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CIS 725. Telephone protocols. Telephone systems. Circuited switched system Call control software. 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. - PowerPoint PPT Presentation
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CIS 725
Telephone protocols
Telephone systems
• Circuited switched system• Call control software
• 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
Off_hook
dialtone
numberringing
ringback
Off_hook
connectconnect
Off_hook
dialtone
number
busy
- C places a call to A- An OCM for C is instantiated- A TCM for A is instantiated with state as busy
- C places a call to B- An OCM for C is instantiated- A TCM for B is instantiated with state as busy
Off_hook
dialtonenumber ringing
ringbackOff_hook
connectconnectOff_hook
dialtone
numberringback
CW_alertFlash_hookOn_hold
connectconnect
Call waiting feature
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
• Before listening to the tone, B presses # to call D using 3WC CW will intercept # and connect to B to C instead.
• Call control interactions: 911 calls: only emergency operator can
terminate the call
• Distributed Interactions: Call number delivery vs call number blocking Call screening vs call forwarding
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
• Explore all reachable states and check whether si and sj are simultaneously reachable
• If you find interaction then avoid them
Assign priorities
• Assign priorities and use layering• Example: CW and CF: CW has priority over CF
• 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
- CW has priority over CF
- CW has priority over 3WC
Multimedia systems
• Different streams of data• What are the new requirements• QoS requirements: - picture quality, brightness, color, tint - jitter, glitches, lip_sync, delay
Intra-media requirements
• Latency: - elapsed time from packet generation to packet playback - low latency: real-time applications - high latency: email
• 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
Intermedia requirements
• Asynchrony: - synchronization between different streams - tight synch for tele-conferencing
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
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
• Asynchrony: Async_negthres, Async_posthres
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
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
• Marker-based synchronization: - periodically insert markers in the streams; - wait for markers to arrive on all streams
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
Playback time
• Each QoS parameters influences the playback time.
• Latency• Jitter• Asynchrony
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