Sierra Wireless DART 300 manual Protocols and Packet Assembly Features, OSI Model

6.Protocols and Packet Assembly Features

This section covers:

•Communication protocols and protocol stacks.

•Protocol Features of UDP, TCP, SLIP, and PPP in DART 300, their advantages and disadvantages.

•Packet Assembly and Disassembly (PAD) features.

The details of the features of the DART 300 are each covered with:

•A brief description of the feature

•A detailed discussion of it with respect to configuration and impact on other features

•Sample(s) of AT command sequences to implement the feature

6.1.Communication Protocols and Stacks

Data communication protocols are generally discussed in relation to the Open Systems Interconnection (OSI) model. The OSI model was introduced in 1978 as a long-term project of the International Organization for Standardization (ISO). It divides the communications process into seven distinct layers, stacked one above the other, hence the term protocol stack. This is very different from the computer processing use of the term stack for a LIFO buffer.

Table 6-1: The OSI Model

Layer		Description	Common Protocols
7	– Application	This is the window between the end-user application and the	Application programs
		communications process. It includes functions such as login and	for file transfer,
		password checks, and resource allocation.	e-mail, etc.
6	– Presentation	Responsible for terminal management such as character set	Telnet, FTP, SMTP,
		interpretation (i.e. ASCII, ANSI) and code conversion.	etc.
5	– Session	Session management includes data-flow synchronization, mapping
		addresses with names, handling graceful and abrupt disconnections,
		and data buffering. This layer organizes data into Session Protocol
		Data Units (SPDUs).
4	– Transport	Transport Protocol Data Units (TPDUs) are assembled at this layer.	UDP, TCP
		This can include multiplexing and de-multiplexing, error detection
		and recovery, and packet reordering where the network can receive
		packets along different routes, which can arrive out of sequence.
3	– Network	Data is organized into packets, which are data frames with network	IP, SLIP, PPP
		headers and trailers added including network addressing. The
		duties include flow control, handling network service data units,
		notifying the transport layer of errors, and possibly sequenced
		delivery.
2	– Link	This layer arranges the bits into frames. It also establishes and	Network Interface
		releases one or more link connections.	Cards, Modems
1	– Physical	This is the physical connection layer concerned with carrying the	Transmission Media:
		communication of digital data.	Twisted Pair, Fiber
			Optics, Cellular
			Radio, etc.

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