Studying ApplicationLayer Performance Implications

1
Studying Application-Layer Performance
Implications

• Of various manipulations oflayer 4/3
  (TCP/IP)and layer 2 (802.11, 802.3, and
  T-1)tuning parameters

2
Scenario 1 Comparison of wired connections
(Ethernet and Serial Wan) with 802.11 wireless
bridge (no client station associations)
AP1
AP2
802.11b wireless bridge
Crossover Ethernet cable
Router 1
Router 2
PC1
PC2
Hub
Hub
Serial WAN cable
Router 4
Router 3
Hub
3
Scenario 2 Comparison of wired connections
(Ethernet and Serial Wan) with associations to
one 802.11 wireless access point (no other
associated client stations)
AP1
802.11b wireless association
802.11b wireless association
Crossover Ethernet cable
Router 1
Router 2
PC1
PC2
Hub
Hub
Serial WAN cable
Router 4
Router 3
Hub
4
Scenario 3 Comparison of wired connections
(Ethernet and Serial Wan) with association to one
802.11 wireless access point (additional
associated client stations)
AP1
PC3
PC4
802.11b wireless association
802.11b wireless association
802.11b wireless association
802.11b wireless association
Crossover Ethernet cable
Router 1
Router 2
PC1
PC2
Hub
Hub
Serial WAN cable
Router 4
Router 3
Hub
5
802.11 Performance-Related Parameters

• Not considered/studied?
• Authentication overhead
• Association overhead
• Uncontrollable, fixed?
• Inherent half-duplex mode
• Header overhead
• Interframe spacing
• AP relay overhead
• Uncontrollable, variable?
• Acknowledgement overhead
• Retransmissions
• RF signal strength
• RF noise
• Channel nominal data rate

• Tunable
• Beacon interval
• Power management settings
• RTS threshold
• Fragmentation threshold
• Long retry limit
• Short retry limit
• Listen interval
• DTIM interval
• ATIM window
• Active scan timer
• Passive scan timer

Source Gast, 802.11 Wireless Networks The
Definitive Guide, 2nd Ed.
6
Table 25-1. Summary of common tunable parameters

7
Other Performance-Related Parameters (very
preliminary candidate list)

• Layer 2 (Ethernet/802.3)
• Maximum transfer unit (MTUcommonly 1500 bytes)
• Layer 3 (IP)
• IP input queue length max (ipqmaxlen)
• Layer 4 (TCP onlyUDP not considered)
• Maximum segment size (MSS)
• Congestion window (cwnd)
• Slow-start threshold (ssthresh)
• Others tcp_recvspace, tcp_sendspace, rfc1323,
  MTU path discovery, tcp_nodelayack, sb_max