A study of Cross layer work of University of Trento folk

A study of Cross layer work of University of Trento folk

A ResiliNet Group PresentationSarvesh Kumar Varatharajan

A study of Cross layer work of University of Trento folk Cross-Layer Performance Optimization in wireless

Local Area Network (Kliazovich's PhD Thesis) Cross Layer ARQ ( Presented Today) Cross Layer Congestion Control (Present next week)

The Cross-Layer Paradigm In Next Generation Internet: Open Issues & Future Perspectives( Granelli) (Walk through the foils)

Formal Methods in Cross-Layer Modeling and Optimization of Wireless Networks: State of the Art and Future Directions (Skip!!!Difficult for me to understand :-( )

Cross-layer Performance Optimization in wireless Local Area Network

Cross-Layer Performance Optimization in wireless Local Area Network (Kliazovich's PhD Thesis) Cross Layer ARQ


TCP/IP shows poor performance when extended to wireless network environment

Limitations in terms of bandwidth,latency,channel loss and mobility

Available Enhancement Schemes Link Layer Solutions Transport Layer Solutions Cross-Layer Solutions


Link Layer Solutions TCP unaware link layer solution

TCP unaware protocols optimize performance by hiding the differences between wired and wireless mediums

Can cause adverse effects eg.) ARQ on TCP RTO TCP aware link layer solution

Knowledge of transport layer protocol provides certain advantages

eg.) Snoop Protocol Snoop Agents ( at the BS) caches the TCP packets that are sent to

mobile nodes. Retransmissions occur locally Disadvantages

Increases complexity at BS No consideration of packet loss delay during hand-off


Transport Layer Solutions Connection Splitting Solutions

I-TCP (Indirect TCP) Split the connection into two One between FH and BS and the other between BS and MH Conceal the wireless link from the FH Disadvantages:

End-to-end Schematics lost Additional overhead during hand-off Can't operate on encrypted packets

TCP Modifications eg.) SACK Disadvantage:

Modification of the ACK procedure both at sender and reciever


Cross-Layer Design ILC-TCP (Interlayer Collaboration Protocol)

State Manager (SM) in parallel to protocol stack Each layer reports its state to the SM SM can signal TCP to stop sending packets if conditions not

good Disadvantage: Additional Layer added to the sender


Analysis of existing schemes under 3 different scenarios Single-hop wireless connections Multi-hop wireless scenario Wireless-cum-wired scenario

Conclusion from the related work No single best solution for all deployment scenario


TCP with link level ARQ Link layer is not the only layer which

acknowledges packet delivery(TCP ACK's) “A single TCP data packet is acknowledged three

times”( One time at the transport level and two times at the link level)

This along with the fact that the physical layer preamble and header are transmitted at the lowest bit rate penalizes performance at higher bit rate


Optimization of the acknowledgement scheme through interaction between transport and link layer will bring down the medium busy time and will improve the performance

A cross layer Link Layer ARQ Exploitation TCP (LLE-TCP) scheme proposed

The idea is to generate the TCP ACK at the sender instead of sending it through the channel


Cross Layer ARQ Agent Operates between TCP and MAC Interfaces both LLE-TCP is a software module Goes into the firmware of the wireless card driver


Packet delivery diagram


Agent interaction with the Link Layer Success Failure

Agent interaction with the transport layer Sender side

ARQ Agent sniffs the packet generated by the higher layer Access the TCP headers On successful reception of the link layer ACK, it prepares

the TCP ACK Upon SUCCESS TCP ACK is released to the Transport

layer On FAILURE three duplicate ACK's are generated

triggering the fast retransmit


Agent interaction with the transport layer Receiver side

Silently drops all the standalone non-duplicate TCP ACK packets

Advantages Does not interfere with the flow control mechanisms Reduces the Round Trip Time(RTT)


TCP Connection Phases Connection Establishment (Three Way handshake)

ACK suppression is done only for the third handshake as sequence number synchronization,negotiation of the size of the contention window takes place

Data Exchange Full ACK suppression

Connection Termination Full ACK suppression

Bidirectional data exchange If the incoming packet acknowledges a segment

number lower or equal to the already acknowledged one,the ACK flag is cleared


Infrastructure Network Scenario TCP performs poorly in such environments due to

completely different characteristics between fixed and wireless parts


TCP ACK's generated at the BS is delivered to the fixed host

TCP ACK's avoided in the wireless part(bottleneck)

LLE-TCP at BS accesses the rwnd of the ACK header and replaces it with empty buffer space left at the BS. Improves Congestion control

Drawback: Complexity at the BS Mobile station switches off its LLE-TCP if the BS

does not support it


Multi hop Networks


Congestion Control at the sender side LLE-TCP congestion control module (LLE-TCP_CC) LLE-TCP_CC buffer stores the packet till it gets the

ACK from LHR rwnd set to 1 packet if LLE-TCP buffer not full rwnd set to 0 packets if LLE-TCP buffer is full Send a duplicate ACK for the last transmitted data

packet with positive value for the advertised window


Experimental Results Simulation in ns-2 Implementation of single-hop scenario in 802.11b

testbed( Fedora Core Linux with 802.11b Orinoco Silver Cards)

TCP Reno is chosen for comparison


Single-hop Network (Simulation Results)


Single-hop Network ( Implementation Results) These results are 3% less than simulation results??


Multi hop Scenario (Simulation Results) TCP Segment size=1000 bytes 10 % to 20 % improvement for small # of hops


Multi hop Scenario (Simulation Results) Three hops, static environment


Multi hop Scenario (Simulation Results) 30 nodes placed randomly in 100*100 m With node mobility ,AODV,RTS/CTS ON 1000s simulation Fewer route errors Fewer route discovery messages Reduced RTT


Infrastructure Network Scenario (Simulation Results)

IEEE 802.11b Wired link 100Mbps,2ms Results close to single hop This is bec ACK is sent on wired link Congestion control at BS

A study of Cross layer work of University of Trento folk Cross-Layer Performance Optimization in wireless

Local Area Network (Kliazovich's PhD Thesis) Cross Layer ARQ ( Presented Today) Cross Layer Congestion Control (Present next week)

The Cross-Layer Paradigm In Next Generation Internet: Open Issues & Future Perspectives( Granelli) (Walk through the foils)

Formal Methods in Cross-Layer Modeling and Optimization of Wireless Networks: State of the Art and Future Directions (Skip!!!Difficult for me to understand :-( )

A study of Cross layer work of University of Trento folk

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