ISO-OSI Layers

The network layers (Fig 25.1) defined by the International Standards Organization for the Open Systems Interconnection (OSI) are:

OSI 7 Layer Model

1.

Physical Layer

• defines electrical and mechanical interfaces (e.g., fiber optics) to the network and determines the maximum bandwidth

  Needs for higher bandwidth for audio/video data

2.

Data Link Layer

• transmission of data frames, access protocol to physical layer, error correction, flow control and frame synchronization

  Retransmission as a method for error correction is not useful for multimedia (a late frame in audio/video is a lost frame). Error control at this layer will be dropped in e.g., ATM networks.

3.

Network Layer

• switches and routes packets, establishes logical association of remote stations. Providing services such as addressing, internetworking, error handling, congestion control, and sequencing of packets.

  Quality of Service (QoS) parameters defined for multimedia data transmission. Resource reservation based on QoS will guarantee small jitter and correct packet ordering.

4.

Transport Layer

• provides a process-to-process connection.

  QoS and synchronization of the multimedia data is enhanced.

5.

Session Layer

• coordinates interaction between user application processes on different hosts. Types of sessions: point-to-point, multicast (one to many), multidrop (many to one).

  Multimedia sessions (for audio, video, etc.) often use more than one transport connection. TCP/IP and ATM networks do not explicitly have this layer

6.

Presentation Layer

• Manages abstract data structures, conversion of different data formats/codes

  Greater need for support and conversion between various audio, video formats (incl. various compression standards).

  In TCP/IP and ATM networks, this layer is merged into Application Layer

7.

Application Layer

• contains various protocols, e.g., ftp, telnet, SMTP (e-mail), etc.

  Increasing varieties of applications, e.g., HTTP, MIME, video-on-demand