Specifications

SLAA137A
MSP430 Internet Connectivity 5
Every network node has its own unique physical address. It is 48 bits long and called the media
access control (MAC) address. The maximum length possible for an Ethernet frame is 1518
bytes. This size covers the whole frame, excluding the preamble. The preamble consists of
alternating zeros and ones and is used for synchronization purposes. It is followed by the real
frame. The first 48 bits are the destination and the second 48 bits are the source MAC address.
After that, a 2-byte value indicating the type of the frame is sent. This type field is used to decide
in the receiving stack to which upper level protocol the frame will be handed over. Afterwards a
maximum of 1500 data bytes can be transferred followed by a 4-byte automatically generated
cyclic redundancy check (CRC) value. Using this CRC, the ethernet ensures data integrity, but it
does not ensure the delivery or in-order delivery of a packet. There are many resources on the
Internet about ethernet.
2.2 Address Resolution Protocol
The address resolution protocol (ARP) is usually used in ethernet networks. Its main purpose is
to determine a physical network address (e.g., Ethernet/MAC address) from a logical one (e.g.,
IP address) for sending packets of higher-level protocols. The network station that wants to
exchange data with another peer broadcasts a packet to the LAN that is received and processed
by all other peers. If a station finds out that the target’s protocol address matches the one of its
own TCP/IP stack, it sends back an answer frame to the sender. Now the sender knows the
MAC address of its partner and can continue sending with Unicast packets. This protocol is
described in detail in RFC 826 [5].
2.3 Internet Protocol
The Internet protocol (IP) is designed for use in packet-based networks such as the Internet. It
provides mechanisms for transmitting datagrams from a source to a destination (addressing),
and for fragmentation if necessary for transmission through small-packet networks. The
demonstration board uses today’s most common protocol, version four (IPv4).
The communication partners are identified by fixed-length addresses (IP addresses). But there
is no guarantee for a reliable end-to-end data transmission, flow control, sequencing and other
services commonly found in host-to-host protocols. If such features are needed, a higher-level
protocol must be used for data transfer (the most common in Internet is the TCP). There is no
assurance that the data sent along the datagram is error free, however the IP header is
protected by a checksum. Each IP packet has a protocol field indicating to which upper layer
protocol the carried data belongs.
Each Internet datagram is treated as a fully independent entity unrelated to any other datagram
and therefore IP is connectionless. A datagram can fail to reach its destination for different
reasons:
• The destination host is not connected to the network
• The datagram was damaged
• A router misdirected the datagram
Upper level protocols (for example, TCP) can compensate for these types of failures. Refer to
RFC 791 [2] for a more detailed description.