1. 1 Design a Simple Two Telephone System Concentrating on the control aspects (not data). Requirements Each phone has a handset that may be picked up and replaced, a button that can be used to alert the other the other telephone of the desire to communicate, and a bell that alerts a user of an incoming call. A call be terminated by either party. Suggested States Idle - handset is down, there is no connection. Dial- ready to dial Ringing - handset is ringing Calling - call has been placed, but not yet connected Talk- connection is established.

2. 2 Informal Description of Tel. Behaviour Idle 1. Pickup handset(become caller); proceed to Dial. or 2. Phone rings; proceed to ringing. Dial 1. Replace handset; proceed to Idle. or2. Press button; proceed to Calling. Calling 1. Abandon call-replace handset; proceed to Idle. or2. Called party picks up; proceed to Talk. Ringing 1. Pickup handset(become called); proceed to Talk. or 2. Caller rings out - receive signal; proceed to idle. Talk 1. Replace handset; proceed to Idle. or 2. Other party disconnects - receive signal; proceed to Dial.

3. 3 Informal Description of Tel. Behaviour Idle 1. Pickup handset (become caller); proceed to Dial. or 2. Phone rings; proceed to Ringing. Dial 1. Replace handset; proceed to Idle. or2. Press button; generate signal to called party; proceed to Calling. Calling 1. Abandon call-replace handset; generate a signal to called party; proceed to Idle. or2. Called party picks up - receive signal; proceed to Talk. Ringing 1. Pickup handset(become called); generate a signal to caller; proceed to Talk. or 2. Caller rings out - receive signal; proceed to Idle Talk 1. Replace handset; generate a signal to other party; proceed to Idle. or 2. Other party disconnects - receive signal; proceed to Dial.

4. 4 Process Diagram (generic) channelsignal handset{up,down} button{press} bell{ring} link{alert,answer, cutoff} Phone 1 Phone 2 bell1 button1 bell2 button2 handset1handset2 link21 link12

5. 5 Parameterised Description of Tel. Behaviour of Phone n Call link from phone n to phone m linkout. Call link from phone m to phone n linkin.

6. 6 Idle 1. Pickup handset; proceed to Dial. or 2. Phone rings on receipt of alert on linkin; proceed to Ringing. Dial 1. Replace handset; proceed to Idle. or2. Press button; generate alert on linkout; proceed to Calling. Calling 1. Abandon call-replace handset; generate a cutoff on linkout; proceed to Idle. or2. Called party picks up - receive answer on linkin; proceed toTalk. Ringing 1. Pickup handset(become called); generate answer on linkout; proceed to Talk. or 2. Caller rings out - receive cutoff on linkin; proceed to Idle Talk 1. Replace handset; generate cutoff on linkout; proceed to Idle. or 2. Other party disconnects - receive cutoff on linkin; proceed to Dial.

7. 7 Message Sequence Charts Describe communication in a particular scenario. vertical axis defines ordering of events indication of state horizontal axis defines message/signal occurrences

8. 8 Message Sequence Charts Example: Call setup and clear down (1 to 2) Phone 1 Phone 2 Idle Idle handset1.up Dial button1.press link12.alert bell2.ring Calling Ringing handset2.up link21.answerTalk handset1.down link12.cutoff IdleDial handset2.down Idle

9. 9 Message Sequence Charts More Examples: Call setup and clear down (2 to 1) 1 calls 2, but rings out before 2 answers. 1 picks up handset, but puts down handset before dialing. Call setup and clear down (1 to 2) but 2 terminates call. 1 calls 2 and 2 calls 1.

10. 10 Message Sequence Charts Example: 1 calls 2 and 2 calls 1 Phone 1 Phone 2 Idle Idle handset1.up handset2.up DialDial button1.press button2.press link12.alert link21.alert Calling DEADLOCK ! (except to return to Idle)

11. 11 A Solution: Communication Protocol When a process needs to output a control signal, then it does so in parallel with receiving a response. When a process receives an unsolicited signal, it must eventually acknowledge it. Why? To prevent infinite blocking of signal sender.

12. 12 Telephone Process Specification Assume all sends on linkout, all receives on linkin. Idle choosehandset.up:Dial linkin.alert :send ack; ring bell; Ringing Dial choosehandset.down : Idle button.press : || send alert || receive response; if response is ack then Calling else if response is alert then Talk Calling choosehandset.down : || send cutoff || receive response; Idle linkin.answer: send ack; Talk

13. 13 Ringing choosehandset.up: || send answer || receive response; if response is ack then Talk else if response is cutoff then Dial linkin.cutoff:send ack; Idle Talk choosehandset.down : || send cutoff || receive response; Idle linkin.cutoff : send ack; Dial

14. 14 Message Sequence Charts Examples with Acks: Call setup and clear down (2 to 1) 1 calls 2, but rings out before 2 answers. 1 picks up handset, but puts down handset before dialing. Call setup and clear down (1 to 2) but 2 terminates call. 1 calls 2 and 2 calls 1.

15. 15 Inadequacies of FSA/Transition Systems Inherently sequential no representation of concurrency. No outputs no explicit way to deal with the generation of events. No state variables e.g. “remembering” the number dialled. No depth/hierarchy/modularity needed for structuring large systems. State and edge explosion no way to deal with “interrupts”, common behaviour.