17. Communications Devices
17.1. Communications Device Model
A communications device provides a facility to send and receive information
over some medium (usually an electrical or fiber-optic cable) using a defined
protocol to one or more other systems which support the same protocol. The
media and protocols are often specified in national or international
standards, although some are proprietary. Within this section, the protocol
used on the external medium is called the "external protocol".
The SCSI communications device model assumes that information to control the
external protocol is embedded within the data transferred by the SEND and GET
MESSAGE commands. The structure of the data transferred by these commands is
not defined by this standard. Vendor-unique MODE pages may be defined to
provide additional control over the external protocol.
The SCSI communications device is a target with the characteristics of a
network access device, typically attached to a local area network (LAN),
public telecommunications network, private telecommunications network, or
packet switching network, etc., with no theoretical limits. Such a
communications device is often called a network node. The communications
device transmits or receives packets of data as requested by the initiator.
The contents and meaning of the data packets is not defined by this standard.
The bytes in the data packets may or may not contain addressing, path
selection, or path control information identifying separate data streams.
Additional information may be provided by the stream selection identifier
found in the ten and twelve byte SEND MESSAGE and GET MESSAGE commands.
In the SCSI communications device, the target accepts and provides the data
packets transferred in accordance with commands received from the initiator.
There is an assumption that the initiator and the communications device know
the rules by which information is exchanged between them, how the information
is to be interpreted by the communications device, and when it is allowable to
exchange the information. These rules are not specified by this standard.
The initiator requests that the communications device accept a packet of
data by transmitting a SEND MESSAGE command. The initiator requests that the
communications device return a packet of data by transmitting a GET MESSAGE
command. The communications device also supports commands related to
configuration of the network access device and network management of the
particular environment. MODE SENSE and MODE SELECT are examples of these
configuration commands.
If a communications device temporarily has no resource available to handle a
data packet from the initiator, or has no data packet to provide to the
initiator, or has no resources assigned to perform the operation, the device
may then choose one of the following responses:
(1) Terminate the command with CHECK CONDITION status. The sense key shall
be set to NOT READY and the appropriate additional sense code for the
condition.
(2) Disconnect until the necessary resource or data packet becomes
available, and then reconnect to the initiator and resume the operation.
(3) Terminate the command with BUSY status.
More than one logical unit can be implemented by a communications device.
logical units can serve as multiple paths to a single network access device,
and/or each logical unit can serve as a discrete path to different resources
within the device. A single logical unit may also serve as a path to multiple
resources if the communications device can interpret information within the
data packet and route the packet to the appropriate resource. If the logical
unit addressed by the initiator does not have an available resource or data
packet associated with it, the communications device may choose to treat the
logical unit as an invalid logical unit (see section 6.5.3) or respond as
described in the previous paragraph.
If the communications device determines that an error or unusual condition
has occurred while performing an operation specified by the contents of a data
packet, the information describing the condition is normally returned as a
part of a data packet from another network device. If the communications
device determines that an error or unusual condition has occurred while either
executing the SCSI command from the initiator, or during a network medium
access transaction, the command is terminated with a CHECK CONDITION. The
failures are identified through a REQUEST SENSE command.
The SCSI communications device is distinguished from a SCSI processor device
by the fact that the primary destination of the data packets transferred to
the communications device is not the target device itself, but another network
node. A SCSI communications device passes the data on to an ultimate
destination outside the target through a network. In contrast, the SCSI
processor device is itself the primary destination of the data packets.
Devices requiring protocols and command sets that are totally incompatible
with the communications device protocols should be examined carefully to be
sure that the incompatibilities are based on functional requirements.
17.1.1. Implementation Examples
Several examples of communications device implementations are provided to
clarify the range of utility of this device type.
17.1.1.1. Host to Host Communications
In this example of host to host communications only the SEND MESSAGE command
is used.
A communications device (Comm A) is attached to a network (Net A) and
another communications device (Comm B) is attached to a network (Net B).
Comm A takes the initiator role and selects a communications device (Comm B),
transmitting a packet to Comm B using the SEND MESSAGE command. After
transmitting the packet on the attached network medium (Net B), the action
required by the SEND MESSAGE command, Comm B assumes the initiator mode and
selects Comm A as a communications device and uses a SEND MESSAGE command to
transmit a packet on the network (Net A) attached to Comm A. Note that the
SEND MESSAGE command is sufficient to perform complete transactions between
communications devices if Comm A and Comm B act as initiators. This provides
the function of a network bridge or gateway for high bandwidth
intercommunication among nearby host processors.
17.1.1.2. Host to Device Communications
In this example of host to device communications the SEND MESSAGE and GET
MESSAGE commands are used.
A host system, Host A, takes the initiator role and selects a communications
device (Comm A) attached to a network (Net A), transmitting a packet on
network Net A to some other network node(s) using the SEND MESSAGE command to
Comm A. Host A then assumes that a result will be obtained by the other
network node(s) consistent with rules understood by all involved network
devices. Host A generates a GET MESSAGE command to obtain packets from other
network nodes. If there are no packets that need handling pending at Comm A,
Comm A may disconnect until a packet arrives. Comm A can then complete the
transaction, reconnect to Host A and the requested data packet can be
returned to Host A. Note that Host A need not support target mode and Comm A
need not support initiator mode to successfully complete an exchange between
the two devices. The host system (Host A) can be replaced by communications
device that is capable of acting as an initiator.
