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2110212 Rev 1.0 Page 37
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
communications process. It includes functions such as login and
password checks, and resource allocation.
Application programs
for file transfer,
e-mail, etc.
6 – Presentation
Responsible for terminal management such as character set
interpretation (i.e. ASCII, ANSI) and code conversion.
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).
Telnet, FTP, SMTP,
etc.
4 – Transport
Transport Protocol Data Units (TPDUs) are assembled at this layer.
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.
UDP, TCP
3 – Network
Data is organized into packets, which are data frames with network
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.
IP, SLIP, PPP
2 – Link
This layer arranges the bits into frames. It also establishes and
releases one or more link connections.
Network Interface
Cards, Modems
1 – Physical
This is the physical connection layer concerned with carrying the
communication of digital data.
Transmission Media:
Twisted Pair, Fiber
Optics, Cellular
Radio, etc.