CSCI Computer Networking: Link Layer Programming George Blankenship. Programming George Blankenship 1

Transcription

1 CSCI 6431 Computer Networking: Link Layer Programming George Blankenship Programming George Blankenship 1

2 Communication Environment Physical Layer Interface Physical Layer Interface (invisible) Computer Parallel/Serial Interface Modem Telephone Telephone circuit Link Layer Interface Programming George Blankenship 2

3 Lecture Outline Programming concepts General concepts Programming constructs components Modem interface Modem/telephone interface System-to-system messages gexamples Programming George Blankenship 3

4 Programming Concepts Basic concepts Finite state machine Interrupts gconstructs Server programming Client programming State variable processing Messages Programming George Blankenship 4

5 Finite State Machine Problem is decomposed Tasks Events State is a task with an input and output Events are the input or output of a task Programming George Blankenship 5

6 FSM Definition State Input Output Next state Initial i Initialize i DTR WiDSR WaitDSR WaitDSR DSR IF setup RTS WaitCTS WaitCTS CTS Reset WaitOK ~DSR ~DTR Initial WaitOK OK Dial WaitConnect ~DSR ~DTR Initial Programming George Blankenship 6

7 ~DSR ~DTR FSM Map Programming George Blankenship 7

8 Interrupts Inputs are capture and processed by ad hoc processes Current state defines applicability of the interrupt Current state defines processing associated with the interrupt Interrupt processing centered on current state Programming George Blankenship 8

9 Programming Two components to each I/O Recipient, waiting entity, server S /Cli t Generator, active entity, client Server Device or service provider Must wait for service request Client Requestor of service Performs read/write as needed d Programming George Blankenship 9

10 Server Programming The server is the entity that allows the client to perform a write whenever it wishes The server must be in a waiting state, waiting for an action by the client The server performs a read (complex action) The server receives the first write by the client Programming George Blankenship 10

11 Client Programming The client behaves like the normal program (simple action) S /Cli t The client performs a write to initiate i i an interaction i (dialog) The client performs a read to capture the servers response. Programming George Blankenship 11

12 State Variable Current state maintained i in single shared variable S /Cli t Rf Reference to the state variable ibl must be protected by (mutex or monitor) PDU/SDU Coding model lis switch statement Switch(state variable) Case state1: Case state2: Programming George Blankenship 12

13 Waiting Problem Solution Non-blocking I/O for reads S /Cli t Separate process (thread) for blocking I/O All I/O is able to an associated inactivity timer Systems are able to perform other processing while waiting for I/O Programming George Blankenship 13

14 Process (Thread) Assignments Main process Computation (compilation) of input Maintenance of GUI Monitor the health of system Input process Collect input from each input source Output process Deliver output to each destination Programming George Blankenship 14

15 PDUs and SDUs APDUi is a message between endsystems S /Cli t An SDU is a message between an endsystem and an interface PDU/SDU Since we will be writing i both sides of the communication, we will be able to define the contents of the PDU The SDU format is fixed by the interface Programming George Blankenship 15

16 Communication Environment Computer Parallel/Serial Interface Modem Telephone Telephone circuit Link Layer Interface Programming George Blankenship 16

17 Communication Interfaces Telephone (modem/telephone) interface Telephone number Speaker level Controlled by normal I/O In-band signaling Character oriented data and control Programming George Blankenship 17

18 Modem (Serial Link IF) (S Registers) S0 - Automatic answer ring number S1 - Ring counter S2 - AT escape character (43) S3 - Command line termination character (13) S6 - Wait before dialing (2) S7 - Connection completion time-out (50) S8 - Comma dial modifier time (2) S10 - Automatic disconnect delay (20) S30 - Inactivity timer (0) Programming George Blankenship 18

19 Modem/telephone Interface In-band signaling (sent on data path) AT starts all commands Individual command Letter (%Letter, &Letter) with modifier L speaker level M speaker operation Sx=value (set an S register) Response is a text string Programming George Blankenship 19

20 Signal Sequence +++<CR> - switch to control mode Set operating environment ATZ (factory defaults) ATL3M1E1F1 (loud until connect, echo commands, do not echo data) ATDT<phone number><cr> - dial a phone number and switch to data mode when connected Connection signaled by Connect ATO0 return back to data mode Programming George Blankenship 20

21 System-to-system messages System-to-system messages are sent after connection has been made Content of the message is transparent to the interfaces (with the exception of +++) Message content is link protocol For class problem, the message will contain hello <called telephone number> from <sending system id> Programming George Blankenship 21

22 General Program Flow Capture (assign, acquire) the communications port S /Cli t Initialize the UART/modem interface is proper PDU/SDU Initialize the modem/telephone interface Dial Exchange data Terminate call Programming George Blankenship 22

23 Initialize the Modem/telephone Interface Switch to control mode by sending +++ slowly WriteFile() I/O function for all commands Send one character at a time waiting for I/O completion After I/O completion control mode is active Programming George Blankenship 23

24 Control Messages (AT command set defined in the references at the end of the notes) S /Cli t Standard I/O is used for the telephone interface PDU/SDU The I/O is an ASCII string The first two characters of each message are AT Individual id commands require one or more intervening characters The last character of each message is <CR> Programming George Blankenship 24

25 Making a Call ATE0F1L3M1DT <CR> E0 echo control F1 do not echo normal data L3 speaker on high (to hear dialing) M1 speaker on during dial until connection DT tone dial (800) Programming George Blankenship 25

26 Receiving a Call Call will generate a ring-in on receiving side Receiving modem will answer S0 is non-zero (ATS0=x) ATA command Modem will present CD to phone Phone will present a connect message Programming George Blankenship 26

27 Sending/Receiving Data WriteFile(), ReadFile() are the I/O functions to send/receive data S /Cli t Protocol encoding and rules must supply all attributes of QoS PDU/SDU Exercise is to send a message and display received message What is the form of the message? How does receiver know message has been received properly? How does sender know the message has been received? Programming George Blankenship 27

28 Terminate When the message has been displayed the call may be S /Cli t disconnected Call disconnect ATH<CR> on modem/telephone control interface Modem will drop carrier CD loss will be reported in on other side Programming George Blankenship 28

29 Reading and Thought /Di l /B i d l Programming George Blankenship 29