CIS 725 Telephone protocols Telephone systems Circuited switched

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 ringing ringback busy Off_hook connect

-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

Call waiting feature Off_hook dialtone number ringback ringing Off_hook connect Off_hook dialtone number ringback On_hold CW_alert Flash_hook connect

Feature Interaction • Limited terminal equipment • Same key used for different purposes • Example: CW and 3 WC 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 3 WC 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 3 WC

Multimedia systems • Different streams of data • What are the new requirements • Qo. S 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 Qo. S 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 Qo. S parameters • Latency - two thresholds: lat_max, lat_min • Jitter Gap_max • Packet loss pktloss_max over time_pktloss % of successive packet loss

• Asynchrony: Async_negthres, Async_posthres

Qo. S 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

Qo. S Protocol Design • Receiver-based Qo. S control • Qo. S control module: - computes the playback time for each pkt - monitors Qo. S packets - adjusts the playback time - may buffer packets or drop them

Playback time • Each Qo. S 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