HL7V2.x and Low Layer Protocols

Minimal LLP (MLLP) and the Hybrid LLP (HLLP) are the two widely used lower layer protocols (LLP) for HL7 versions 2.x transmission. They are efficient and extremely simple to implement and use simple socket based communication. MLLP and HLLP does not really qualify as a "protocol" as they make use of TCP's reliability. MLLP and HLLP are simply a means of framing individual HL7 messages which is needed at the 7th level.

There are two types of LLP connectivity

Ø Persistent - In this type of connectivity socket is cached based on the endpoint and only one socket per endpoint is created and reused for the next set of messages.

Ø Temporary - In this type of connectivity a new socket is created for each message. When a message is sent the sender waits for the ACK and once it is received socket is closed.

The processing rules for both MLLP and HLLP are more of less the same; however, processing rules for HLLP include additional logic to signal transmission errors.

MLLP Wrapper

The MLLP wire format looks like <SB>dddd<EB><CR>

Where <SB> is Start Block, 1 Byte, ASCII <VT>, HEX <OxOB> and should not be confused with ASCII characters SOH or STX.

dddd is the HL7 variable length HL7 message. The data can contain any displayable ASCII characters and the carriage return character, . <CR>

<EB> is End Block, 1 byte, ASCII<FS> , HEX <Ox1C> and should not be confused with ASCII characters ETX or EOT.

<CR>is Carriage Return, 1 byte, ASCII , HEX <OxOD>

HLLP Wrapper

The HLLP wire format looks like <SB>tvv<CR>ddddCcccc<EB><CR>

Where <SB> is Start Block, 1 Byte, ASCII<VT> , HEX <OxOB>and should not be confused with ASCII characters SOH or STX.

t is Block Type, 1 Byte. ‘D’ is data block. This is used for HL7application message and HL7 ACK response message.‘N’ is NAK block and is used only as NAK to signal transmission errors

vv is Protocol Id, 2 Bytes. The characters ‘2’’1’ for this version

<CR>is Carriage Return, 1 byte, ASCII<CR> , HEX <OxOD>

dddd is is the HL7 variable length HL7 message. The data can contain any displayable ASCII characters and the carriage return character, <CR>.

In a NAK block, this field contains a 1-byte reason code as follows:
‘C’ is character count wrong in previous data block received.
‘X’ is checksum wrong in previous data block received.
‘B’ is data too long for input buffer in previous block received.
‘G’ is error not covered elsewhere.

Ccccc is Block Size, 5 bytes and is the character count of all characters so far in the data block up to and including the last data character. For this version of the protocol this is 5 + the size of the dddd field. An encoded HL7 message ends with a <CR>character. This character is considered part of the data.

Xxx is Checksum, 3 bytes an X-OR checksum of all character in the block up to and including the last data character. The checksum is expressed as a decimal number in three ASCII digits. If the value is 999, the checksum should not be computed. Processing will proceed as if the checksum were correct. This feature is used for applications where the messages will be translated from one character set to another during transmission.

<EB>is End Block, 1 byte, ASCII<FS> , HEX <Ox1C> and should not be confused with ASCII characters ETX or EOT.

<CR>is Carriage Return, 1 byte, ASCII , HEX <OxOD>