cis 725 telephone protocols. telephone systems circuited switched system call control software

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