qos in wireless systems preetam patil leena chandran-wadia

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


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 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 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


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


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


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


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


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


DiffServ Components Edge functions

Flow classification and packet marking Traffic conditioning

Core functions Enforcement of Per Hop Behaviours

Boundary functions Conformance enforcement


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.


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


MPLS Architecture


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 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


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)


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


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



23 1 3 1 2

2 4

21

input 1conn Id 2output 2conn Id 1

Host AHost C

Host B


ATM Logical Connections

Transmission Path

Virtual Path

Virtual Channels


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)


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


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


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


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!!


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 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


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


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


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 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


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


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


Traffic Flow TemplatesAssign different QoS-profiles to different applications -Signaling done using RSVP API


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 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


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


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


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


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…


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

qos in wireless systems preetam patil leena chandran-wadia

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