IEC 60728-7-2:2003

INTERNATIONAL IEC
STANDARD
60728-7-2
First edition
2003-10
Cable networks for television signals,
sound signals and interactive services –
Part 7-2:
Hybrid Fibre Coax Outside Plant
Status Monitoring –
Media access Control (MAC)
Layer Specification
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INTERNATIONAL IEC
STANDARD
60728-7-2
First edition
2003-10
Cable networks for television signals,
sound signals and interactive services –
Part 7-2:
Hybrid Fibre Coax Outside Plant
Status Monitoring –
Media access Control (MAC)
Layer Specification
 IEC 2003  Copyright - all rights reserved
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– 2 – 60728-7-2  IEC:2003(E)
CONTENTS
FOREWORD . 4
INTRODUCTION .6
1 Scope . 7
2 Normative references. 7
3 Terms, definitions and abbreviations. 8
4 Reference architecture forward and return channel specifications.10
5 Media access control layer specification .10
5.1 Overview .10
5.2 MAC packet transport .11
5.3 MAC packet structure .13
5.4 MAC packet delimiters .18
5.5 MAC protocol data units (PDUs) .18
6 MAC protocol operation .29
6.1 Non-volatile parameters.29
6.2 Duplex capabilities.29
6.3 Packet priorities.29
6.4 Packet reception.29
6.5 NE responses .30
6.6 Message sequence numbers and transaction synchronization .30
6.7 Solicited messages.31
6.8 Autonomous (unsolicited) messages.31
6.9 Return channel transmissions.35
6.10 MAC state machines.35
Annex A (informative) Operational details .38
A.1 Introduction.38
A.2 Time of day.38
A.3 Firmware downloads.38
A.4 NE addressing .38
A.5 Alarm processing HMS MAC protocol .39
A.6 Automatic channel discovery.43
A.7 Auto-registration .44
A.8 Configuration changes and SNMP trap generation .45
Figure 1 – Reference architecture diagram.10
Figure 2 – Bit transmission order.12
Figure 3 – MAC packet structure .13
Figure 4 – MAC header control byte – Bit definition.13
Figure 5 – MAC header sequence byte – Bit definition .16
Figure 6 – MAC PDU structure .18
Figure 7 – STATRESP STATUS byte – Bit definition .20
Figure 8 – Return channel transmission permitted.35

60728-7-2  IEC:2003(E) – 3 –
Figure 9 – Contention state diagram .36
Figure 10 – Backoff state diagram.37
Figure A.1 – Property MIB usage .39
Table 1 – Transponder type classifications. 7
Table 2 – Generic MAC packet structure .13
Table 3 – Protocol field values .14
Table 4 – MAC PDUs .19
Table 5 – Possible MAC protocol transactions.19
Table 6 – NAK PDU format.20
Table 7 – ACK PDU format.20
Table 8 – STATRQST PDU format .20
Table 9 – STATRESP PDU format.20
Table 10 – CHNLRQST bit settings .21
Table 11 – CNTNRM bit settings .21
Table 12 – CNTCUR bit settings.21
Table 13 – MAJOR bit settings .21
Table 14 – MINOR bit settings .22
Table 15 – TALKRQST PDU format.22
Table 16 – TALK PDU format .23
Table 17 – CONTMODE PDU format.23
Table 18 – CONTMODE: MODE settings.23
Table 19 – NE message retrieval example .24
Table 20 – REG_REQ PDU format .25
Table 21 – SET_ADDR PDU format.25
Table 22 – REG_END PDU format .26
Table 23 – REG_END: STATUS settings.26
Table 24 – CHNLDESC PDU format .27
Table 25 – INVCMD PDU format .28
Table 26 – INVCMD: REASON codes.28
Table 27 – TIME PDU format .29
Table 28 – Non-volatile parameters.29
Table 29 – MAC sequence field example (non-contention mode).31
Table 30 – Contention state settings versus forward channel packets .32
Table 31 – Backoff state machine parameters .34
Table A.1 – Properties .40
Table A.2 – Alarm notification and retrieval – Polled mode.42
Table A.3 – Alarm notification and retrieval – Contention mode.43
Table A.4 – Auto-registration implementation example .45

– 4 – 60728-7-2  IEC:2003(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CABLE NETWORKS FOR TELEVISION SIGNALS,
SOUND SIGNALS AND INTERACTIVE SERVICES –
Part 7-2: Hybrid Fibre Coax Outside Plant status monitoring –
Media Access Control (MAC) layer specification
FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60728-7-2 has been prepared by technical area 5: Cable networks
for television signals, sound signals and interactive services, of IEC technical committee 100:
Audio, video and multimedia systems and equipment.
This standard was submitted to the national committees for voting under the IEC Fast Track
Procedure as the following documents:
CDV Report on voting
100/577/CDV 100/684/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

60728-7-2  IEC:2003(E) – 5 –
The committee has decided that the contents of this publication will remain unchanged until
2006. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
The following differences exist in some countries:
The Japanese de facto standard (NCTEA S-006) concerning requirements for the HFC
outside plant management, which was published in 1995, has already been available in
Japan. The purpose of this standard is to support the design and implementation of
interoperable management systems for HFC cable networks used in Japan.

