COP 5611 Operating Systems Spring 2010 Dan C. Marinescu Office: HEC 439 B Office hours: M-Wd 2:00-3:00 PM

Lecture 9 Reading Assignment: Chapter 7 from the online textbook HW1 due today. Remember: A progress report for the project is due on every Monday till week 12. Last time: Thread coordination and scheduling  Multi-level memories  I/O bottleneck Today:  Network properties  Layering  Data link layer Next time  Network layer

3 Properties of Networks Physical limitations:  Speed of light  finite communication latency  Hostile environments  Limited channel capacity  limited bandwidth Channels are shared - multiplexed  Why: Support any-to-any communication Share costs  How Isochronous multiplexing – scheduled access  TDM  FDM Asynchronous multiplexing

4 Communication channels are multiplexed

5 Data flow on an isochronous link

6 A data communication network

7 Asynchronously multiplexed link

8 Communication Continuous versus bursty  The old phone network versus data networks  Human versus computer communication Connection-oriented versus connectionless communication Packet-forwarding networks  Routing problem  Delays

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10 Packet forwarding (store and forward) networks

11 Problems in packet forwarding networks Delay  Propagation delay  Transmission delay  Processing delay  Queuing delay Resources are finite and a worst case design is not feasible  heavy tail distributions of resource needs Buffer overflow and discarded packets  Adaptive rate modulated by information regarding network congestion  Timers and packet retransmission  Duplicate packets

12 Queuing delays versus utilization.

13 Recovery of lost packets

14 Duplicate requests

15 Delays and recovery lead to duplicate response

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17 Layering Simplify the design Example- RPC

18 Client-server communication based on RPC

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21 Example of layered design

22 Data link layer

23 Network layer

24 End-to-end (transport) layer

25 How many layers should a network model have? OSI –has 7 layers Internet is based on a model including  Application  Transport  Network  Data Link  Physical Layer Applications are very diverse and it makes no sense for a lower layer to implement functions required by higher layers. The end-to-end argument  application knows best

26 Network composition Mapped composition  some layers of a network are composed of basic data-link, network, and transport layers of another network. Overlay networks Internetworking  interconnect several networks together, e.g., the Internet

27 Network composition. The overlay network Gnutella uses for its link layer an end-to-end transport protocol of the Internet. In turn, the Internet uses for one of its links an end-to-end transport protocol of a dial-up phone system

28 More about the link layer Function: push bits from one place to another Analog worlds Capacity of a communication channel Capacity of a noisy communication channel C= B x log (1+ signal/noise) B is the bandwidth in Hz signal/noise – ratio of signal power to noise power Signals attenuation Signals are distorted over long distances

29 Serial transmission

30 How to push bits from A to B which do not share the same clock? First raise the READY line

31 Signal attenuation and shape distortion

32 Framing A pattern of bits serve as a frame delimiter – e.g., seven 1’s Bit stuffing:  The sender: add a 0 whenever it encounters a pattern of six 1’s in data  The receiver: remove a 0 following a pattern of six 1’s in data Add a frame header Add a frame trailer

33 Sender bit stuffing procedure

34 Receiver bit stuffing procedure

35 A network protocol may use multiple data link protocols

36 Multiple transport and data link protocols

37 Sending a frame

38 Receiving a frame