Cross-Layer Design -Kalpana Uppalapati -Kalpana Uppalapati

Agenda:  Introduction  Problem definition (3 issues)  Background  Description of the 3 issues  Solutions for the issues  Results of the methods used  Future Research  Conclusion

Introduction:  Wireless Networking: o Applications: wireless Internet access, ad hoc networks, sensor networks o Diverse requirements: high-bandwidth video and data, low-bandwidth voice and data o Goal: reliable communication-on-the-move in highly dynamic environments, QoS provisioning  A central problem: How to efficiently transmit heterogeneous traffic over wireless links?

 Design Approach: Layered design Layered design Each layer can be optimized independently Changes in one layer do not require changes in the other layers Cross-layer design Cross-layer design  Layered approach is not adequate in wireless!! Inflexible Inflexible No Adaptation No Adaptation Would the advantages of cross-layer design lead to new network architecture ? Why CLD?

Looking at the Status of Lower Layer May Improve Performance send receive Expose lower layer status may improve performance status get

Configuring the Lower Layer to Improve Performance send receive status get set Control lower layer behavior may improve performance

Application+Link:perf. gain in e2e SNR by joint channel coding and compression for video over wireless Examples of Cross-layer Design APPLICATION TRANSPORT ROUTING/CONTROL PLANE MAC LINK PHYSICAL MAC+Link: multiuser diversity gain of throughput if scheduling is based on link availability instead of FIFO (2x) Routing+Link: perf. improvement in percentage of packets not meeting a delay deadline through joint rate allocation and routing (10x) (MAC+PHY) perf. gain in channel utilization through channel reservation based on physical layer parameters MAC+Routing: perf. gain in channel capacity via joint MAC/routing protocol design. 3x difference for Application+Link+Physical: instead of using “target bER” power adaptation, consider modifying the target bER based on application requirements

Workdone:  At application-layer, a media server can track packet losses and adjust media source rate accordingly.  At transport-layer, several cross-layer approaches, such as EBSN, Snoop TCP, and freeze TCP, have been proposed as TCP alternatives to distinguish congestion loss from non-congestion loss and invoke different flow control mechanisms.  At link and network-layer, the persistence level of the MAC layer ARQ mechanism should adapt to each application’s latency and reliability requirements.

3 ISSUES: Issue 1: Cross-layer Design to achieve Quality of Service and resource efficiency in a multi-user context(PHY+MAC+App) Issue 2: TCP–over Wireless CDMA Links;SnoopTCP(Link+Transport) Issue 3: Cross-Layer Approach for Video over Time-Varying CDMA Channels

ISSUE 1 ISSUE: A joint cross-layer design for achieving QoS (Quality of Service) and Resource Efficiency Proposed Scheme: QoS-awareness scheduler and power adaptation scheme QoS-awareness scheduler and power adaptation scheme is used at both uplink and downlink MAC layer to coordinate the behavior of the lower layers for resource efficiency and QoS Central to the proposed cross-layer design is the concept of adaptation Central to the proposed cross-layer design is the concept of adaptation

The conventional protocol stack is inflexible and the layers doesn’t adapt to the changing conditions. The conventional protocol stack is inflexible and the layers doesn’t adapt to the changing conditions. Adaptation represents the ability of the network protocols to observe and respond to channel variation Adaptation represents the ability of the network protocols to observe and respond to channel variation Central to adaptation is the cross-layer design Central to adaptation is the cross-layer design The proposed QoS-awareness scheduler and power adaptation scheme deals with Application, MAC and physical layers The proposed QoS-awareness scheduler and power adaptation scheme deals with Application, MAC and physical layers

Purpose of a Scheduler:   MAC scheduler selects appropriate transmission power/format and priorities of the packets for each user depending on its present channel condition and the associated QoS requirements

  At MAC layer, different QoS-aware MAC states for both uplink and downlink transmission are used. The IP-base station dynamically schedules users in different MAC states based on resource availability and overall QoS   QoS (combined QoS criteria) = QoS class * QoS stream QoS class is determined by service pricing QoS stream is determined by characteristics of data traffic

