CIS 725 Telephone protocols. Telephone systems Circuited switched system Call control software.
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Transcript of CIS 725 Telephone protocols. Telephone systems Circuited switched system Call control software.
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CIS 725
Telephone protocols
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Telephone systems
• Circuited switched system• Call control software
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• OCM = originating call machine• TCM = terminating call machine• A places a call to B • An OCM for A is instantiated with initial
state as idle• When signal reaches B, a TCM for B is
instantiated
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Off_hook
dialtone
number
ringingringback
Off_hook
connect
connect
Off_hook
dialtone
number
busy
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- C places a call to A- An OCM for C is instantiated- A TCM for A is instantiated with state as busy
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- C places a call to B- An OCM for C is instantiated- A TCM for B is instantiated with state as busy
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Off_hook
dialtone
numberringing
ringback
Off_hook
connectconnectOff_hook
dialtone
number
ringbackCW_alert
Flash_hookOn_hold
connect
connect
Call waiting feature
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Feature Interaction
• Limited terminal equipment• Same key used for different purposes• Example: CW and 3WC
A is talking to B
C calls B
B receives the call;
Call is accepted by CW; issues a tone to B
Now, CW is in state ready to accept # from B
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• Before listening to the tone,
B presses # to call D using 3WC
CW will intercept # and connect to B
to C instead.
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• Call control interactions:
911 calls: only emergency operator can terminate the call
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• Distributed Interactions:
Call number delivery vs call number
blocking
Call screening vs call forwarding
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Interaction Detection
• Model each feature using a formal model• Depending on the current state, a feature
may or may not accept a signal• Fi = accepts a signal S in state si• Fj = accepts a signal S in state sj• Can Fi and Fj be in the states si and sj
simultaneously
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• Explore all reachable states and check whether si and sj are simultaneously reachable
• If you find interaction then avoid them
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Assign priorities
• Assign priorities and use layering• Example: CW and CF:
CW has priority over CF
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• Feature at layer N processes a signal before a feature at layer N - 1
• If feature N does not accept a signal, it passes it to the next layer
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- CW has priority over CF
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- CW has priority over 3WC
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Multimedia systems
• Different streams of data• What are the new requirements• QoS requirements:
- picture quality, brightness, color, tint
- jitter, glitches, lip_sync, delay
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Intra-media requirements
• Latency:
- elapsed time from packet generation to
packet playback
- low latency: real-time applications
- high latency: email
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• Jitter:
- disruption in continuous playback
- low jitter for real-time applications• Packet loss:
- % of packets lost
- video = packet loss could be high
- text = low packet loss
Successive packet loss
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Intermedia requirements
• Asynchrony:
- synchronization between different streams
- tight synch for tele-conferencing
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Conflicts between QoS parameters
• Jitter and latency conflict
- To control jitter,
value of latency must be large enough
to smooth out variations in network• Asynchrony conflicts with latency
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Specifying QoS parameters
• Latency
- two thresholds: lat_max, lat_min• Jitter
Gap_max• Packet loss
pktloss_max over time time_pktloss
% of successive packet loss
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• Asynchrony:
Async_negthres, Async_posthres
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QoS Control
• Can be implemented inside the network
- streams are synchronized
- virtual circuit-based systems• Can be implemented at end-systems• Combination of both techniques
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Inter-media synchronization
• Use a single channel to send all types of data
- perfect synchronization• Master/slave channels
- use one of the media as the master and synchronize others with respect to this channel
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• Marker-based synchronization:
- periodically insert markers in the streams;
- wait for markers to arrive on all streams
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QoS Protocol Design
• Receiver-based QoS control• QoS control module:
- computes the playback time for each pkt
- monitors QoS packets
- adjusts the playback time
- may buffer packets or drop them
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Playback time
• Each QoS parameters influences the playback time.
• Latency• Jitter• Asynchrony
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Real-time Transport protocol(RTP)
• Application Level Framing: - application knows its own needs - application knows how to segment data * Defines format for data packets (RTP) and
control packets (RTCP) * Provides timing-related information to the
application to process packets and make decisions