17.1.1.3. Multiple Role Communications
In this example of multiple role communications the SEND MESSAGE and GET
MESSAGE commands are used. The device acts as both a host and a
communications device depending on its needs and the requests made in the
network.
A communications device (Comm A) is attached to a network (Net A) and
another communications device (Comm B) is attached to a network (Net B).
Comm A takes the initiator role and selects a communications device (Comm B).
Comm A transmits a packet for network Net B using the SEND MESSAGE command to
Comm B. Comm A then requests data packets from Net B by issuing a GET MESSAGE
command to the Comm B device. Following these actions, Comm B assumes the
initiator role and transmits a packet using the SEND MESSAGE command to Comm A
for transmission to network node(s) located on Net A. Comm B then requests
data packets from Net A by issuing a GET MESSAGE command to Comm A.
17.2. Command Descriptions for Communications Devices
The commands for communications devices shall be as shown in Table 17-1.
Table 17-1: Commands for Communications Devices
==============================================================================
Operation
Command Name Code Type Section Page
------------------------------------------------------------------------------
CHANGE DEFINITION 40h O 7.2.1
GET MESSAGE(6) 08h M 17.2.1
GET MESSAGE(10) 28h O 17.2.2
GET MESSAGE(12) A8h O 17.2.3
INQUIRY 12h M 7.2.5
LOG SELECT 4Ch O 7.2.6
LOG SENSE 4Dh O 7.2.7
MODE SELECT(6) 15h O 7.2.8
MODE SELECT(10) 55h O 7.2.9
MODE SENSE(6) 1Ah O 7.2.10
MODE SENSE(10) 5Ah O 7.2.11
READ BUFFER 3Ch O 7.2.12
RECEIVE DIAGNOSTIC RESULTS 1Ch O 7.2.13
REQUEST SENSE 03h M 7.2.14
SEND DIAGNOSTIC 1Dh M 7.2.15
SEND MESSAGE(6) 0Ah M 17.2.4
SEND MESSAGE(10) 2Ah O 17.2.5
SEND MESSAGE(12) AAh O 17.2.6
TEST UNIT READY 00h M 7.2.16
WRITE BUFFER 3Bh O 7.2.17
==============================================================================
Key M = Command implementation is mandatory.
O = Command implementation is optional.
All other operation codes for communications devices are reserved for future
standardization.
17.2.1. GET MESSAGE(6) Command
Table 17-2: GET MESSAGE(6) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (08h) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | (MSB) |
-----|--- ---|
3 | Allocation Length |
-----|--- ---|
4 | (LSB) |
-----|-----------------------------------------------------------------------|
5 | Control |
==============================================================================
The GET MESSAGE(6) command (Table 17-2) transfers data from the target to
the initiator.
17.2.2. GET MESSAGE(10) Command
Table 17-3: GET MESSAGE(10) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (28h) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | Reserved |
-----|-----------------------------------------------------------------------|
3 | Reserved |
-----|-----------------------------------------------------------------------|
4 | (MSB) |
-----|--- Stream Selection ---|
5 | (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | (MSB) |
-----|--- Allocation Length ---|
8 | (LSB) |
-----|-----------------------------------------------------------------------|
9 | Control |
==============================================================================
The GET MESSAGE(10) command (Table 17-3) transfers data from the target to
the initiator.
The stream selection field specifies a further level of addressing for the
data, so that it can be accessed by the target from the appropriate data
stream.
17.2.3. GET MESSAGE(12) Command
Table 17-4: GET MESSAGE(12) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (A8h) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | Reserved |
-----|-----------------------------------------------------------------------|
3 | Reserved |
-----|-----------------------------------------------------------------------|
4 | (MSB) |
-----|--- Stream Selection ---|
5 | (LSB) |
-----|-----------------------------------------------------------------------|
6 | (MSB) |
-----|--- ---|
7 | |
-----|--- Allocation Length ---|
8 | |
-----|--- ---|
9 | (LSB) |
-----|-----------------------------------------------------------------------|
10 | Reserved |
-----|-----------------------------------------------------------------------|
11 | Control |
==============================================================================
The GET MESSAGE(12) command (Table 17-4) transfers data from the target to
the initiator.
The stream selection field specifies a further level of addressing for the
data, so that it can be accessed by the target from the appropriate data
stream.
17.2.4. SEND MESSAGE(6) Command
Table 17-5: SEND MESSAGE(6) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (0Ah) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | (MSB) |
-----|--- ---|
3 | Transfer Length |
-----|--- ---|
4 | (LSB) |
-----|-----------------------------------------------------------------------|
5 | Control |
==============================================================================
The SEND MESSAGE(6) command (Table 17-5) transfers data from the initiator
to the target.
The transfer length specifies the length in bytes of data that shall be sent
during the DATA OUT phase. A transfer length of zero indicates that no data
shall be sent. This condition shall not be considered as an error.
