9Human Speech:Nyquist Theory (Dr. Harry Nyquist, Bell labs.)- To accurately recreate an electrical pulse the sampling rate must be twice the frequency of the original.Human speech typically ranges up to 9000 Hz therefore the sampling rate must be 18,000 samples per second!
10Converting Analog to Digital: Sample the signalQuantize the signalEncode the quantized value into binary format:Optionally compress the sample to save bandwidth.
11Sample the Signal:How often to Sample?Nyquist – 18,000 Samples per second!Realistically to recognize voice and mood 8,000 Samples per second.Result less quality less bandwidthProcess referred to as Pulse Amplitude-Modulation (PAM)
13Quantize the Signal:How many Digits?Known as QuantizationDivided into sixteen (16) segments. 0 through 7 positive and 0 through 7 negativeValues are not evenly spaced to allow for more accurate recreation of voice patterns
15Encode the Quantized Signal: How many Digits?Each Quantized value is encoded into an eight bit (8) binary number.Total bandwidth is equal to eight bits for each sample times eight thousand samples per second.8 X 8000 = 64Kbps
18Circuit-Switched Hierarchical Network of Central Office Switches
19Public Switched Telephone Network (PSTN): The Pieces:Analog Telephone: Able to connect directly to the PSTN.Local loop: Connection between the customer premises and the phone company central office.Center Office (CO) Switch: Provides services to the devices on the local loop.
20Public Switched Telephone Network (PSTN) continued: The Pieces:Trunk: Provides a connection between central office switches.Private Switch (PBX): Allows a business to operate an “in-house” phone company.Digital Telephone: Typically connects to a PBX converts audio into binary
22Address Signaling:Dual-tone multifrequency (DTMF)-Each button on the keypad of a touch-tone pad or push-button telephone is associated with a pair of high and low frequencies. On the keypad, each row of keys is identified by a low-frequency tone and each column is associated with a high-frequency tone. The combination of both tones notifies the telephone company of the number being called, thus the term dual-tone multifrequency (DTMF).Pulse-The large numeric dial-wheel on a rotary-dial telephone spins to send digits to place a call. These digits must be produced at a specific rate and within a certain level of tolerance. Each pulse consists of a “break” and a “make”, which are achieved by opening and closing the local loop circuit, The break segment is the time during which the circuit is open. The make segment is the time during which the circuit is closed. The break-and-make cycle must correspond to a ratio of 60 percent break to 40 percent make.
23Multiple calls over a single line: Time Division Multiplexing (TDM)each call has a “time-slot”T1 has twenty-four (24) time slots known as a Digital Signal. IE: Digital Signal 0 is DS0E1 has thirty (30) DS0
26Signaling:Channel Associated Signaling (CAS): Uses the same bandwidth as the voice. IE: In-band signaling, as in telnet. Because it uses bits of the voice for signaling it is referred to as “Robbed Bit Signaling” (RBS).Common Channel Signaling (CCS): Uses a separate dedicated channel for signaling. IE: Out-of-band signaling as in a console connection or ISDN “D” channel.
27Robbed Bit Signaling (RBS): Uses the eighth (8th) bit on every sixth (6th) sample.Uses the least significant bit (binary 1) to limit change in quality of voice transmission
29T1 Frame:Each T1 frame consists of:Twenty-four (24) DS0’s of eight (8) bitsOne framing bit8 X 24 = = 193 bitsAt 8000 frames per second (Nyquist)Total is 193 X 8000 = Mbps
30Super Frame (SF):Each Super Frame sends twelve (12) T1 frames at a time.Uses the twelve framing bits only for synchronization.
31Extended Super Frame (ESF): Sends groups of twenty-four (24) T1 frames at a time.Of the 8000 framing bits sent every second:Two-thousand (2000) are used for framing.Two-thousand (2000) are used for error checking.Four-thousand (4000) are used as a supervisory channel (Out-of-band)
37Supervisory Signaling: On-hook Signal: When the phone is on-hook there is no connection between tip and ring.Off-hook Signal: When the phone is off-hook the connection between tip and ring is made and electrical current (signal) is present.Ringing: To cause a phone (on-hook) to ring an AC (Alternating Current) signal is sent.
38Informational Signaling: Dial Tone: Indicates the phone company is ready to receive digits.Busy: Indicates the remote phone is in use.Ringback: Indicates to the originator that the receiving phone is ringing.Congestion: Indicate the long distance network is not able to complete the call.Reorder: Indicates the local network is not able to complete the call.Receiver 0ff-hook: Indicates the local phone has been off-hook for an extended period of time.
39Informational Signaling (Continued): No Such Number: Indicates the dialed number is invalid.Confirmation: Indicates the telephone company is attempting to complete the call.
40Glare (Loop Start Signaling, Most common in Home): When a user attempts to dial an outgoing call at the same time an incoming call is received, the two connect without ring or dial-tone.More frequent in business where multiple incoming calls are received and multiple outgoing calls are made
41Telephone Services: Call Waiting Call Forwarding Three-way Calling DisplayCall BlockingCalling Line ID BlockingAutomatic CallbackCall ReturnCircuit Switched Long DistanceCalling Cards800/888/877 NumbersVirtual Private NetworksPrivate Leased LinesVirtual Circuits
53International Telecommunications Union (ITU) accepted in 1996. H.323:International Telecommunications Union (ITU) accepted in 1996.Designed to carry multimedia over Integrated Services Digital Network (ISDN)Based or modeled on the Q.931 protocolCryptic messages based in binaryDesigned as a peer-to-peer protocol so each station functions independentlyMore configuration tasksEach gateway needs a full knowledge of the systemCan configure a single H.323 Gatekeeper that has all system informationEach end system can contact the gatekeeper before making a connectionGatekeeper can perform Call Admission Control (CAC) to determine if resources are available before a call is acceptedGatekeeper and Gateway can be the same device
56SIP was designed by the IETF as an alternative to H.323 SIP is a single protocol whereas H.323 is a suite of protocols as FTP is a single protocol within the TCP/IP protocol suiteSIP is designed to set up connections between multimedia endpointsUses other protocols (UDP, RTP, TCP….) to transfer voice or video dataMessaging is in clear ASCII textVendors can create their own “add-ons” to the SIP protocolSIP is still evolvingSIP is destined to become the only voice and video protocol
58IETF standard with developmental aid from Cisco MGCP:IETF standard with developmental aid from CiscoAll devices under a central controlVoice gateway becomes a dumb terminalAllows minimal local configurationSingle point of failureUses UDP port 2427
60Often called “skinny” protocol SCCP:Often called “skinny” protocolCisco proprietarySimilar to MGCP in that it is a stimulus/response protocolAllows Cisco to deploy new features in their phonesCisco phones must work with Cisco systems (CME, CUCM,CUCME…)Cisco phones can also use other protocols such as SIP or MGCP with downloaded firmware