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Call Admission Control In Mobile Wireless Networks


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This is a presentation I did at the pre-Master studies at Ain shams University. It\'s about algorithms and techniques of call admission when a mobile node moves among cell in a wireless network.

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Call Admission Control In Mobile Wireless Networks

  1. 1. <ul><li>الدهر يومان ذا أمن وذا خطر ** والعيش عيشان ذا صفو وذا كدر </li></ul><ul><li>أما ترى البحر تعلو فوقه جيف ** وتستقربإقصى قاع ـــــــــ ه الدرر </li></ul><ul><li>وفي السماء نجوم لاعداد لها ** وليس يكسف إلا الشمس والقم ـ ر </li></ul><ul><li>أخي لن تنال العلم إلا بستة ** سأنبيك عن تفصيلها ببيان </li></ul><ul><li>ذكاء وحرص وإجتهاد وبلغة ** وصحبة أستاذ وطول زمان </li></ul>
  2. 2. Admission Control Handoff Amira Rezk Mervat Fahmy
  3. 3. Handoff Strategy
  4. 4. Mobile Computing <ul><li>A technology that allows transmission of data, via a computer, without having to be connected to a fixed physical link . </li></ul>
  5. 5. Mobile data communication <ul><li>Allows users to transmit data from remote locations to other remote or fixed locations . </li></ul><ul><li>It is a solution to the biggest problem of business people on the move - Mobility . </li></ul>
  6. 6. Cellular Network Architecture <ul><li>consists of mobile units linked together to switching equipment, which interconnect the different parts of the network and allow access to the fixed Public Switched Telephone Network ( PSTN ( </li></ul>
  7. 7. Cellular Network Architecture <ul><li>Each cell has a number of channels associated with it . These are assigned to subscribers on demand . </li></ul><ul><li>When a Mobile Station ( MS ) becomes 'active' it registers with the nearest BS . The corresponding MSC stores the information about that MS and its position . This information is used to direct incoming calls to the MS . </li></ul>
  8. 8. When a mobile user want to communicate with another user or a base station it must first obtain a channel from the base stations that hears it. If a channel is available, it is granted to the user else the new call is blocked . Cellular Wireless Technology
  9. 9. Cellular Wireless Technology <ul><li>The user releases the channel under either of the following scenarios: </li></ul><ul><ul><li>The user complete the call </li></ul></ul><ul><ul><li>The user moves to another cell before the call is completed ( Handoff ) while performance the handoff the mobile unit requires that the base station in the cell that it move into will allocate it a channel. If no channel available in the new cell, (the handoff call blocked ) </li></ul></ul>
  10. 10. Handoff <ul><li>The process of reconfiguring the mobile host, wireless network and backbone wired network to support communication after a user enters a different cell of the wireless network. </li></ul><ul><li>The objective of handoff </li></ul><ul><ul><li>Is to maintain end-to-end connectivity in the dynamically reconfigured network topology. </li></ul></ul>
  11. 11. Handoff <ul><li>During a handoff </li></ul><ul><ul><li>Assign a channel in the new cell. </li></ul></ul><ul><ul><li>The route of data through the wired network to the MH must be updated to pass through this new BS. </li></ul></ul><ul><ul><li>Any state in the old BS associated with the MH must somehow be transferred to the new BS. </li></ul></ul>
  12. 12. Problems with handoff <ul><li>High speed vehicles can cross many “small” cells in a short time. </li></ul><ul><ul><li>Umbrella cell. Large cell with a powerful base station to handle high speed vehicles </li></ul></ul><ul><li>Another problem is called cell dragging. </li></ul><ul><ul><li>Happens when the user moves slowly away from the cell and the base station didn’t recognize it due to strong average signal. </li></ul></ul><ul><li>Macrocells for fast movers </li></ul><ul><li>Microcells for slow movers </li></ul>
  13. 13. Implementation Goals <ul><li>Minimize handoff latency </li></ul><ul><li>Minimize frequency of handoff and its effects on QoS </li></ul><ul><li>Minimize probability of dropping connections across handoffs </li></ul><ul><li>Minimize “call blocking”/effects of admission control </li></ul>
