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Network Support for Multimedia Applications
in Mobile Networks
Major Area ExamKimaya Sanzgiri
MOMENT Lab
Computer Science Dept., UCSB
Motivation Growing popularity of multimedia applications
Streaming music/videos Internet telephony Media-rich messaging
Growing popularity of mobile wireless networks Infrastructured Multi-hop (ad hoc)
Increasing support for multimedia content on wireless devices
Characteristics of Real-time Multimedia Applications
Sensitive to end-to-end delay and jitter Many applications can tolerate some
packet loss Different needs from other types of
applications, such as bulk data transfers
Network Support Due to diverse needs, packets belonging to
different types of applications need to be handled differently by the network
Network needs to offer different qualities of service
Availability of sufficient resources must be ensured in order to meet application requirements
Characteristics of Wireless Networks Shared nature of medium
Resource availability influenced by activities of neighboring nodes
Mobility and dynamic topology Resource constrained devices Higher error rates No defined network boundary Lack of central authority
Supporting Multimedia Applications
Solutions have been proposed for both wired and wireless networks that operate at different levels of the network stack
In this talk, we focus on network layer solutions
At the end, we will mention some proposed solutions at other layers
QoS support at the Network layer QoS-aware routing Admission control Resource reservation Packet classification and QoS-sensitive
packet forwarding Monitoring/Policing
Wired Network Solutions Often not directly applicable to wireless
networks due to the inherent difference in characteristics
Provide insight into the problem Are a good starting point to address the
problem in the wireless environment
Prominent Network-layer QoS Solutions for Wired Networks
IP Precedence and TOS Integrated Services (IntServ) Differentiated Services (DiffServ)
IP Precedence and Type of Service (TOS) Field in the IPv4 header Indicates that the need for QoS support was
recognized since the early days of the Internet The TOS field can be used to specify a
precedence value (0-7) or a TOS (delay/throughput/reliability/cost) for each IP packet
Interpretation of this field was left ambiguous Field remained largely unused
Integrated Services Attempt to modify Internet service model to
support diverse application requirements Any data flow that desires better than best-effort
delivery requests and reserves resources at routers along the path RSVP is the recommended reservation protocol
If insufficient resources are available, the flow is denied admission into the network
Integrated Services (cont.) Each router
Maintains reservation state for each flow Classifies every packet and decides forwarding
behavior Monitors the flow to ensure that it does not consume
more than the reserved resources Advantages
Enables fine-grained QoS and resource guarantees Disadvantages
Not scalable, harder to administer
Differentiated Services Moves admission control and flow monitoring to
the edge of the network Edge nodes classify and mark packets to receive a
particular type of service Diff Serv Code Point (DSCP) Finite set of DSCPs defined
Interior nodes determine the type of service for forwarded packets based on their DSCP values
Differentiated Services (cont.) Advantages
More scalable No per-flow state Easier to administer
Disadvantages Cannot provide the same per-flow guarantees
as IntServ
QoS support at the Network layer QoS-aware routing Admission control Resource reservation Packet classification and QoS-sensitive
packet forwarding Monitoring/Policing
Applicability of Wired Approaches to Wireless Networks Some ideas may be applicable directly,
while some need modification and others may be inapplicable
Additional challenges in wireless networks that are not encountered in wired networks have to be addressed
Applicability of Wired Approaches to Wireless Networks IntServ
Effectiveness of reservations in highly dynamic environment questionable
Per-flow state and monitoring may be resource exhaustive (depends on traffic)
DiffServ DSCP idea may be useful With dynamic topology and no defined network
boundary, some admission control/monitoring may be necessary at each node
Admission Control in Wireless Networks Determining available bandwidth at a
wireless node is a complex task due to the nature of the wireless medium Wireless medium is shared among multiple
nodes Bandwidth is affected by transmissions of
nodes that are not within transmission range Each node potentially has a different view of
the medium
Bandwidth is affected by nodes that are not within transmission range
A
Interference/Carrier-Sensing
Range
TransmissionRange
B
CC’s transmissions affect bandwidth at A
Different views of the wireless medium at different nodes
Y
XZ
P
R
S
Q
Carrier-Sensing Range of X
Carrier-Sensing Range of Q
Making an Admission Control Decision
Y
XZ
P
R
S
Q
T
400 kbps
400 kbps
Total bandwidth = 1 Mbps
If X admits a 400 kbps flow to Z, the medium will getcongested at Q
?
Contention-Aware Admission Control Protocol (CACP) [Yang 2003] Each node senses the medium to determine the
fraction of time that the medium is idle Local bandwidth availability is determined from
the idle fraction Further, each node queries all nodes within its
carrier sensing range for their local bandwidths. The minimum of these is the neighborhood available bandwidth
Admission control decisions are based on the neighborhood available bandwidth
CACP Admission Control
Y
XZ
P
R
S
Q
T
400 kbps
400 kbps
Total bandwidth = 1 Mbps
Neighborhood available bandwidth at X is 200 kbps, so X will not admit the 400 kbps flow
?
