Computer Networks VOIP Preshetha Kanagaiah VoIP ● Voice over Internet Protocol, aka Internet telephony is a technology that allows a user to use the internet as a medium of transmission of voice communication to make and receive phone calls instead of using the traditional PSTN (public switched telephone network lines). ● Converts the voice from an analog signal to a digital signal over the internet using codecs, digital information is packetized, transmission occurs as IP packets. ● Communication platforms that use VoIP: ○ Skype, Vonage, Viber, Google Hangouts, Whatsapp (end-to-end encryption) Traditional PSTN vs. VoIP PSTN (POTS): Network: - Circuit switching - Packet switching - Dedicated path b/w sender & receiver - No dedicated path b/w sender & receiver - Reserved bandwidth in advance - Bandwidth is adjusted based on necessity - Cost based on distance and time - Cost is NOT based on distance and time - Very Costly for additional features such as - Additional features are covered under the VM, Caller ID, call waiting, call forwarding, service (VM, CID, call waiting, forwarding, conferencing, additional lines conferencing) - Better voice quality - Transmit data, video, and voice - Local and international communication - can be converged with PSTN VoIP Signaling Protocols ❏ SIP (Session Initiation Protocol) : ❏ open standard, application layer protocol ❏ Similar to HTTP- adopts client/server architecture ❏ Based on text-based messages for requests/responses ❏ INVITE request, ACK, BYE terminates connection, (others include: CANCEL, OPTIONS, REGISTER) ❏ Skype protocol ❏ proprietary so details are hard to procure, difficult to compare to other protocols ❏ Utilizes TCP for signaling, and transports media via UDP/TCP, sent on different ports ❏ 2 nodes: ❏ ordinary hosts: Skype application on user’s computer ❏ super nodes: endpoint with a public IP address Challenges of VoIP ❏ QoS type guarantees such as Capacity & available bandwidth ❏ Latency - delay in conversation ❏ Packet loss - packets of conversation missing due to congested network ❏ Jitter - poor audio quality due to queuing delays/network congestion ❏ Security vulnerabilities Network Bandwidth ❏ Voice data is much bulkier than conventional text data, uses more BW ❏ BW consumption dependent on codec utilized (compresses voice data), packet overhead, and network protocol used. ❏ Available BW is limited resource for desirable QoS ❏ Multiple applications utilizing network limits availability of BW for VoIP Network Latency ❏ Delay as a result of conversion of voice data from one format to another format when using a voice codec so packets arrive late ❏ Incoming traffic exceeds the capacity of the link that connects the gateway to the internet (packets get dropped) ❏ Packets are buffered and are subject to queuing delay ❏ Affects voice performance: echo impairment and hesitation ❏ Solution: ❏ Prioritization of voice packets Packet Loss ❏ IP Packet loss can occur due to: ❏ Congestion in the IP network ❏ Packet dropped if it arrives too late to destination ❏ Wireless systems contain variations in received signal power - may be faded/reduced ❏ Solution ❏ FEC (forward error correction) techniques : reduce # of errors in data transmission ❏ Adds redundancy to transmitted data using a predefined algorithm, allows receiver to detect and correct errors within a bound Jitter ❏ Result of network congestion (sharing of load b/w many access links) and improper queuing delays ❏ Sender transmits voice packets at a constant rate while packets are received at receiving side at an uneven rate. ❏ Delayed packets get dropped from network congestion ❏ When packets get dropped, it results in poor audio quality that is unintelligible at the receiving end ❏ Solution: ❏ Jitter buffer: buffer packets and play them out at a steady rate ❏ Reserve bandwidth and utilize faster links Security in VoIP ❏ VoIP is vulnerable to all intrinsic security problems in IP ❏ Threats: ❏ DDos ❏ network connections to the outside world open servers to unknown access, little centralized control, more prone to address spoofing leading to DDoS ❏ Open TCP/UDP ports ❏ Eavesdropping ❏ Packet sniffers can intercept traffic/data that is unencrypted ❏ Deciphering a conversation out of IP transmission requires peeling away layers of the protocol stack ❏ Limited gateway security: securing NAT and firewalls References ❏ Websites: ❏ http://searchunifiedcommunications.techtarget.com/tutorial/VoIP-models-and-services-Comple te-guide ❏ Journals: ❏ “VoIP: A comprehensive survey on a promising technology” by Stylianos Karpantazis, Fotini-Niovi Pavlidou. Computer Networks. Volume 53, Issue 12, 13 August 2009, Pages 2050-2090..