qos in wireless systems preetam patil leena chandran-wadia
TRANSCRIPT
QoS in wireless systems
Preetam Patil
Leena Chandran-Wadia
QoS in Wireless Systems 2
Contents QoS in wired systems
technologies - ATM, IP/MPLS mechanisms - scheduling, routing, admission
control…. architecture – DiffServ
QoS in wireless Wireless ATM GPRS MANETS
Perspective
QoS in Wireless Systems 3
Case for QoS “QoS is a means to convergence but a
goal in itself from network point of view.” Over provisioning of resources is not
enough… Different applications have different QoS
requirements. Particularly important from the point of how
TCP reacts to packet losses and delays.
QoS in Wireless Systems 4
QoS in Wired Networks What is QoS? - “Better than best effort” Associated metrics include
Guarantees on bandwidth Bounds on delay (queuing, multiplexing) Bounds on delay variation (jitter) Bounds on loss probability Minimize cost
Ideally we would like to have “end-to-end QoS” and associated pricing
QoS in Wireless Systems 5
QoS Mechanisms support for real-time flows in the n/w
marking such flows - precedence (ToS) admission control assign to different queues priority scheduling buffer management constrained routing mechanisms for signaling - within n/w as
well as between users and n/w
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Performance measures QoS services (depending on the level)
generally involve putting all or at least a few of these mechanisms into place Fairness - access to excess capacity Isolation - protection from excess traffic
from other users Efficiency - number of flows
accommodated per service level complexity - implementation, control
overhead
QoS in Wireless Systems 7
IP QoS Approaches Two broad families:
Per-flow service Integrated Services and RSVPSince per-flow information needs to be
maintained, too complex and not scalable Aggregated service
Differentiated servicesOnly class-based information required, hence
more scalable, and easier to implement
QoS in Wireless Systems 8
Differentiated Services(DiffServ) Goals and motivations
Data path scalability Coarse granularity service classes (no
per-flow state) Minimum impact on packet forwarding
performance Realizable through simple
mechanisms
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DiffServ… - continuedRapid deployment
Standardize service codepoints in IP header and associated expected local behaviour (Per Hop Behaviour - PHB)
Wide range of possible implementations
Avoid chicken and egg problem of signalling deployment and application/user support
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How it works - IP TOS field in IPV4 or Traffic Class field in
IPV6 used to mark packets Pre-configured set of service classes
(behaviours) Expedited Forwarding (local behaviour only)
Virtual leased line type of service Assured Forwarding (local behaviour only)
Several service classes with drop precedence within each class
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DiffServ Components Edge functions
Flow classification and packet marking Traffic conditioning
Core functions Enforcement of Per Hop Behaviours
Boundary functions Conformance enforcement
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DiffServ Components… continued Components
ClassifiersSelect packets and assigns DS code
point Traffic conditioners
Enforces rate limitations Per Hop Behaviours
Differentiated packet treatments
QoS in Wireless Systems 13QoS in Wireless Systems 13
Multi-Protocol Label Switching (MPLS)
An attempt to exploit benefits of ATM label-switching and flexibility of IP routing.
Has roots in IP tag-switching. MPLS works between L2 and L3. Designed to work over different link-layer
technologies- Ethernet, Frame-relay, etc. Different network protocols supported.