Issues with CACP approach How does a node communicate with its carrier-
sensing neighbors? High power transmissions
May increase collisions Local multi-hop flood
May reach nodes that are outside CS range May not reach some nodes in CS range
Considering neighborhood bandwidth as defined by CACP may sometimes be overly conservative and prevent spatial reuse
Y
XZ
P
R
S
Q
T
700 kbps
?
Preventing Spatial ReuseTotal bandwidth = 1 Mbps
Neighborhood available bandwidth at X is 700 kbps, so X will not admit the 400 kbps flow, although it could be admitted
Bandwidth Availability Determination Other approaches have been proposed
Different trade-offs between accuracy and efficiency Perceptive Admission Control (PAC) [Chakeres
2004] reduces overhead and improves spatial reuse compared to CACP
However, even PAC could be overly conservative in some situations
Open Question: How can bandwidth availability be determined more accurately with low overhead?
Multi-hop Admission ControlIn a multi-hop route, there could be interference between multiple hops
BC
X
D
E
UP
Q
RS
Y
T
A
FCS Range of X CS Range of Y
Multi-hop Admission Control Due to the interference between multiple
hops, the bandwidth required at a node is some multiple of that requested by the application
The exact value depends on the Contention Count at the node Contention Count at a node is the number of
other nodes on the route that are contending with this node for medium access
Multi-hop Admission ControlContention Count at a node is determined by the number of nodes on the route that are within the nodes carrier-sensing range
BC
X
D
E
UP
Q
RS
Y
T
A
FContention Count at X = 5 Contention Count at Y = 7
V
Determining Contention Count Node must know the identities of its carrier-sensing
neighbors CACP does either high power periodic broadcasts or
multi-hop broadcasts - Collision, overhead and inaccuracy problems
Node must know the identities of all other nodes on any route Requires source routing or path accumulation in routing
packets – overhead Open Questions: Is there a better way? Can
approximations be made that could reduce overhead?
QoS Routing Several QoS routing protocol have been
proposed. Each exhibits one or more of the following features Extend a corresponding best-effort routing protocol
(AODV/DSR/DSDV) Find one or more QoS-satisfactory paths between
source and destination Admission control may be integrated with route
discovery Resource reservations may be established along the
route
QoS-sensitive extensions of AODV
S D
RREQ
RREP
• QoS information is added to the RREQ packet
• Intermediate nodes forward the RREQ only if they have sufficient resources to meet
the QoS requirement• Resource information is updated in the RREQ by intermediate nodes
• Destination sends resource information back to source in the RREP message
Other Challenges for QoS Routing and Admission Control
X
R S
P Q
Simultaneous Intersecting Requests
Simultaneous Parallel Requests
QoS Monitoring Resource availability may change over
time due to mobility and changing topology
There is a need for monitoring and renegotiation of QoS parameters
Monitoring can be performed in various ways
QoS Monitoring Approaches INSIGNIA [Lee 2000] uses in-band
signalling QoS parameters added to every data packet
using IP options in the IP header Intermediate nodes appropriately set the
values for the parameters based on their current resource availability
Destination gathers the information from the data packets and gives feedback to the source
QoS Monitoring Approaches SWAN [Ahn 2002] does monitoring at
intermediate nodes and uses Explicit Congestion Notification (ECN) to regulate flow
AQOR [Xue 2003] does no monitoring at intermediate nodes. Destination does the monitoring based on the received data characteristics and gives feedback to the source
Open Questions: Can these approaches be used in combination for an effective solution? Is there a better new approach?
Hybrid networks A hybrid network is formed when the mobile
network extends the wired Internet To run multimedia applications in hybrid
networks, QoS needs to be ensured in both the wired and wireless parts of the network
QoS mechanisms in wired and wireless networks can be very different
Open Question: How can this be addressed? Network layer QoS gateways? Needs exploration
Solutions at other layers MAC layer
Priority-based medium access Transport layer
QoS monitoring, rate control Application layer
Adaptive streaming, layering techniques Open Question: How do mechanisms at
different layers interact?
Other Open Questions Most of the proposed QoS solutions have
been validated through simulations or analytical models. Do the observations and results hold true in real deployments?
Can special characteristics of wireless networks, such as mobility, be leveraged in any way to improve QoS?
Conclusions Multimedia applications require QoS support
from the network This is particularly difficult in wireless networks
owing to their special characteristics Several solutions have been proposed at the
network layer for admission control, QoS routing and monitoring in wireless networks
Many open questions still remain and there is significant scope for further research