Information Formats

Data and control information transmitted across internetworks takes a variety of formats, depending on which layer of the OSI Model is being discussed. The terms describing these information formats are not used consistently in the internetworking industry and sometimes are used interchangeably.

Common information formats are listed in Table 1-1.

Table 1-1. Information Formats

Information Format	OSI Layer (if applicable )
Data Units / Datagrams	N/A
Frames	Layer-2
Packets	Layer-3
Segments	Layer-4
Cells	N/A

The following sections discuss these formats of information transmission.

Data Units/Datagrams

Data unit, sometimes known as a datagram, is a generic term referring to a variety of information units found in network environments. Some common data units are as follows :

Service data units (SDUs) - Information units from upper-layer protocols that define a service request to a lower-layer protocol.
Protocol data units (PDUs) - OSI terminology for a packet.
Bridge protocol data units (BPDUs) - Used by the spanning-tree algorithm as "hello" messages.

Frames

Frames are data-link layer (OSI Layer-2) datagrams and are made up of the following:

Data-link layer header
Payload (upper-layer data)
Trailer

The frame header and trailer contain control information, such as addressing, link management, and error-checking mechanisms, to be used by Layer-2 devices, such as LAN bridges or Frame Relay WAN switches.

The length of a frame, measured in bytes, is determined by the maximum transmission unit (MTU) setting, which can be adjusted manually or by the protocol default. Figure 1-2 illustrates a data-link layer frame.

Figure 1-2. Layer-2 Datagram (Frame)

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Packets

Packets are network layer (OSI Layer-3) datagrams and are made up of a network layer header, payload (upper-layer data), and a trailer. The packet header and trailer contain control information, such as addressing, to be used by Layer-3 devices, such as WAN routers and Layer-3 switches.

The length of a packet, measured in bytes, is determined by the MTU setting, which can be adjusted manually or by the protocol default. Figure 1-3 illustrates the basic components of a network layer packet.

Figure 1-3. Layer-3 Datagram (Packet)

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Segments

Segments are transport layer (OSI Layer-4) datagrams and are made up of a transport layer header, payload (upper-layer data), and a trailer. The segment header and trailer contain control information, such as sequencing and error checking, to be used by Layer-3 devices, such as routers. Routers use the sequencing information found in the header to ensure that all packets in a data stream have been received.

The length of a segment, measured in bytes, is determined by the MTU setting, which can be adjusted manually or by the protocol default. Figure 1-4 illustrates the basic components of a transport layer segment.

Figure 1-4. Layer-4 Datagram (Segment)

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Cells

Cells are a fixed size and used in switched environments, such as ATM networks. Cells are made up of a header and payload; cells do not use trailers . The header contains control information, such as addressing, used by cell switching devices, such as ATM switches.

The length of a cell, measured in bytes, is a fixed length. For example, ATM cells are 53 bytes in length; 5 bytes for the header and 48 bytes for the payload. Figure 1-5 illustrates an ATM cell.

Figure 1-5. ATM Cell

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Technical Note: Transmitting Large Datagrams

If the payload of a datagram is larger than the MTU will allow, the datagram must be broken down into fragments for transmission. For example, if the MTU of a frame (Layer-2) is 1500 bytes and the data-link layer is presented with a packet of 4500 bytes in length, the packet is fragmented into three frames for transmission. These fragments are then reassembled by the receiving device for handing off to the next upper-layer.