Bcn Midterm Dec2003 Solution Sheet 34

Lehrstuhl für Kommunikationsnetze Technische Universität München

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Prof. Dr.-Ing. J. Eberspächer

K BCN Midterm, Dec. 2003

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Solution Sheet

Problem 3: Circuit Switching and Packet Switching 3.1 (a) TS,Packet =

(b) n Packet =

L Packet 90 ⋅ 8 bit = 2.4 s = R 300 Mbit/s

60 Kbyte 60 Kbyte = 740.74 = L Packet − L Header 81 byte

741 complete packets

100 km = 500 s 200000 km/s TT = (nPacket + 1) ⋅ TS,Packet + 2 ⋅ TL + TV,Packet = 742 ⋅ 2.4 s + 2 ⋅ 500 s + 5 s = 2.786 ms

(c) TL.Link =










(d) TCircuitSet up = 4 ⋅ TL,Link + 4 ⋅ TV,Circuit = 4 ⋅ 500 s + 4 ⋅ 50 s = 2.2 ms 



60 ⋅ 8 Kbit = 1.6 ms 300 Mbit/s = TCircuitSet up + TS + 2 ⋅ TL,Link = 2.2 ms + 1.6 ms + 1 ms = 4.8 ms

(e) TS,300Mbit/s = TTransfer

3.2 (a)

nPacket 30 Kbyte = = 370.37 2 81 byte 742 packets = nPacket + 1

371 complete packets

(b) TT = (n Packet + 2) ⋅ TS,Packet + 2 ⋅ TL + TV,Packet = 743 ⋅ 2.4 s + 2 ⋅ 500 s + 5 s = 2.788 ms 30 ⋅ 8 Kbit (c) TS,100Mbit/s = = 2.4 ms 100 Mbit/s TTransfer = TCircuitSet up + TS,100Mbit/ s + 2 ⋅ TL,Link = 2.2 ms + 2.4 ms + 1 ms = 5.6 ms 

BCN – Midterm Dec. 2003 – Solution Sheet





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Problem 4: Token Ring (a) A terminal uses the free Token and sends a packet. Single Packet: Token is released after the complete packet is received by the same terminal. Multiple Token: Token is released immediately after the packet is sent. (b) Terminals B and C are in “Repeat and Stand By” state Terminal A is in “Active” state (because it has the highest hardware address). A free token is circulating on the ring. (c) TVX is necessary at the monitor (terminal A) in order to enable fast reaction for recovery (d) Yes, the timer has to be greater than or equal to the ring round trip time. Otherwise the timer would expire permanently even if there is no error. TVX TR = 3·T0 (e) See figure on next page. (f) State of terminal A: Active State of terminal B: Active State of terminal C: Repeat and Standby (g) There are two free Tokens on the ring. So two stations can each take one free Token and transmit at the same time. Therefore collisions can happen.

BCN – Midterm Dec. 2003 – Solution Sheet

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(e)

A

B

C Ring

Terminal A (Addr.=3)

Terminal B (Addr.=3 !)

Terminal C (Addr.=1)

R

R

R

t=0 T0

TNT expired

TNT expired

C

C(3)

C

TNT expired C(3) C P

P(3)

C R

P(3)

Tk

C(1)

1<3

C(3)

P(3)

P A

(Terminal A)

A

Tk Tk

Tk

Tk Tk

t = 11 T0

BCN – Midterm Dec. 2003 – Solution Sheet

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