QoS-aware & Power-adaptive MAC States a. High-QoS state: Users are actively sending and receiving traffic. In the high-QoS state, users have a dedicated control channel with both control and traffic channel power and timing controlled. Traffic segments are instantaneously assigned to any high-QoS user when there is data to send or receive b. Media-QoS state: While in media-QoS state, users have contention-free uplink request slots to indicate to an IP- base station that they have data to send. Users also have shared downlink message slots that are timing c. Low-QoS state: Users only maintain connectivity to an IP-base station. Low QoS mode users have shared downlink paging slots, where they know to wake up periodically and listen for incoming pages from the IP-base station

Transition among different QoS states

QoS-aware & Power-adaptive Transmission:   Different applications have different QoS network requirements. QoS vector constitutes Cost network of a stream.The network parameters combined with QoSclass andQoSstream compose a multi criteria decision for a given user and overall cost measurement is defined as: Cost = QoSclass ∗ QoSstream ∗ Costnetwork

QoS-aware and Power-adaptive Transmission:   The MAC scheduler selects appropriate transmission power/format and priorities of the packets for each user depending on its present channel condition and the associated QoS requirements.   MAC scheduler dynamically computes Cost real based on the current channel condition.

Simulation Results: Four different SNR environments, Location A(Dis=0.5Miles & SNR=20dB) B (Dis=1.4 Miles & SNR=20dB) C (Dis=2.0 Miles & SNR=15dB) D(Dis=1.6 Miles & SNR=10dB), were located within one cell. Three applications are chosen to run the test: Mobile1: A FTP download of 50 distinct 2MB files (B-C) Mobile 2: A webpage of 205kb was periodically refreshed (C-D) Mobile 3: A 128 kb/s media stream served from within the core network (D-A)

 Test #1: Equal QoS assignment, they simultaneously request resources. QoS “Gold” user (mobile#1) with FTP download Mobile #1 has throughput 2.5Mb/s initially Mobile#2 has 160Kb/s initially When Mobile #3 comes, MAC scheduler arbitrates resource assignments so that all users are supported. So, Mobile#1-dropped to 1.4Mb/s When Mobile #3 completes,Mac Scheduler reallocates bandwidth back to Mobile#1

 Test #2: When different QoS are assigned to mobiles, Mobile #1 was assigned low QoS Low QoS=> never receive more than 150Kb/s Figure shows throughput drop The available throughput to the user was reduced from 2.6Mb/s to 150Kb/s

Issue-2:TCP Over CDMA Wireless Links TCP has been optimized for wired networks. Any packet loss is considered as a congestion and hence the window size is reduced dramatically as a precaution, however wireless links are known to experience sporadic and usually temporary losses due to fading, shadowing, handoff etc. which cannot be considered as congestion. TCP has been optimized for wired networks. Any packet loss is considered as a congestion and hence the window size is reduced dramatically as a precaution, however wireless links are known to experience sporadic and usually temporary losses due to fading, shadowing, handoff etc. which cannot be considered as congestion. Problem: wireless corruption mistaken for congestion Solution: Snoop Protocol

Channel Errors Internet Router Loss  Congestion Loss ==> Congestion Burst losses lead to coarse-grained timeouts Result: Low throughput

Performance Degradation Time (s) Sequence number (bytes) TCP Reno (280 Kbps) Best possible TCP with no errors (1.30 Mbps) 2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN

Cross-layer protocol design & optimizations At the transport-layer, several cross-layer approaches such as freeze TCP, snoop TCP have been proposed as TCP alternatives to distinguish congestion-loss from non- congestion loss. At the transport-layer, several cross-layer approaches such as freeze TCP, snoop TCP have been proposed as TCP alternatives to distinguish congestion-loss from non- congestion loss. In snoop TCP, TCP layer knowledge is used by link layer schemes In snoop TCP, TCP layer knowledge is used by link layer schemes Transport Network Link Physical Link-aware transport (Explicit Loss Notification) Transport-aware link (Snoop agent at BS)

Solution: Snoop Protocol Shield TCP sender from wireless vagaries Shield TCP sender from wireless vagaries Eliminate adverse interactions between protocol layers Eliminate adverse interactions between protocol layers Congestion control only when congestion occurs Congestion control only when congestion occurs Fixed to mobile: transport-aware link protocol Mobile to fixed: link-aware transport protocol