QoS in Wireless Systems 14
MPLS Features Packets are forwarded based on a 20-bit
fixed-length label in packet-header instead of destination IP address
A path (LSP - Label Switched path) is first established using a signalling protocol Label Distribution Protocol extensions to RSVP
QoS in Wireless Systems 15
MPLS Architecture
QoS in Wireless Systems 16
MPLS Architecture- contd.. LSR- routers supporting MPLS are called
Label Switching Routers Ingress LSR - LSR where packets in a
flow enter the MPLS domain Egress LSR - LSR where packets in a
flow leave the MPLS domain FEC - packets to be forwarded in same
manner are assigned to same Forwarding Equivalence Class (FEC)
QoS in Wireless Systems 17
QoS and Traffic Engineering in MPLS
MPLS and DiffServ similar in the way packets are looked up and classified at the Ingress
LSPs can be set up for Different Service classes, or bits in MPLS header can be used to mark flows for QoS
LSPs can be explicitly set up based on QoS and Traffic-Engg objectives (CR-LSPs)
Many extensions to MPLS for QoS and TE proposed
QoS in Wireless Systems 18
ATM Reference Model Complete protocol stack, alternative to
TCP/IP - fully QoS capable!! 4 layer (upper, adaptation, ATM and
physical), 3 dimensional model Different from both OSI and TCP/IP User Plane (data transport, flow, error control)
and Control Plane (connection management) Plane and Layer Management (RM and
interlayer coordination)
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Service Differentiation Two major components
Data path: identifies packets eligible for services and enforces themPacket classifiersscheduling and Buffer management
Control path: determines if and how guarantees can be providedsignalingadmission controlQoS routing
QoS in Wireless Systems 20
ATM - Connection Oriented Cell Switching
Call setup: synchronization before data transfer
input 3 3conn Id 1 2output 2 2conn Id 1 2
SwitchS1
SwitchS2
SwitchS4
SwitchS3
input 1 1conn Id 1 2output 4 3conn Id 1 1
input 1 1conn Id 1 2output 4 3conn Id 1 2
23 1 3 1 2
2 4
21
input 1conn Id 2output 2conn Id 1
Host AHost C
Host B
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ATM Logical Connections
Transmission Path
Virtual Path
Virtual Channels
QoS in Wireless Systems 22
ATM Connection Terminology Virtual Channel Connection (VCC),
also called VC identified by one VPI/VCI at an interface
Virtual Channel Link Virtual Channel Identifier
no global identifier Two types
Switched - SVCs (need connection setup) Permanent - PVCs (service provider)
QoS in Wireless Systems 23
More Connection Terminology
Virtual Path Connection, also called VP
identified by one VPI at one interface Virtual Path Link Virtual Path Identifier
no global identifier Virtual paths make it possible for CPN
to have closed user groups, with a network of VPs
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ATM Cells - NNI
VPIVPI VCI
VCIVCI PT
HEC
48 bytes
VPI
PT
CLP
HEC
Virtual Path Identifier
Virtual Channel Identifier
Payload Type
Cell Loss Priority
Payload
Header Error Control
QoS in Wireless Systems 25
Service Categories CBR - Constant Bit Rate (T1/E1 circuit) VBR - Variable Bit Rate
rt VBR - real-time Video conferencing nrt VBR - multimedia E-mail
ABR - Available Bit Rate (Browsing the web)
UBR - Unspecified Bit rate (Background file transfer). Useful for sending IP packets
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ATM Perspective Standardization took too much time no native ATM applications were written meanwhile, runaway success of the
Web and of MBone meant that killer applications were all running IP
this meant LANs would remain Ethernet and WANs would run IP over ATM But... ATM Hardware is selling as much
as IP switches and routers today!!
QoS in Wireless Systems 27
Wireless ATM User (data) plane largely unchanged Control plane
MATM adapter (handsets): UNI + Mobility WATM & AP: support control of Radio
Access (signal strength etc.) Switches: Signaling to support mobility
QoS Wireless QoS: reservation adds to delay Handover QoS: blocking, re-negotiation
QoS in Wireless Systems 28
QoS in Wireless Networks What’s different in Wireless ?
