6610-l3- - Scheduling 1 Puzzle You have an unfair coin...

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1 Scheduling
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Puzzle You have an unfair coin (prob(H) = p != ½) How will you generate a fair toss?
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3 Scheduling When a station gets to transmit, the protocol that determines which packet gets to be transmitted Fairness the primary consideration Weighted fairness … an extension Default scheduling in today’s Internet?
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4 Scheduling Policies … FCFS Packets queued on a first-come-first-served basis No fairness Priority Multiple queues with different priorities Packets belonging to queue k served only when queues with higher priorities are empty
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5 Scheduling (Contd.) Generalized Processor Sharing (GPS) Ideal fair queuing approach based on a fluid flow system Complex, idealistic Packetized GPS (WFQ) Packetized version of GPS (practical) Finish times in a correponding GPS system used for prioritizing packets
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6 Scheduling - WFQ 3 flows A, B, C Weights: A(1), B(2), C(3) Assume same packet sizes 2 5 8 1 3 4 7 6 Tx Schedule: C B C C B A C B …   Complex overhead due to  computation of finishing times  Simpler approach?
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7 Scheduling (Contd.) Weighted Round Robin Simpler approximation of WFQ Assumes constant packet sizes Can cause unfair delay A B C Fixed Tx Schedule: C C C B B A A 
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8 Scheduling (Contd.) WRR with WFQ Spread For each flow k with weight wk generate wk elements of the form (1/wk,k), (2/wk,k), …, (wk/ wk,k) Sort all such elements from all competing flows in lexicographic order Extracting the second element of each pair from the resulting sorted list gives the WRR with WFQ spread
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9 WRR with WFQ Spread A (1)  B (2) C (3) (1,A) (1/2,B), (1,B) (1/3,C), (2/3,C), (1,C) Lexicographically Sorted:  (1/3,C), (1/2,B), (2/3,C), (1,A), (1,B), (1,C) Schedule:  C B C A B C
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10 Other scheduling policies… Deficit Round Robin (DRR) Handles varying size packets Frame of N bits split among the competing flows in the ratio of their weights Flows get to transmit HOL packet only when they have accumulated packet-length number of bits Class based queuing (CBQ) General scheduling and link-share scheduling Greater flexibility to control scheduling policy
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11 Internet Protocol (IP)
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12 TCP/IP Protocol Suite Physical layer Data-link layer – ARP, RARP, SLIP Network layer – IP, ICMP, IGMP, BootP Transport layer _ TCP, UDP, RTP Application layer – http, smtp, ftp
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13 Internet Protocol (IP) Addressing Routing Fragmentation and Reassembly Quality of Service Multiplexing and Demultiplexing
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14 Addressing Need unique identifier for every host in the Internet (analogous to postal address) IP addresses are 32 bits long Hierarchical addressing scheme Conceptually … IPaddress =(NetworkAddress,HostAddress)
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15 Address Classes Class A Class B Class C 0  netId hostId 7 bits 24 bits 1  0   netId hostId 14 bits 16 bits 1 1  0  netId hostId 21 bits 8 bits
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16 IP Address Classes (contd.) Two more classes 1110 : multicast addressing 1111 : reserved Significance of address classes?
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This note was uploaded on 12/14/2009 for the course ECE 6610 taught by Professor Staff during the Fall '08 term at Georgia Institute of Technology.

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6610-l3- - Scheduling 1 Puzzle You have an unfair coin...

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