Network Layers

OSI Model Layers Application Presentation Session Transport Network Data Link Physical Multimedia Presentation Layer Standards JPEG - Picture standard form the Joint Photographic Experts Group. MIDI - Musical Instrument Digital Interface is used to digitize music. MPEG - Motion Picture Expert's Group standard for encoding video. PICT - Picture format for QuickDraw graphics used by Macintosh programs. TIFF - Tagged Image File Format for bitmapped high resolution images. Session Layer Offers three communications modes Simplex - Only one device transmits. Half-duplex - Each side takes turns transmitting from one side at a time. Full-duplex - Devices on both sides of the communications channel can talk at the same time. Connection split into the following three phases: Connection establishment Data transfer Connection termination Session Layer Protocols

• NFS - Network File System
• SQL - Structured Query Language
• RPC - Remote Procedure Call
• X Window
• ASP - AppleTalk Session Protocol used for Apple client to server communications.
• DNA SCP - Digital Network Architecture Session control Protocol. A Digital DECnet protocol.

Transport Layer

Maintains flow control which prevents buffer overflow and data loss. Reliable data transmission provides for:

• Acknowledgement of received segments.
• Retransmission of segments not acknowledged.
• Proper re-sequencing of segments.
• Flow control to manage the data so no data is lost.

In reliable communications, data is transmitted and received in the same order. If the data is received out of order, the communication fails. Rather than transmitting a packet and having an acknowledgement sent for each packet a window is established. A window specifies the number of packets the transmitting device can send before an acknowledgement is received. Then the acknowledgement is sent the number of the next packet that has not yet been received is sent. If no acknowledgement is received by the sender in a given period of time, the packet or packets are re-transmitted. Two main protocols:

• TCP - Reliable with establishment of virtual circuit and numbering and and acknowledgement of packets. Also establishes the connection before sending data.
• UDP - Unreliable with no virtual circuit and numbering without acknowledgement of packets.

Network Layer

Responsible for controlling the path of the data.

Data Link Layer

The data packets are encapsulated into frames. A header with a hardware (MAC) destination and source address are added. Parts of a data frame include:

• Start indicator (preamble) - Bit pattern indicating the start of a frame.
• Destination address
• Source address
• Type field in ethernet II frames
• Length field in 802.3 frames
• Data
• Frame Sequence Check (FSC) field which includes the Cyclic Redundancy Checksum (CRC)

Data Link Layer WAN protocols:

• HDLC - High Level Data Link Control - Implemented by the INternational Standards Organization (ISO).
• SDLC - Synchronous Data Link Control uses a master slave relationship between primary and secondary devices. The master always initiates communication. This protocol was started by IBM to communicate between offices with mainframe computers.
• LAPD - Link Access Procedure Balanced - Can detect missing or out of sequence frames and is used with X.25.
• X.25 - Defines specification between a DTE and a DCE and is a packet switching network.
• SLIP - Used to support TCP/IP communications over a slow serial interface.
• PPP - Used to support several protocols over a slow serial interface with several enhancements over SLIP.
• ISDN - Integrated Services Digital Network - Digital phone lines for voice and data transmission.
• Frame Relay - Higher speed than X.25 packet switching network.

Physical Layer

Standards:

• EIA/TAI-232
• EIA/TIA-449
• V.24
• V.35
• X.21
• G.703
• EIA-530
• HSSI - High speed serial interface

DOD Model

These are the names of the layers according to Microsoft documentation with corresponding OSI layers:

• Application - Application, Presentation, and Session.
• Host-to-Host - Transport
• Internet - Network
• Network Access - Data Link and Physical