  14. 14. Handling Handoffs <ul><li>Here, an important issue is to limit the probability of forced call termination. </li></ul><ul><li>Therefore, the system must reduce the chances of unsuccessful handoffs by reserving some channels explicitly for handoff calls. </li></ul><ul><li>Handoff prioritizing schemes are channel assignment strategies that allocate channels to handoff requests more readily than new calls. </li></ul><ul><ul><li>Guard Channels Schemes </li></ul></ul><ul><ul><li>Handoff Queuing Schemes </li></ul></ul><ul><ul><li>New Call Queuing Schemes </li></ul></ul>
  15. 15. Handoff prioritizing schemes <ul><li>Guard Channels Schemes </li></ul><ul><ul><li>Reserving a number of channels exclusively for handoffs in each cell. The remaining channels can be shared equally between handoffs and new calls. </li></ul></ul><ul><li>Handoff Queuing Schemes </li></ul><ul><ul><li>In this scheme no new call is granted a channel before the handoff requests in the queue are served. </li></ul></ul><ul><li>New Call Queuing Schemes </li></ul><ul><ul><li>The delay insensitivity of new calls makes it more feasible to queue new call attempts instead of handoff attempts. </li></ul></ul>
  16. 16. Re-routing Connection Protocol • Virtual connection trees • User tracking • Multicast routing The most important difference between the mobile and stationary user is that the routing of data to mobile users must be more dynamic. The route data takes in a network must change each time a mobile user moves between cells of a wireless network. There have been many past efforts to support wireless mobile data communication and re-routing connection such as:
  17. 17. Virtual Connection Trees The objective of the VC Tree approach is to reduce the number of router updates caused by handoffs. Virtual Connection Numbers: Defines a path between the root node of the tree and one of the base stations in the region.
  18. 18. Virtual Connection Trees
  19. 19. Virtual Connection Trees Have some weaknesses include : • The virtual connection tree handoff is a long and expensive procedure. • The approach does not attempt to eliminate data loss. While the root node is being notified, the data is incorrectly routed and never forwarded. • The approach has possible scaling problems. There is a single root node per region. However to reduce inter-region handoffs, regions must be made as large as possible. As a result, the root node must process a large number of routing updates as well as route a large amount data.
  20. 20. User tracking (groups-based routing) <ul><li>This design introduces a three level hierarchy. This hierarchy consists of: </li></ul><ul><li>Mobile Hosts (MH) </li></ul><ul><li>Mobile Support Stations (MSS) </li></ul><ul><li>Supervisor Hosts (SH) </li></ul>
  21. 21. User Tracking • Connection Establishment – MH S to communicate with MH D ; Reflect request to MSS S and SH S – Assigns local VCN – SH S locates MH D (VCN, ID of MH S , ID of MH D ) » Looks for MH D within local subnet first (ACTIVE LOCAL) » If no response, broadcast locate request to other SHs; When located, assign local VCN, respond to SH S Connection status becomes ACTIVE REMOTE – Connection is now established
  22. 22. Multicast routing There are three basic parts to the routing system : 1. Delivery to the home agent: 2. Determine the physical location of the mobile host. 3.The delivery of packets from the home agent to the mobile host .
  23. 23. Do not Forget <ul><li>From the point of a mobile user forced termination of an ongoing call is less desirable than blocking a new call </li></ul><ul><li>Need handoff mechanism that achieves very low latencies with little disruption in traffic at the receiver, and to do this with as little data loss as possible </li></ul>
  24. 24. Thanks
  25. 25. Call Admission Control in Mobile Wireless Networks What? Why? and How? By Mervat M. Fahmy Mansoura University Faculty of Computer & Information Sciences Information Systems Dept.
  26. 26. <ul><li>The rapid development in wireless cellular communications dictates that the next generation of networks carry multimedia traffic: voice, video, images, or data, or combinations of them. </li></ul><ul><li>The QoS guarantee is an essential issue in wireless mobile networks supporting multimedia. </li></ul><ul><li>One of the key elements in providing QoS guarantee is an effective Call Admission Control (CAC) policy. </li></ul>CAC…..what?
