Black Box LB3104A series, Configuration Manual

Page: 42 / 99

CHAPTER 5: Advanced Setup Menu
39
Link Protocol
Select this parameter to assign the link protocol. Link protocols available for asynchronous link are: SLIP,
CSLIP, or PPP.
• SLIP — SLIP stands for Serial Line Internet Protocol. It is a simple form of encapsulation for IP datagrams
on serial lines. SLIP is often used in connecting home systems to the Internet, through the RS-232 serial
port. The following rules specify the framing used by SLIP. The IP datagram is terminated by the END
character (0xc0). Most datagrams transmit an END character at the beginning of a datagram as well.
Placing an END at the beginning prevents interpreting line noise as being part of the datagram. Any data
before the END character is removed and erased.
• If a byte of the IP datagram equals the END character, the 2-byte sequence 0xdb, 0xdc is
transmitted instead. This special character, 0xdb, is called the SLIP ESC character. (This is not
the same as the ASCII ESC character.
• If a byte of the IP datagram equals the SLIP ESC character, the 2-byte sequence is transmitted
instead.
SLIP has several drawbacks:
• Each end must know the other’s IP address. There is no method for one informing the other of
its IP address.
• There is no type field. If a serial line is using SLIP, the line cannot be used for some other
protocol.
• There is no checksum added by SLIP. If a noisy phone line corrupts a datagram being transferred
by SLIP, the higher levels must detect the corruption.
SLIP is specified in RFC 1055.
• CSLIP — CSLIP stands for Compressed Serial Line Internet Protocol. CSLIP is used to solve a problem
associated with SLIP. SLIP lines are often slow (19200 bits/second or less). They are often used for
interactive traffic, such as Telnet, which uses TCP. TCP adds many small packets to the data. To carry 1 byte
of data requires a 20-byte IP header on a 20-byte TCP header, an overhead of 40 bytes. To overcome this
drawback, CSLIP reduces the header from 40 bytes to 3 or 5 bytes. By reducing the header size, response
time is improved. CSLIP maintains the state of up to 16 TCP connections on each end of the CSLIP link.
CSLIP is specified in RFC 1144.
• PPP — PPP stands for Point-to-Point Protocol. PPP consists of 3 components:
• A way to encapsulate IP datagrams on a serial link. PPP supports either an asynchronous link with
8 bits of data and no parity, or bit-oriented synchronous links.
• A link control procedure (LCP) to establish, configure, and test the data-link connection. Having
an LCP allows each end to negotiate various options.
• A family of network control protocols (NCPs) specific to different network-layer protocols. The
NCPs allow each end to configure network control parameters.