Snoop Protocol: FH to MH FH Sender Mobile Host Base Station Snoop agent: active interposition agent Snoops on TCP segments and ACKs Snoops on TCP segments and ACKs Detects losses by duplicate ACKs and timers Detects losses by duplicate ACKs and timers Suppresses duplicate ACKs from FH sender Suppresses duplicate ACKs from FH sender Cross-layer protocol design: snoop agent state is soft Snoop agent

Snoop Protocol: FH to MH Mobile Host 1 Base Station Snoop Agent FH Sender

Snoop Protocol: FH to MH Mobile Host 1234 Base Station 5 FH Sender

Snoop Protocol: FH to MH Mobile Host Base Station FH Sender

Snoop Protocol: FH to MH Mobile Host Base Station Sender

Snoop Protocol: FH to MH Mobile Host Base Station ack 0 Sender Duplicate ACK

Snoop Protocol: FH to MH Mobile Host Base Station Sender Retransmit from cache at higher priority ack 0 65

Snoop Protocol: FH to MH Mobile Host Base Station 1 1 Suppress Duplicate Acks Sender 5 ack 0 ack 4

Snoop Protocol: FH to MH Base Station Sender ack 4 ack 5 Clean cache on new ACK

Snoop Protocol: FH to MH Mobile Host Base Station Sender ack 4 6 ack 6 ack 5

Snoop Protocol: FH to MH Mobile Host Base Station Active soft state agent at base station Transport-aware reliable link protocol Preserves end-to-end semantics 6 Sender ack 5 ack

Best possible TCP (1.30 Mbps) Snoop Performance Improvement Time (s) Sequence number (bytes) Snoop (1.11 Mbps) TCP Reno (280 Kbps) 2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN

ISSUE 3:Cross-Layer Approach for Video over Time-Varying CDMA Channels Problem: Applying multi-user diversity to real-time traffic is very challenging due to the delay requirement of such traffic Solution: A Cross-Layer approach,DWGPS(Dynamic Weight Generalized Processor Sharing) is proposed. The proposed cross-layer approach can benefit from information in both the application and physical layers

Existing Technology:GPS GPS is an ideal fair scheduling discipline originally proposed for wire line networks GPS is an ideal fair scheduling discipline originally proposed for wire line networks Principle of GPS: Principle of GPS: A fixed-weight is assigned to each session and bandwidth is allocated to all sessions according to the weights and traffic loads. A fixed-weight is assigned to each session and bandwidth is allocated to all sessions according to the weights and traffic loads. GPS can provide each session with a minimum service rate. GPS can provide each session with a minimum service rate.

The minimum service rate and tight delay bound guaranteed in GPS may seem attractive to real-time video transmission The minimum service rate and tight delay bound guaranteed in GPS may seem attractive to real-time video transmission A large weight should be assigned to a video session in order to guarantee the peak rate A large weight should be assigned to a video session in order to guarantee the peak rate This means a video session will get a large portion of the This means a video session will get a large portion of the total capacity whenever it has traffic to transmit, thus leading to service degradation of other sessions total capacity whenever it has traffic to transmit, thus leading to service degradation of other sessions In order to apply GPS discipline to video transmission and extend it to wireless networks, dynamic weights in GPS are used(DWGPS ) In order to apply GPS discipline to video transmission and extend it to wireless networks, dynamic weights in GPS are used(DWGPS )

The link layer resource allocation benefits from the video compression information such as the batch class and batch arrival size from the application layer; and in return, the link layer provides the video compression with the desired service differentiation. This DWGPS scheduling procedure in the link layer can achieve low computation complexity and small signaling overhead. LINK Layer Application Layer Provides video compression for the desired service Video compression information

Conclusion:  Though Cross-layer design has many advantages the Achilles heel of cross-layer design is its potential to destroy modularity, and make the overall system fragile. Tradeoff between efficiency and modularity  Future work on CLD implementation can be aiming at replacing the traditional layered structures completely. However this might not even be possible because of the demand for compatibility with every other network using the IP.

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