  27. 27. CAC…..what? <ul><li>Call admission control is the process of making a decision for new call admission according to the amount of available resources versus user’s QoS requirements and the effect upon the QoS of the existing calls imposed by new admission. </li></ul><ul><li>CAC has to ensure that the network meets the QoS of newly arriving calls (if accepted) while guaranteeing that the QoS of existing calls does not deteriorate . . </li></ul>
  28. 28. CAC…..what? <ul><li>Admission control decision is made using a traffic descriptor that specifies traffic characteristics and QOS requirements </li></ul><ul><li>Traffic characteristics: </li></ul><ul><ul><li>peak cell rate (PCR), sustained cell rate (SCR), maximum burst size (MBS),... </li></ul></ul><ul><li>QOS requirements: </li></ul><ul><ul><li>tolerable cell loss, cell delay, delay variation </li></ul></ul>
  29. 29. <ul><li>Two more QoS requirements are considered for wireless networks: </li></ul><ul><ul><li>Graceful degradation of service: reducing allocated bandwidth to the existing calls, </li></ul></ul><ul><ul><li>Guarantee of seamless service: providing connectivity on the move, based on the minimum requirement criteria provided by the user. </li></ul></ul>CAC…..what?
  30. 30. CAC…..what? <ul><li>When a mobile moves across cells during its lifetime, dropping is caused by unavailability of the channels in the new cell. </li></ul><ul><li>Dropping a call in progress has more negative impact from users’ perception than rejecting a newly requested call. </li></ul><ul><li>A key design goal is to minimize the call dropping probability . </li></ul><ul><ul><li>This leads to poor channel utilization due to admitting less new calls. </li></ul></ul>
  31. 31. CAC…..what? <ul><li>The main challenge in the design of an efficient admission control scheme is to balance these two conflicting requirements. </li></ul><ul><li>The major performance parameters of interest are the call dropping probability , channel utilization , and new call blocking probability . </li></ul>
  32. 32. CAC…..what? <ul><li>Example (Uncontrolled TDMA Network) </li></ul><ul><li>Assumptions: </li></ul><ul><li>Number of channels per cell=20. </li></ul><ul><li>The network consists of 3 cells. </li></ul><ul><li>At instant (t 0 ), number of users N i (t 0 ) (i=1,2,3) is equal to (18, 17, 19) </li></ul><ul><li>At instant (t 1 ), a new user arrived at cell 3. Then N i (t 2 ) will be (18, 17,20). </li></ul>
  33. 33. CAC…..what? <ul><li>At instant (t 2 ), one of the 17 active users in cell 2 is leaving cell 2 heading cell 3. </li></ul><ul><li>This user has to be handed over to cell 3, but cell 3 is fully loaded. </li></ul><ul><li>Result: This user will be rejected by cell 3 and the call will be dropped. </li></ul><ul><li>Conclusion: Admitting the new user in cell 2 at instant (t 1 ) was not a good decision! </li></ul>
  34. 34. CAC…..why difficult? <ul><li>Typical traffic sources are bursty </li></ul><ul><li>Some traffic sources are VERY bursty </li></ul><ul><li>Traffic can be highly unpredictable </li></ul><ul><li>Accurate traffic descriptors may not be known in advance </li></ul><ul><li>Traffic may not conform to its descriptor </li></ul>
  35. 35. CAC… <ul><li>CAC strategies can be categorized according to: </li></ul><ul><li>Decision Level </li></ul><ul><ul><li>Distributed </li></ul></ul><ul><ul><li>Semi-Distributed </li></ul></ul><ul><ul><li>Central. </li></ul></ul><ul><li>Information Level </li></ul><ul><ul><li>Local </li></ul></ul><ul><ul><li>Semi-local </li></ul></ul><ul><ul><li>Global </li></ul></ul>
  36. 36. CAC… -Less information -No comm. -Simple (Distributed , Local) -More BS-to-switch comm. -Most Complex -All information (Optimum) (Central , Global) -BS-to-switch comm. -More complex -More information (Sub-optimum) (Semi-distributed , Semi-local) -BS-to-BS comm. -More information (Sub-optimum) (Distributed , Semi-local) (Cooperative) Cons Pros (Decision , Information)
