cis 725 telephone protocols. telephone systems circuited switched system call control software
TRANSCRIPT
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
number
ringingringback
Off_hook
connect
connect
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
dialtone
numberringing
ringback
Off_hook
connectconnectOff_hook
dialtone
number
ringbackCW_alert
Flash_hookOn_hold
connect
connect
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