These notes are based on reading the official certification guide “CCNA Collaboration CICD 210-060 Official Cert Guide“.

 

CICD – Ch1 – Traditional Voice vs. Unified Voice

Analog Voice

Analog Signal

• Electrical voltage, frequency, current and charge used to convey properties of voice

Analog Phone Lines

• Uses properties of electricity to convey changes in voice over cabling
• Must convey signaling
  • Dial tone, dialed digits, busy

Analog Circuit

• Pair of wires
  • Ground (positive) – TIP
  • Negative (battery) – RING
  • Where concept of TIP and RING comes from

Loop Start Signaling

• 2 wire phone with 48v DC current
• Typically found in home environments
• Problem – Glare
  • Occurs when going to pick up phone off hook and at the same time a call comes in before the phone can ring

Key System – PBX

• Have chance of glare occurring

Modern PBX

• Found in larger corporate environments
• Ground start signaling
  • Originated by the pay phone
  • Allows PBX to separate an answering phone from incoming phone line

Supervisory Signaling

• On hook, off hook, ringing
• Informational signaling
  • Dial tone, busy, ring back
• DTMF – Dual Tone Multi Frequency

Analog Signal Problems

• Degraded over distances, signal loss
• Repeaters installed to increase distance
  • Regenerated the signal
  • Could not differentiate between voice on wire vs. line noise
  • Line noise would be amplified
• Number of wires the phone company needed to run and maintain
  • Each phone required 2 wires
  • Bundles of wires hard to maintain

Digitizing Voice

• Process of changing analog voice signal into a series of numbers
• Digital transmission eliminated need for individual pair of wires required for analog connectors

Traditional Digital

• TDM, Time Division Multiplexing
• Digital encode multiple conversations over a single 4 wire path
• Numeric value transmitted in specific time slots
  • Different conversations

T1 Circuit (US, Canada, Japan)

• 24 separate 64 kbps channels
  • DS0
• Each channel supported 1 voice call

Channel Associated Signaling – CAS

• Signaling information is transmitted in the same channel as voice
• Robber bit signaling – RBS
  • Steals a bit from voice channel to transfer signaling information
• T1 uses 8th bit of every 6th sample / frame

Common Channel Signaling – CCS

• Dedicated one DS0 from T1/E1 for signaling
• Out of Band (OOB) signaling
  • Signal is completely separate from voice traffic
• T1 has 23 usable DS0’s for voice
• Signaling protocol – Q.931
  • Used on ISDN circuits
  • SS7 signaling for CCS between CO’s
• Allows PBX vendors to communicate proprietary messages and features between PBX systems using ISDN
• T1 uses 24th slot
• E1 uses 17th slot

PSTN – Public Switches Telephony Network

• Establish world wide pathways to allow people to easily connect, converse and disconnect
• Components
  • Analog phone
    • converts audio to electrical and vice versa
    • Connects to PSTN
  • Local Loop
    • Link between customer and service provider
  • CO Switch
    • Provides services to devices on local loop
    • Signaling, digit collection, call routing, setup and teardown
  • Trunk
    • Connection between CO Switches
  • Private Switch
    • Miniature PSTN inside company
    • PBX?
  • Digital Phone
    • Connects to PBX
    • Converts audio into binary

PBX – Private Branch Exchange

• Manage in-house phones (business)
• Allow internal calls without using PSTN resources
• Connects internal phones and connects to PSTN
• Components
  • Line Cards
    • Provides connection between phones and PBX
  • Trunk Card
    • Provides connection from PBX to PSTN or other PBX’s
  • Control Card
    • Intelligence of PBX
    • All call setup, routing and management functionality
• Key System
  • Geared for smaller environments (50 users or less)
  • Fewer features, more of a shared line feel
• Connections to PSTN

• • CCS style signaling
    • Call setup, routing, billing, informational messages
    • Call > CO > # lookup > forward to destination

PSTN Number Plans

• Must use valid E.164 standard address
• E.164 is international number plan created by International Telecommunications Union (ITU)
  • Limited to max of 15 digits
• Components
  • Country Code
  • National Destination Code
  • Subscriber number

VOIP – Voice over IP

• Send voice traffic over data network
• Concerns
  • Ensuring packets get to destination intact and quickly (QoS)
  • Coding / Decoding (Codec)
  • Security, not snooped or changed in transit (Encryption)
• Benefits
  • Reduced cost of communications
    • Use existing WAN/Internet for calls
    • No toll charges
  • Reduced cabling
    • Run single ethernet to desk
  • Seamless Voice Networks
    • Uses business network, not PSTN
    • Central control of all voice devices
    • 4 digit dialing across the world
  • Reduced cost of MACD
  • Softphone
  • Unified email, voicemail and fax
  • Increased productivity
    • Ring all devices
  • Feature rich
  • Open standards

Converting voice to packets

• Dr Harry Nyquist – created mathematical foundation to convert analog signals (flowing) waveforms into digital format (binary)
• 3 Step Process (optional 4th)
  • Samping
  • Quantization
  • Encoding
  • (optional) Compression
• Audio Frequencies
  • Human ear can hear > 20 – 20,000 Hz
  • Human speech > 200 – 9,000 Hz
  • Traditional phone transmits > 300 – 3,400 Hz
  • Standard equipment to digitize human speech > 300 – 4,000 Hz
• Nyquist
  • Sample twice the highest frequency (2×4000)=8000
  • Sample = 1 byte, 8 bits
  • 8000 samples per second, times 8 bits for each sample
    • Product = 64,000 bits per second
    • This is uncompressed audio
    • G.711 – 64kbps
  • Once sampling assigns a numeric value to all analog signals traffic gets encapsulated
    • UDP and RTP (real-time transport Protocol)

Codecs

• 2 main codecs on all Cisco IP Phones
• Compression, reduce amount of bandwidth required for a call
• MOS – Mean Opinion Score
  • Quality of various voice codecs

• G.711
  • Common on all VOIP devices
  • 64 kbps per call
• G.729
  • Compressed audio
  • 8 kbps
  • 2 Variants
    • G.729a (annex A) – sacrifices audio quality for better processor efficient coding
    • G.729b (annex B) – introduces VAD (voice activity detection), makes voice transmission more efficient
• G.722
  • Default on new Cisco phones
  • Wideband codec
    • Reproduces a wider range of frequencies
    • Better audio
  • 64 kbps

Digital Signal Processor – DSP

• Offload processing for voice related tasks from the routers processor
• Chip that performs all sampling, encoding and compression functions on audio / video coming into the router
• Packet Voice DSP Module (PVDM)
  • Memory stick
  • Comes in different sizes
• Codecs consume DSP resources
  • Some consume more than others
  • DSP resources can handle (approx) double the number of medium complexity calls per DSP than high complexity

Real-time Transport Protocol – RTP

• Operates at transport level of OSI model
  • UDP based traffic, does not require acknowledgement
• UDP provides port numbers and header checksum
• RTP adds timestamps and sequence numbers to header information
• Random port – 16,384 <> 32,767
  • Always even number
• Devices setup point to point RTP stream, one in each direction

Real-time Transports Control Protocol – RTCP

• Reports statistics between 2 devices in the call
  • Packet count
  • Packet delay
  • Packet loss
  • Jitter (delay variations)
• UDP based traffic
  • Random port – 16,384 <> 32,767
    • Always odd number
• Separate session from RTP
• Devices send RTCP packet once every 5 seconds