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1. Provided the transmission medium accommodates X/YMODEM.
Chapter 3 Rev 10-27-87 Typeset 10-27-87 5
Chapter 3 ZMODEM Protocol 6
3.3 IIIInnnntttteeeeggggrrrriiiittttyyyy aaaannnndddd RRRRoooobbbbuuuussssttttnnnneeeessssssss
Once a ZMODEM session is begun, all transactions are protected with 16 or
32 bit CRC.[2] Complex proprietary techniques such as CCCCyyyybbbbeeeerrrrnnnneeeettttiiiicccc DDDDaaaattttaaaa
RRRReeeeccccoooovvvveeeerrrryyyy((((TTTTMMMM))))[3] are not needed for reliable transfers.
An optional 32-bit CRC used as the frame check sequence in ADCCP (ANSI
X3.66, also known as FIPS PUB 71 and FED-STD-1003, the U.S. versions of
CCITT's X.25) is used when available. The 32 bit CRC reduces undetected
errors by at least five orders of magnitude when properly applied (-1
preset, inversion).
A security challenge mechanism guards against "Trojan Horse" messages
written to mimic legitimate command or file downloads.
3.4 EEEEaaaasssseeee ooooffff IIIImmmmpppplllleeeemmmmeeeennnnttttaaaattttiiiioooonnnn
ZMODEM accommodates a wide variety of systems:
o+ Microcomputers that cannot overlap disk and serial i/o
o+ Microcomputers that cannot overlap serial send and receive
o+ Computers and/or networks requiring XON/XOFF flow control
o+ Computers that cannot check the serial input queue for the presence of
data without having to wait for the data to arrive.
Although ZMODEM provides "hooks" for multiple "threads", ZMODEM is not
intended to replace link level protocols such as X.25.
ZMODEM accommodates network and timesharing system delays by continuously
transmitting data unless the receiver interrupts the sender to request
retransmission of garbled data. ZMODEM in effect uses the entire file as
a window.[4] Using the entire file as a window simplifies buffer
management, avoiding the window overrun failure modes that affect
MEGAlink, SuperKermit, and others.
ZMODEM provides a general purpose application to application file transfer
protocol which may be used directly or with with reliable link level
__________
2. Except for the CAN-CAN-CAN-CAN-CAN abort sequence which requires five
successive CAN characters.
3. Unique to Professional-YAM and PowerCom
4. Streaming strategies are discussed in coming chapters.
Chapter 3 Rev 10-27-87 Typeset 10-27-87 6
Chapter 3 ZMODEM Protocol 7
protocols such as X.25, MNP, Fastlink, etc. When used with X.25, MNP,
Fastlink, etc., ZMODEM detects and corrects errors in the interfaces
between error controlled media and the remainder of the communications
link.
ZMODEM was developed _f_o_r _t_h_e _p_u_b_l_i_c _d_o_m_a_i_n under a Telenet contract. The
ZMODEM protocol descriptions and the Unix rz/sz program source code are
public domain. No licensing, trademark, or copyright restrictions apply
to the use of the protocol, the Unix rz/sz source code and the _Z_M_O_D_E_M
name.
4. EEEEVVVVOOOOLLLLUUUUTTTTIIIIOOOONNNN OOOOFFFF ZZZZMMMMOOOODDDDEEEEMMMM
In early 1986, Telenet funded a project to develop an improved public
domain application to application file transfer protocol. This protocol
would alleviate the throughput problems network customers were