Controlled Handoff (NCHO) •Used by low-tier CT-2 plus and by high-tier AMPS •BS monitors the signal strength and quality from the MS •When deteriorate below some threshold, the network arranges for a handoff to another BS •The network asks all surrounding BSsto monitor the signal from the MS and the measurement results back to the network •The network then chooses a new BS for the handoff and informs both the MS (through old BS) and the new BS Network uses multiple (current and surrounding) BSsto supervise the quality of all current connections by making measurements of RSSI •MSC will command surrounding BSsto occasionally make measurements of these links •MSC makes the decision when and where to effect the handoff •Heavy network signaling traffic and limited radio resources at BSsprevent frequent measurements of neighboring links long handoff times •Handoff time: upto10sec or more 3>Mobile-Assisted Handoff (MAHO) •Used in GSM, IS-136 and IS-95 •The network asks the MS to measure the signal from the surrounding BSsand report back to old BS •The network makes the handoff decision based on the reports from the MS. •Handoff process is more decentralized •The MS and the BS supervise the quality of the link, RSSI and WEI values •In GSM the MS transmits measurements twice a second •GSM handoff execution time ~ 1sec



•In both NCHO and MAHO -if the network can’t tell the mobile about the new channel/time slot/… to use before the link qualityhas decayed too far, the call may be terminated Handoff Failures •No channel on selected BS •Insufficient resources as determined by the network (for example, no available bridge, no suitable channel card {for example, none supporting the voice CODEC or radio link coding} •It takes too long for the network to set up the new link •Target link fails during handoff Channel Assignment •Goals: –to achieve a high degree of spectrum utilization for a given grade of service –use a simple algorithm –require a minimum number of database lookups •Unfortunately it is hard to do all of these at once! •If there is no available channel, then –new calls are blocked –existing calls that can’t be handed over forced terminations Schemes introduced to reduce the number of forced terminations, at the cost of increased block or decreased efficiency: –Nonprioritizedscheme (NPS) -handoff call treated the same as a new call –Reserved Channel scheme (RCS)-reserves some resources for handoffs –Queuing Priority scheme (QPS) -exploit the over lap (handoff area) –Subratingscheme (SRS) -switching codes of one or more calls to free resources Flowchart for Non-prioritized Scheme: Flowchart for Reserved Channel Scheme



Flowchart for Queuing Priority Scheme (for Handoff Calls)

Implementation Issues •RCS is easy to implement •It reduces the forced termination probability more effectively than NPS •New call-blocking probability for RCS is larger than that of NPS



•RCS is desirable when reducing forced termination is much more important than reducing new call blocking Implementation for the measurement-based priority scheme (MBPS) is more complex than that for the FIFO scheme, but performance is identical •QPS reduce forced terminations, at the expense of increased new call blocking •Probability of incomplete calls for FIFO and MBPS is slightly lower than that for NPS •QPS add hardware/software complexity for both BSsand MSsto manage waiting queues SRS has the least forced termination probability and the probability of incomplete calls when compared with other schemes. •This benefit is gained at the expense of the extra hardware/software complexity required to subratea channel. Implementation Issues: Conclusion •Trade-off between implementation complexity and performance •If reducing forced termination is more important than reducing total call incompletion, then RCS, QPS, and SRS are all better than NPS •If implementation cost is major concern, then RCS and NPS should be considered. •To achieve the best performance with a slight voice quality degradation, SRS should be selected. •If BS density is high in a given PCS service area, then QPS may be good choice. Handoff Management: Radio Link Transfer Link Transfer Procedure •Hard Handoff-Oriented –MS connects with only one BS at a time –Some interruption in the conversation during the link transition –Used in TDMA and FDMA systems •Soft Handoff-Oriented –MS receives/transmits the same signal from/to multiple BS simultaneously •Soft Handoff is more complicated than Hard Handoff Link Transfer Types •Two Operations –The radio link is transferred from the old BS to the new BS. –The network bridges the trunk to the new BS and drop the trunk to the old BS.



•Link transfer cases: 1.Intracell •Link transfer is between two time slots or channels in the same BS •In TDMA system, this is referred as Time Slot Transfer (TST) 2.Intercellor inter-BS 3.Inter-BSC 4.Intersystem or inter-MSC 5.Intersystem between two PCS networks •Focus on Inter-BS Handoff

Hard Handoff MCHO Link Transfer •New radio channel is selected by MS •Handoff request message is transmitted by the MS to the new BS •MS is responsible to choose the best BS



MCHO Inter-BS handoff message flow MAHO/NCHO Link Transfer



MAHO Inter-BS handoff message flow SubratingMCHO Link Transfer •Procedure of subratinga full-rate channel into subratedchannels for handoff request consists of three parts: 1.Requesting the Handoff 2.Subratingan existing call 3.Assigning the newly created subratedchannel to the MS requesting the Handoff

Message flow for subratingautomatic link transfer Soft Handoff CDMA allows many users to share a common frequency/time channel for transmission, and the user signals are distinguishedby spreading them with different PN sequences.



•Also, an MS can transmit/receive the same information to/from several BSs if they have same PN sequence •Signaling and voice information from multiple BSs are combined (or bridged) at the MSC, and the MSC selects the highest-quality signals from the BSs. •Signaling and voice information must be sent from the MSC to multiple BSs, and the MS must combine the results •Thus, within the overlap area of two cells, an MS can simultaneously connect to both the old and new BSs, and the link transfer procedure is no longer time-critical •Focus on adding and removing BSs with MAHO soft handoff

Adding a new BS Dropping the old BS



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Mobile Network Notes