A premium on efficiency (due to limitations in spectrum resource)
Low reliability in the worst case Traffic limited by interference
Similar to congestion, but more easily controllable
“Cost” of one stream related not only to rate parameters, but also to reliability(energy per bit) and acceptable delay
Best error- control coding techniques are at the physical and media- access layers
QoS in Wireless Systems 29
Wireless Systems – GPRS Varying Conditions of Radio interface QoS profile consists of parameters like
precedence: delay: includes radio access delay (uplink)
or radio scheduling delay (downlink), radio transit delay, GPRS-network transit delay
reliability: error rates much higher throughput: specified by maximum bit rate
and mean bit rate
QoS in Wireless Systems 30
GPRS (1) Each GPRS subscription will be
associated with one QoS profile (HLR) SGSN will negotiate QoS for the flow
Based on subscribed default in HLR The requested profile from the MN Current availability of GPRS resources
SGSN must distribute resources fairly among flows, it may renegotiate QoS if necessary
QoS in Wireless Systems 31
QoS in Wireless Systems 32
GPRS (2) QoS Classes Four traffic classes
Conversational,streaming, interactive, background
(1) Conversational, streaming: for carrying real-time flowsdifference is the extent of delay sensitivityForward error correction
(2) interactive, background: for traditional internet traffic interactive class has higher responsebetter error recovery using retransmissions
QoS in Wireless Systems 33
QoS Profile Parameters Eight other parameters are used for defining
the specific QoS-profile MAX bit rate, Guaranteed bit rate Delivery order, Reliability PDU size information, Transfer delay Traffic handling priority, Allocation priority
Values will depend on main traffic class More complex, but will reflect different
applications better Applications must signal QoS requirements
QoS in Wireless Systems 34
Conversational Class Assumed to be relatively non-bursty Real time, low delay - Voice Characterized by
maximum bit rate guaranteed bit rate guaranteed transfer delay
rest optional, but usually specified lower classes specify fewer parameters
QoS in Wireless Systems 35
Re-negotiation of QoS MN, BSS & SGSN have the capability
to trigger a modification of the QoS profile associated with an ongoing data flow due to congestion or shortage of radio
resources in order to map QoS parameters of the
packet data network into the GPRS network
QoS in Wireless Systems 36
Traffic Flow TemplatesAssign different QoS-profiles to different applications -Signaling done using RSVP API
QoS in Wireless Systems 37
QoS in MANets Availability of link state information and
its management is difficult QoS of wireless link is apt to change in
dynamic environment mobility of hosts resource limitations (time varying)
DiffServ a possible solution what are the boundary routers? concept of SLA does not exist
QoS in Wireless Systems 38
QoS in MAC protocols MAC protocol design goals
solve medium contention deal with hidden/exposed terminal problem improve throughput
QoS MACs must provide resource reservation and QoS guarantees to real-time traffic Wireless LANs – Black burst contention etc Manets – MACA/PR
QoS in Wireless Systems 39
MACA/PR Multiple Access Collision Avoidance with
Piggyback Reservations Rapid and reliable transmission to non-real time
datagrams Guaranteed b/w support to real-time traffic
NRT traffic waits for “free” window in reservation table plus additional random time equivalent to single hop round-trip delay
proceed with RTS-CTS-PKT-ACK dialogue Reservation table records all reserved send
and receive windows of all stations in range
QoS in Wireless Systems 40
MACA/PR - RT To send first data packet of a RT connection,
sender initiates RTS-CTS and then proceeds with PKT-ACK
For subsequent data packets only PKT-ACK is needed
If sender fails to receive several ACKs then restarts RTS-CTS dialogue
MACA/PR does not retransmit after collisions To reserve b/w for real-time traffic, RT
scheduling information is carried in headers of PKTS and ACKs
QoS in Wireless Systems 41
MACA/PR -RT Sender piggybacks reservation information
for its next data packet transmission on the current data PKT
Receiver inserts reservation in its Reservation table and confirms it with the ACK to the sender
Neighbors of receiver R will defer their transmission on receiving the ACK
ACK also tells them next scheduled receiving time of R, so they can avoid transmission
QoS in Wireless Systems 42
MACA/PR -RT Real-time packets are protected from
hidden hosts by the propagation of reservation tables among neighbors, not by RTS-CTS dialogues
Thus, through piggybacked reservation of information and the maintenance of reservation tables, bandwidth is reserved and guaranteed for real-time traffic…
QoS in Wireless Systems 43
Perspective Essentially, concept of QoS must be
accepted and supported by every element in the value chain Infrastructure and terminal developers Mobile network operators Application developers End users