Transcript Data Communications and Computer Networks

QoS in wireless systems
Preetam Patil
Leena Chandran-Wadia
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
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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.
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QoS in Wired Networks
is QoS? - “Better than best effort”
 Associated metrics include
 What
 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-toend QoS” and associated pricing
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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
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IP QoS Approaches
 Two
broad families:
 Per-flow
service
 Integrated
Services and RSVP
 Since per-flow information needs to be
maintained, too complex and not scalable
 Aggregated
service
 Differentiated
services
 Only class-based information required, hence
more scalable, and easier to implement
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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… - continued

Rapid 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
 Classifiers

Select packets and assigns DS code
point
 Traffic

Enforces rate limitations
 Per

conditioners
Hop Behaviours
Differentiated packet treatments
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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.
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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
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MPLS Architecture
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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)
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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

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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 them
 Packet
classifiers
 scheduling and Buffer management
 Control
path: determines if and how
guarantees can be provided
 signaling
 admission
control
 QoS routing
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ATM - Connection Oriented Cell
Switching
 Call
setup: synchronization before data
transfer
input
conn Id
output
conn Id
3
1
2
1
3
2
2
2
input
conn Id
output
conn Id
2
3
1
1
4
1
1
2
3
12
1
Switch
S1
input 1
conn Id 2
output 2
conn Id 1
3
Switch
S2
2
Host A
1 Switch
S3
1
2
Switch
S4
Host C
4
2
QoS in Wireless Systems
Host B
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ATM Logical Connections
Transmission Path
Virtual Channels
Virtual Path
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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)
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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
VPI
VPI
VCI
VCI
VCI
PT
VPI
HEC
Virtual Channel Identifier
PT
CLP
48 bytes
Virtual Path Identifier
HEC
Payload Type
Cell Loss Priority
Header Error Control
Payload
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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!!
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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
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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
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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
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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
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GPRS (2) QoS Classes
 Four
traffic classes
 Conversational,streaming,
interactive,
background
 (1) Conversational, streaming: for carrying
real-time flows
 difference
is the extent of delay sensitivity
 Forward error correction
 (2)
interactive, background: for traditional
internet traffic
 interactive
class has higher response
 better error recovery using retransmissions
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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

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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
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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
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Traffic Flow Templates
Assign different QoS-profiles to different applications Signaling done using RSVP API
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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
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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 realtime traffic
LANs – Black burst contention etc
 Manets – MACA/PR
 Wireless
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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

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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

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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

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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…
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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
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