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This section describe how µCμC/TCP-IP uses buffers to receive and transmit application data and network protocol control information. You should understand how network buffers are used by µCμC/TCP-IP to correctly configure your interface(s).
Network Buffers
µCμC/TCP-IP stores transmitted and received data in data structures known as Network Buffers. Each Network Buffer consists of two parts: the Network Buffer header and the Network Buffer Data Area pointer. Network Buffer headers contain information about the data pointed to via the data area pointer. Data to be received or transmitted is stored in the Network Buffer Data Area.
µCμC/TCP-IP is designed with the inherent constraints of an embedded system in mind, the most important being the restricted RAM space. µCμC/TCP-IP defines network buffers for the Maximum Transmission Unit (MTU) of the Data Link technology used, which is most of the time Ethernet. Default Ethernet’s maximum transmit unit (MTU) size is 1500 bytes.
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On transmission, the number of bytes to transmit is always known, so it is possible to use a Network Buffer size smaller than the maximum frame size. µCμC/TCP-IP allows you to reduce the RAM usage of the system by defining small buffers. When the application does not require a full size frame to transmit, it is possible to use smaller Network Buffers. Depending on the configuration, up to eight pools of Network Buffer related objects may be created per network interface. Only four pools are shown below and the remaining pools are used for maintaining Network Buffer usage statistics for each of the pools shown.
In transmission, the situation is different. The TCP/IP stack knows how much data is being transmitted. In addition to RAM being limited in embedded systems, another feature is the small amount of data that needs to be transmitted. For example, in the case of sensor data to be transmitted periodically, a few hundred bytes every second can be transferred. In this case, a small buffer can be used and save RAM instead of waste a large transmit buffer. Another example is the transmission of TCP acknowledgment packets, especially when they are not carrying any data back to the transmitter. These packets are also small and do not require a large transmit buffer. RAM is also saved.
µCμC/TCP-IP requires that network buffer sizes configured in net_dev_cfg.c satisfy the minimum and maximum packet frame sizes of network interfaces/devices.
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All μC/TCP-IP device drivers require a configuration structure for each device that must be compiled into your driver. You must place all device configuration structures and declarations within a pair of files named net_dev_cfg.c and net_dev_cfg.h.
Micriµm Micriμm provides sample configuration code free of charge; however, most sample code will likely require modification depending on the combination of compiler, processor, evaluation board, and device hardware used.
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The first step in creating a device driver configuration for µCμC/TCP-IP begins with the memory configuration structure. This section describes the memory configuration settings for most device drivers, and should provide you an in-depth understanding of memory configuration. You will also discover which settings to modify in order to enhance the performances of the driver.
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Configuring the loopback interface requires only a memory configuration, as described in µC?C/TCP-IP Network Interface Configuration.
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The device configuration will directly impact the Network Task Queues Configuration.
The µCμC/TCP-IP stack includes two queues. The first one is the Rx queue and is used to store the Rx buffer that have been filled and are ready to be process. The second queue is the Tx deallocation and is used to store the Tx buffers that are ready to be deallocate.
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