  37. 37. Static CAC Strategies <ul><li>Guard Channel Scheme (Trunk reservation): </li></ul><ul><li>Preferential treatment to the handoff calls. </li></ul><ul><li>Reserves a fixed long-term number of guard channels exclusively for handoffs . </li></ul><ul><li>Assumptions are needed for the call arrival processes, call lifetime, and channel holding time to calculate the number of guard channels. </li></ul><ul><li>uses a threshold to decide whether to accept new call requests or not, the threshold is a maximum number of busy channels in each cell. </li></ul>
  38. 38. Static CAC Strategies <ul><li>Pros: </li></ul><ul><ul><li>It is easy to decide whether to accept a new connection or not. </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>Bandwidth underutilization </li></ul></ul>
  39. 39. Dynamic (Statistical)CAC Strategies <ul><li>Shadow Cluster Scheme: </li></ul><ul><li>Every mobile terminal with an active wireless connection exerts an influence upon the cells in the vicinity of its current location and along its direction of travel. </li></ul><ul><li>The coverage of a shadow cluster for a given active mobile consists of the cell where the mobile is currently present (center of shadow cluster) and all its adjacent cells. </li></ul><ul><li>The area moves as the mobile call is handed off to other cells, thus a shadow cluster needs to be implemented for every new call as well as for handoff call. </li></ul><ul><li>Status information is exchanged among base stations upon each call arrival (new call and handoff). </li></ul>
  40. 40. <ul><li>Distributed call admission control (D-CAC): </li></ul><ul><li>Status information exchange is done periodically. </li></ul><ul><li>Calculates the maximum number of calls that can be admitted to a given cell without violating the QoS of the existing calls in the cell and its adjacent cells. </li></ul><ul><li>D-CAC determines the admission threshold (maximum number of admitted calls) for a cell to admit new calls during a CAC period T by considering the number of active calls in both the local and adjacent cells. </li></ul>Dynamic CAC Strategies
  41. 41. Dynamic CAC Strategies <ul><li>This admission test is applied for newly requested connection in a cell: </li></ul><ul><li>Where C is the wireless link capacity, B r is the target reservation bandwidth for hand­offs in the cell, b i is the bandwidth being used by an existing connection i , and b new is the bandwidth required by the newly requested connection. </li></ul><ul><li>The approach is based on the estimated mobility during the time window [ t o , t o + T ], where t o is the current time. </li></ul><ul><li>The mobility of an active mobile with connection C 0,j is estimated with P h ( C 0,j  i ), the probability that C 0,j hands off into cell i within T . </li></ul>
  42. 42. <ul><li>The required bandwidth to be reserved in cell 0 for the expected hand­offs from cell i is obtained as: </li></ul><ul><li>  </li></ul><ul><li>where C i is the set of indices of the connections in cell i and b ( C i,j ) is connection C i,j 's bandwidth. </li></ul><ul><li>The target reservation bandwidth B r,0 in cell 0 , which is the aggregate bandwidth to be reserved in cell 0 for the expected hand­offs from adjacent cells within T , is calculated as: </li></ul><ul><li>  </li></ul><ul><li>where A i is the set of indices of cell i 's neighbors. </li></ul>Dynamic CAC Strategies
  43. 43. <ul><li>The admission test is given by: </li></ul><ul><li>For all i belonging to A 0 (neighbors of cell C 0 ), check if </li></ul><ul><li>  </li></ul><ul><li>Check if: </li></ul><ul><li>If all of the above tests are positive then the connection is admitted. </li></ul>Dynamic CAC Strategies
  44. 44. Dynamic CAC Strategies <ul><li>Pros: </li></ul><ul><ul><li>Bandwidth efficient utilization based on statistical information about call traffic characteristics. </li></ul></ul><ul><li>Cons: </li></ul><ul><ul><li>Difficulties in characterizing a call arrival process and lack of understanding as to how it is shaped. </li></ul></ul><ul><ul><li>CAC Decisions must be made on the fly. </li></ul></ul>
  45. 45. THANK YOU