Voice over IP (VoIP)

• VoIP converts analog voice signals into digital packets that travel over IP networks instead of traditional circuit-switched telephone networks
• Voice quality depends on network conditions - latency, jitter, and packet loss directly impact call quality
• Uses Real-time Transport Protocol (RTP) for actual voice data and RTP Control Protocol (RTCP) for quality monitoring
• Requires Quality of Service (QoS) configuration to prioritize voice traffic over data traffic

VoIP Components

• IP Phones: Hardware endpoints that connect directly to network switches (Power over Ethernet capable)
• Softphones: Software applications on computers/mobile devices that function as virtual phones
• Analog Telephone Adapters (ATAs): Convert traditional analog phones to work with VoIP systems
• VoIP Gateways: Bridge VoIP networks with Public Switched Telephone Network (PSTN) for external calls
• Call Manager/PBX: Central system managing call routing, features, and user accounts

Signaling Protocols

Network Requirements

• Bandwidth: 64-90 Kbps per call (depends on codec used)
• Latency: Must be under 150ms one-way (anything over 150ms causes noticeable delay)
• Jitter: Should be under 30ms (variation in packet arrival times)
• Packet Loss: Must be under 1% (even 3% loss severely degrades quality)

Common Codecs

QoS Implementation

• Voice traffic requires highest priority - typically marked with DSCP EF (Expedited Forwarding)
• Call signaling traffic marked with DSCP AF31 (lower priority than voice bearer traffic)
• Use Low Latency Queuing (LLQ) to guarantee voice traffic gets processed first
• Configure traffic shaping on WAN links to prevent buffer overflow (causes jitter)

Power Considerations

• IP phones typically use Power over Ethernet (PoE) - IEEE 802.3af standard provides 15.4W
• Cisco phones may require PoE+ (802.3at) providing up to 25.5W for advanced features
• Always verify switch PoE budget - calculate total wattage needed for all connected devices
• Deploy UPS systems for switches to maintain phone service during power outages

VLAN Configuration

• Separate voice and data VLANs - prevents data traffic from impacting voice quality
• Voice VLAN typically configured as auxiliary VLAN on access ports
• Cisco Discovery Protocol (CDP) or LLDP-MED tells phones which VLAN to use
• Example: Data on VLAN 10, Voice on VLAN 20 with higher QoS priority

Notes

• VoIP is highly sensitive to network issues - problems invisible to data users become immediately apparent to voice users
• Always test voice quality under full network load conditions, not just during quiet periods
• Consider bandwidth overhead - RTP headers, IP headers, and Layer 2 headers add ~40 bytes per packet
• Jitter buffers in phones compensate for variable delay but add latency - tune appropriately for your network
• Security consideration: VoIP calls can be intercepted more easily than traditional phone calls - implement encryption (SRTP) for sensitive environments
• Emergency services (911) location - VoIP systems must be configured to provide accurate location information for emergency calls
• Backup power planning is critical - unlike traditional phones, VoIP phones don’t work when network equipment loses power
• Consider bandwidth multiplication for WAN planning - a 10Mbps link can theoretically handle 125 G.729 calls, but QoS overhead and bursting reduce this significantly