Transcript Mobility and and Handover Issues
Mobility and Handover Issues
Mobile Communication
• Two aspects of mobility: –
user mobility
: users communicate (wireless) “anytime, anywhere, with anyone” –
device portability
: devices can be connected anytime, anywhere to the network • Wireless vs. mobile Examples stationary computer notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA)
What is Mobility?
• – – A device that moves Between different geographical locations Between different networks • – – – – A person who moves Between different geographical locations Between different networks Between different communication devices Between different applications
Device mobility
• Plug in laptop at home/work on Ethernet – Occasional long breaks in network access – Wired network access only (connected => well-connected) – Network address changes – Only one type of network interface – May want access to information when no network is available: hoard information locally • Cell phone with access to cellular network – Continuous connectivity – Phone # remains the same (high-level network address) – Network performance may vary from place to place
Device mobility, continued
• Can we achieve best of both worlds?
– Continuous connectivity of wireless access – Performance of better networks when available • Laptop moves between Ethernet, WLAN and Cellular networks – Wired and wireless network access – Potentially continuous connectivity, but may be breaks in service – Network address changes – Radically different network performance on different networks
People mobility
• Phone available at home or at work – Multiple phone numbers to reach me – Breaks in my reachability when I’m not in • Cell phone – Only one number to reach me – Continuously reachable – Sometimes poor quality and expensive connectivity • Cell phone, networked PDA, etc.
– Multiple numbers/addresses for best quality connection – Continuous reachability – Best choice of address may depend on sender’s device or message content
Mobility means changes
How does it affect the following?
• Hardware – Lighter – More robust – Lower power • Wireless communication – Can’t tune for stationary access • Network protocols – Name changes – Delay changes – Error rate changes
Changes, continued
• Fidelity – High fidelity may not be possible • Data consistency – Strong consistency no longer possible • Location/transparency awareness – Transparency not always desirable • Names/addresses – Names of endpoints may change • Security – Lighter-weight algorithms – Endpoint authentication harder – Devices more vulnerable
Changes, continued, again
• Performance – Network, CPU all constrained – Delay and delay variability • Operating systems – New resources to track and manage: energy • Applications – Name changes – Changes in connectivity – Changes in quality of resources • People – Introduces new complexities, failures, devices
Mobile and wireless services – Always Best Connected
LAN, WLAN 780 kbit/s GSM 53 kbit/s Bluetooth 500 kbit/s UMTS, GSM 115 kbit/s LAN 100 Mbit/s, WLAN 54 Mbit/s GSM/EDGE 384 kbit/s, WLAN 780 kbit/s GSM 115 kbit/s, WLAN 11 Mbit/s UMTS, GSM 384 kbit/s UMTS, DECT 2 Mbit/s
Example changes
• Addresses – Phone numbers, IP addresses • Network performance – Bandwidth, delay, bit error rates, cost, connectivity • Network interfaces – PPP, eth0, strip • Between applications – Different interfaces over phone & laptop • Within applications – Loss of bandwidth triggers change from color to B&W • Available resources – Files, printers, displays, power, even routing
Effects of device portability
• Power consumption – limited computing power, low quality displays, small disks due to limited battery capacity – CPU: power consumption ~ CV • Loss of data (e.g., defects, theft) • Limited user interfaces • Limited memory 2 – flash-memory or ? as alternative f • C: internal capacity, reduced by integration • V: supply voltage, can be reduced to a certain limit • f: clock frequency, can be reduced temporally – higher probability, has to be included in advance into the design – compromise between size of fingers and portability – integration of character/voice recognition, abstract symbols – limited value of mass memories with moving parts
Mobile devices
Sensors, embedded controllers Pager • receive only • tiny displays • simple text messages PDA • simpler graphical displays • character recognition • simplified WWW Mobile phones • voice, data • simple graphical displays Palmtop • tiny keyboard • simple versions of standard applications
performance
Summing up
Generally, mobility stresses all resources further: • CPU • Power • Bandwidth • Delay tolerance • Radio spectrum • Human attention • Physical size • Constraints on peripherals and GUIs (modality of interaction) • Locations (body parts!) for device placement
Mobility and handover issues
• Mobility allows the possibility for the mobile subscriber of being reachable anywhere and at anytime.
• Managing the mobile terminal mobility is one of the most essential parts of cellular system functionality.
• In a radio communication system Paging, Location Update and Handover Operations provide the User mobility.
Mobility and handover issues
• Handover mechanisms guarantees that whenever the mobile is moving from one base station area (Cell) to another, the radio signal is handed over to the target Base Station.
• Location update and Paging mechanisms guarantee that the mobile station can be reached even though there is no continues active radio link between the mobile and the corresponding Base Station.
– The Paging mechanism is always initiated by the Network.
– The Location Update procedure is always initiated by the Mobile Station.
Mobility and handover issues Handover Control
• Handover is one of the essential means to guarantee the User Mobility in a mobile communications network • The basic concept of handover control is that when the subscriber moves from the coverage area of one cell to another, a new connection with the new target cell has to be set-up and the connection with the old cell may be released.
Mobility and handover issues Handover Control
Frequency F1
Cell 2
Mobility and Handover Issues Reasons behind the Handover
• The basic reason behind the handover is that the air interface connection does not fulfil the desired criteria set for it any more resulting in unacceptable QoS and thus either the Mobile Station or the Network initiates Handover in order to improve the procedure.
• The decision of whether the handover should be performed or not is based on handover criteria.
• Handover may occur due to Signal Quality, User Mobility, Traffic Distribution etc.
Mobility and Handover Issues Reasons behind the Handover
• •
Signal Quality handover
– occurs when the quality or the strength of the radio signal falls below certain parameters specified in handover criteria. – deterioration of the signal is detected by constant signal measurements carried out by both the Mobile Terminal and the Base Station.
