NAT and PAT – network address translation and port address translation . NAT is used in private networks to modify the packet’s IP header to the routing device’s IP address. PAT is an extension to NAT that permits multiple devices on a local area network (LAN) to be mapped to a single public IP address, with the intent of conserving IP addresses.
Proxy firewall – makes it appear as though internal traffic that is communicating with an untrusted host (on the web) is coming from the proxy instead of the trusted internal client.
- Circuit level (Layer 5) – less processing, does not analyze data fields
- Application level (Layer 7) – more processing, analyzes data fields
VOIP Concepts
Voice-over-internet-protocol, or VOIP compresses (with a codec) audio signals and converts them into packets to transmit over a network.
Session initiation protocol (SIP) – starts, maintains, and wraps up the call session.
- Supports encryption, including TLS
- Supports caller ID suppression
Packet loss is when packets of data fail to reach their destination.
Packet loss concealment (PLC) is a method of masking the effects of packet loss in VOIP communications. As the packets travel different routes, they may arrive late, corrupted, or not at all. PLC methods include:
- Zero substitution: the lost packets are replaced with zero, resulting in lower audio quality.
- Waveform substitution, or interpolation: the missing gap is artificially substituted. Easy to implement but uses more computing resources.
Jitter – variation in packet timing that causes delays. Adaptive buffer controls that adjust to network conditions (after a quality of service measurement) are ideal for mitigating jitters. Adjusting buffer size, packet size, and codec can also mitigate jitters.
Sequence error – when packets are received in a different order than they were sent. Disorderly packets can degrade voice quality.