17.2.5. SEND MESSAGE(10) Command
Table 17-6: SEND MESSAGE(10) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (2Ah) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | Reserved |
-----|-----------------------------------------------------------------------|
3 | Reserved |
-----|-----------------------------------------------------------------------|
4 | (MSB) |
-----|--- Stream Selection ---|
5 | (LSB) |
-----|-----------------------------------------------------------------------|
6 | Reserved |
-----|-----------------------------------------------------------------------|
7 | (MSB) |
-----|--- Transfer Length ---|
8 | (LSB) |
-----|-----------------------------------------------------------------------|
9 | Control |
==============================================================================
The SEND MESSAGE(10) command (Table 17-6) transfers data from the initiator
to the target.
The stream selection field specifies a further level of addressing for the
data, so that it can be directed by the target to the appropriate data stream.
Please refer to the SEND MESSAGE(6) command for a description of the fields
in this command.
17.2.6. SEND MESSAGE(12) Command
Table 17-7: SEND MESSAGE(12) Command
==============================================================================
Bit| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
Byte | | | | | | | | |
==============================================================================
0 | Operation Code (AAh) |
-----|-----------------------------------------------------------------------|
1 | Logical Unit Number | Reserved |
-----|-----------------------------------------------------------------------|
2 | Reserved |
-----|-----------------------------------------------------------------------|
3 | Reserved |
-----|-----------------------------------------------------------------------|
4 | (MSB) |
-----|--- Stream Selection ---|
5 | (LSB) |
-----|-----------------------------------------------------------------------|
6 | (MSB) |
-----|--- ---|
7 | |
-----|--- Transfer Length ---|
8 | |
-----|--- ---|
9 | (LSB) |
-----|-----------------------------------------------------------------------|
10 | Reserved |
-----|-----------------------------------------------------------------------|
11 | Control |
==============================================================================
The SEND MESSAGE(12) command (Table 17-7) transfers data from the initiator
to the target.
The stream selection field specifies a further level of addressing for the
data, so that it can be directed by the target to the appropriate data stream.
Please refer to the SEND MESSAGE(6) command for a description of the fields
in this command.
17.3. Parameters for Communication Devices
17.3.1. Diagnostic Parameters
This section defines the descriptors and pages for diagnostic parameters
used with communication devices.
The diagnostic page codes for communication devices are defined in Table 17-
8.
Table 17-8: Diagnostic Page Codes
==============================================================================
Page Code Description Section
------------------------------------------------------------------------------
00h Supported Log Pages 7.3.1.1
01h-3Fh Reserved (for all device type pages)
40h-7Fh Reserved
80h-FFh Vendor-specific pages
==============================================================================
17.3.2. Log Parameters
This section defines the descriptors and pages for log parameters used with
communication devices.
The log page codes for communication devices are defined in Table 17-9.
Table 17-9: Log Page Codes
==============================================================================
Page Code Description Section
------------------------------------------------------------------------------
01h Buffer Over-Run/Under-Run Page 7.3.2.1
02h Error Counter Page (Write) Page 7.3.2.2
03h Error Counter Page (Read) Page 7.3.2.2
04h Error Counter Page (Read Reverse) Page 7.3.2.2
05h Error Counter Page (Verify) Page 7.3.2.2
07h Last n Error Events Page 7.3.2.3
06h Non-Medium Error Page 7.3.2.4
00h Supported Log Pages 7.3.2.5
08h - 2Fh Reserved
30h - 3Eh Vendor-specific pages
3Fh Reserved
==============================================================================
17.3.3. Mode Parameters
This section defines the descriptors and pages for mode parameters used with
communication devices.
The mode parameter list, including the mode parameter header and mode block
descriptor are defined in 7.3.3.
The medium-type code field is contained in the mode parameter header (see
Table 7-61 and 7-62). This field is reserved for communications devices.
The device specific parameter field is contained in the mode parameter
header (see Table 7-61 and 7-62). This field is reserved for communications
devices.
The density code field is contained in the mode parameter block descriptor
(see Table 7-63). This field is reserved for communications devices.
The mode page codes for communication devices are shown in Table 17-10.
Table 17-10: Mode Page Codes
==============================================================================
Page Code Description Section
------------------------------------------------------------------------------
0Ah Control Mode Page 7.3.3.1
02h Disconnect-Reconnect Page 7.3.3.2
09h Peripheral Device Page 7.3.3.3
01h Reserved
03h - 08h Reserved
0Bh - 1Fh Reserved
Fh Returns all pages (valid only for the MODE SENSE command)
00h Vendor specific (does not require page format)
20h - 3Eh Vendor specific
==============================================================================
17.4. Glossary for Communications Devices
communications device. An SCSI device whose principal function is to
communicate with one or more other systems, usually over distances that exceed
the maximum cable length defined for SCSI.
external medium. The medium used by the communications device to send or
receive information to or from one or more communications devices. The other
communications devices may or may not use an SCSI interface.
external protocol. The protocol(s) used by the communications device to
transfer information over the external medium. The external protocol(s) are
not defined by this standard.
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