– The signal quality reason handover may be applied both for the uplink and downlink radio links.
Traffic handover
– occurs when the traffic capacity of a cell has reached its maximum or is approaching it – the UE near the edges of the cell with high load may be handed over to neighbouring cells with less traffic load.
– the system load can be distributed more uniformly.
Mobility and Handover Issues Reasons behind the Handover
• The number of handovers depends on the degree of mobility. • It is obvious that the faster the mobile node (MN) is moving, the more handovers it causes to the Network. • To avoid undesirable handovers the MN with high motion speed may be handed over from micro-cells to macro cells. • On the other hand, if the MN moving slowly or not at all, it can be handed over from macro-cells to micro-cells to improve the radio signal strength and avoid consuming its battery.
Mobility and Handover Issues Reasons behind the Handover
UE Moving from
Micro Cell 1
to
Macro Cell
Overlay Networks - The Global Goal
integration of heterogeneous fixed and mobile networks with varying transmission characteristics regional vertical handover metropolitan area in-house campus-based horizontal handover
Mobility and Handover Issues – Handover Process
• A basic handover process consists of three main phases:
Mobility and Handover Issues – Handover Process
• Handover mobility.
measurement
provision is a very important task for the system performance. This is because the signal strength of the radio channel may vary drastically due to fading and signal path loss, resulting from the cell environment (e.g. buildings, mountains) and user •
Decision phase
consists of assessment of the overall QoS of the connection and comparing it with the requested QoS attributes and estimates measured from neighboring cells. Depending on the outcome of this comparison, the handover procedure may or may not be trigger. • The Network checks whether the values indicated in the measurement reports meet the QoS specified for the end-user service. If not, then it allows
executing
the handover.
Mobility and Handover Issues – Handover Decision
Receive Level BTS old Receive Level BTS old MS BTS old HO_MARGIN MS BTS new
Mobility and Handover Issues – Handover Procedure
MS measurement BTS old BSC measurement old report result HO decision HO required MSC BSC new BTS HO request resource allocation ch. activation HO request ack ch. activation ack new HO command HO command HO command HO access Link establishment HO complete HO complete clear command clear complete clear command clear complete
Mobility and Handover Issues – GSM: 4 Types of Handover
1 2 3 4 MS MS MS MS BTS BTS BSC BTS BSC MSC BTS BSC MSC
Mobility and Handover Issues –
UMTS Radio Access Network Architecture RNS RNC: Radio Network Controller RNS: Radio Network Subsystem UE I ub Node B RNC I u • UTRAN comprises several RNSs • Node B can support FDD or TDD or both Node B Node B I ub I ur RNC CN • RNC is responsible for handover decisions requiring signaling to the UE • Cell offers FDD or TDD RNS
• • • •
Mobility and Handover Issues – Support of mobility in UTRAN
From and to other systems (e.g., UMTS to GSM) – This is a must as UMTS coverage will be poor in the beginning RNS controlling the connection is called SRNS (Serving RNS) RNS offering additional resources (e.g., for soft handover) is called Drift RNS (DRNS) End-to-end connections between UE and CN only via I u – Change of SRNS requires change of I u – Initiated by the SRNS – Controlled by the RNC and CN at the SRNS UE Node B Node B I ub SRNC I ur I ub DRNC I u CN
Mobility and Handover Issues –
Example Handover Types in UMTS/GSM UE 1 UE 2 UE 3 UE 4 Node B 1 Node B 2 Node B 3 RNC 1 I ub RNC 2 I ur I u 3G MSC 1 3G MSC 2 BTS A bis BSC A 2G MSC 3
Mobility and Handover Issues –
Support of mobility in UTRAN: Macrodiversity UE Node B Node B RNC CN • • • Multicasting of data via several physical channels – Enables soft handover – FDD mode only Uplink – Simultaneous reception of UE data at several Node Bs – Reconstruction of data at Node B, SRNC or DRNC Downlink – Simultaneous transmission of data via different cells – Different spreading codes in different cells
Mobility and Handover Issues –
Soft Handover Algorithm (Macrodiversity)
Mobility and Handover Issues –
Soft Handover Algorithm example • By the term
Soft Handover
we mean that the mobile node is maintaining connections with more than one base stations. • The
Active Set
includes the cells that form a soft handover connection to the mobile station. • The
Neighbor/Monitored Set
is the list of cells that the mobile station continuously measures, but their signal strength is not powerful enough to be added to the
Active Set.
SHO Algorithm
1.
• The algorithm samples the signal strength of the surrounding base stations every 1 sec • Uses 3dB as the threshold for soft handover and • Uses 6dB as the threshold for hard handover. • The size of the Active Set is 3 signals. 2.
3.
4.
5.
i.
ii.
Each UE is connected to its
Primary_BS
, and keeps an
Active_ Set
(2 “closest” cells) Each UE measures the SIR received from the surrounding cells.
If (AS1_SIR – Pr_BS_SIR) >3dB OR (AS2_SIR – Pr_BS_SIR) > 3dB UE enters Soft Handover UE keeps a simultaneous connection to the
Primary_BS
and one or both of the
Active_Set
cells i.
ii.
If (AS1_SIR
Primary_BS
– Pr_BS_SIR) > 6dB for three measurements in a row: AS1 becomes the If (AS2_SIR – Pr_BS_SIR) > 6dB for three measurements in a row: AS2 becomes the
Primary_BS
Neighboring cells replace the cells in the
Active_Set Active_Set
if their SIR exceeds the cells’ SIR by 6dB.
SHO in UMTS
Cell 6 Cell 10 Cell 7 Cell 11