Mobile Computing and VoIP

Mobile Computing Overview

  • Instructor: Assoc. Prof. Dr. Awos Kh. Ali
    College of Education – Computer Science Dept. – Mosul University

Voice over IP (VoIP)

  • Definition: VoIP or Internet telephony uses the Internet as a telephone network with enhanced capabilities.
  • Characteristics:
    • Packet-switched: Unlike traditional circuit-switched networks, VoIP transmits data in packets.
    • Protocols:
    • SIP (Session Initiation Protocol)
    • H.323
Session Initiation Protocol (SIP)
  • Type: Application layer protocol designed by IETF.
  • Functions: Establishes, manages, and terminates communication sessions.
  • Operates on: Independent of underlying transport layers such as UDP, TCP, or SCTP.
  • Message Types:
    • INVITE: Initializes a session; initiates a call.
    • ACK: Confirms after the callee answers.
    • BYE: Terminates a session.
    • OPTIONS: Queries about capabilities.
    • CANCEL: Cancels an already initialized session.
    • REGISTER: Makes a connection when the callee is unavailable.
  • Addressing: Identifies sender and receiver for communication.
SIP Modules
  • Components:
    • Establishing: Initiating the session.
    • Communicating: Active session management.
    • Terminating: Ending the session.
Callee Tracking
  • Callee may be at a different terminal or lack a fixed IP address (using DHCP).
  • Registrars: Servers maintaining the callee's IP address.
  • Proxy Server: Sends lookup messages to registrars.

H.323 Standard

  • Overview: An ITU-T standard allowing communication between telephones and computers over the Internet.
  • Components:
    • Gateway: Connects the Internet to the telephone network; translates protocols.
    • Gatekeeper: Acts as a registrar for calls.
H.323 Protocols
  • Compression: Uses protocols like G.711, G.723.1, managed by H.245.
  • Connection Management: Established and terminated using Q.931.
  • Registration: Handled by H.225 protocol.

Real-time Transport Protocol (RTP)

  • Function: Designed for real-time traffic on the Internet, typically used with UDP.
  • Encapsulation: RTP is treated like an application program encapsulated within UDP datagrams.
  • Port Usage: Utilizes temporary even-numbered UDP ports.
RTP Packet Format
  • Structure:
    • Ver | PX | Contr. | Count | M | Payload Type
    • Sequence Number | Timestamp | Sync Source ID | Contributor ID
  • Payload Types: Various audio and video types specified (e.g., PCMM audio, MPEG video).

Real-time Transport Control Protocol (RTCP)

  • Role: Manages and provides feedback on the quality of RTP data streams.
  • Message Types:
    • Sender and Receiver Reports: Report on transmission and reception statistics.
    • Source Description Messages: Provide additional info about the source.
    • Bye Message: Shuts down a stream.
H.323 Call Setup & Operation
  • Components: Includes terminals (endpoints), gatekeepers, MCU (Multipoint Control Unit), and gateways.
  • Process:
    1. Call Setup (Signaling):
    • Endpoint Registration: Registers with the gatekeeper using RAS protocol.
    • Call Signaling: Q.931 setup messages are sent.
    • Call Admission: Gatekeeper checks bandwidth availability.
    1. Capability Negotiation: Exchange of H.245 messages to agree on codecs and transport protocols.
    2. Media Transmission: Established using RTP for streaming and RTCP for monitoring.
    3. Call Termination: H.225 Release Complete message sends to halt communication and free resources.