– 6 – 60728-7-2  IEC:2003(E)
INTRODUCTION
Standards of the IEC 60728 series deal with cable networks for television signals, sound
signals and interactive services including equipment, systems and installations for
• head-end reception, processing and distribution of television and sound signals and their
associated data signals, and
• processing, interfacing and transmitting all kinds of signals for interactive services
using all applicable transmission media.
All kinds of networks like
• CATV-networks,
• MATV-networks and SMATV-networks,
• individual receiving networks,
and all kinds of equipment, systems and installations installed in such networks, are within
this scope.
The extent of this standardization work is from the antennas, special signal source inputs to
the head-end or other interface points to the network up to the system outlet or the terminal
input, where no system outlet exists.
The standardization of any user terminals (i.e. tuners, receivers, decoders, multimedia
terminals, etc.) as well as any coaxial and optical cables and accessories therefore is
excluded.
60728-7-2  IEC:2003(E) – 7 –
CABLE NETWORKS FOR TELEVISION SIGNALS,
SOUND SIGNALS AND INTERACTIVE SERVICES –
Part 7-2: Hybrid Fibre Coax Outside Plant status monitoring –
Media Access Control (MAC) layer specification
1 Scope
This part of IEC 60728 specifies requirements for The Hybrid Fibre Coax (HFC) Outside Plant
(OSP) Media Access Control (MAC) Layer. This standard is part of the series developed to
support the design and implementation of interoperable management systems for evolving
HFC cable networks. The HMS Media Access Control (MAC) layer specification describes the
messaging and protocols implemented at the Data Link Layer (DLL), layer 2 in the 7 layer
ISO-OSI reference model, that support reliable and efficient communications between HMS
compliant transponders interfacing to managed OSP network elements (NEs) and a
centralized head-end element (HE).
This standard describes the MAC layer protocols that must be implemented between all
Type 2 and Type 3 compliant OSP transponders on the HFC plant and the controlling
equipment in the head-end to support bandwidth management and reliable communications.
Any exceptions to compliance with this standard will be specifically noted herein as
necessary. Refer to Table 1 for a full definition of the type classifications.
Transponder type classifications referenced within the HMS series of standards are defined in
Table 1.
Table 1 – Transponder type classifications
Type Description Application
This transponder interfaces with legacy network equipment
through proprietary means.
Refers to legacy transponder
Type 0 equipment, which is incapable of
This transponder could be managed through the same
supporting the specifications
management applications as the other types through
proxies or other means at the head-end.
This transponder interfaces with legacy network equipment
Refers to stand-alone transponder
through proprietary means.
equipment (legacy or new), which can
Type 1
Type 1 is a standards-compliant transponder (either
be upgraded to support the
manufactured to the standard or upgraded) that connects to
specifications
legacy network equipment via a proprietary interface.
This transponder interfaces with network equipment
designed to support the electrical and physical
Refers to a stand-alone, compliant
specifications defined in the standards.
Type 2
transponder
It can be factory or field-installed.
Its RF connection is independent of the monitored NE.
This transponder interfaces with network equipment
designed to support the electrical specifications defined in
the standards.
It may or may not support the physical specifications
Refers to a stand-alone or embedded,
Type 3
defined in the standards.
compliant transponder.
It can be factory-installed. It may or may not be field-
installed.
Its RF connection is through the monitored NE.
2 Normative references
None.
– 8 – 60728-7-2  IEC:2003(E)
3 Terms, definitions and abbreviations
For the purposes of this document, the following definitions apply.
3.1 Terms and definitions
3.2
data link layer (DLL)
layer 2 in the Open System Interconnection (OSI) architecture; the layer that provides
services to transfer data over the physical transmission link between open systems
3.3
forward spectrum
pass band of frequencies in HFC cable systems with a lower edge of between 48 MHz and
87,5 MHz, depending on the particular geographical area, and an upper edge that is typically
in the range of 300 MHz to 860 MHz depending on implementation
3.4
full spectrum
combined forward and return spectrums in HFC cable systems and excludes any guard band
3.5
guard band
unused frequency band between the upper edge of the usable return spectrum and the lower
edge of the usable forward spectrum in HFC cable systems
3.6
network element (NE)
active element in the outside plant (OSP) that is capable of receiving commands from a head-
end element (HE) in the head-end and, as necessary, providing status information and alarms
back to the HE
3.7
open system interconnection (OSI)
a framework of the International Organization for Standardization (ISO) standards for
communication between multi-vendor systems that organizes the communication process into
seven different categories that are placed in a layered sequence based on the relationship to
the user. Each layer uses the layer immediately below it and provides services to the layer
above. Layers 7 through 4 deal with end-to-end communication between the message source
and destination, and layers 3 through 1 deal with network functions
3.8
organizationally unique identifier (OUI)
a 3-octet IEEE assigned identifier that can be used to generate universal LAN MAC addresses
and protocol identifiers per ANSI/IEEE standard 802 for use in local and metropolitan area
network applications
3.9
physical (PHY) layer
layer 1 in the open system interconnection (OSI) architecture; the layer that provides services
to transmit bits or groups of bits over a transmission link between open systems and which
entails electrical, mechanical and handshaking procedures
3.10
return spectrum
pass band of frequencies in HFC cable systems with a lower edge of 5 MHz and an upper
edge that is typically in the range of 42 MHz to 65 MHz depending on the particular
geographical area
60728-7-2  IEC:2003(E) – 9 –
3.11
transponder
device that interfaces to outside plant (OSP) NEs and relays status and alarm information to
the HE. It can interface with an active NE via an arrangement of parallel analogue, parallel
digital and serial ports
3.12 Abbreviated terms
CCITT Comité Consultatif International de Télégraphie et Téléphonie
(ITU – International Telecommunication Union)
CRC Cyclic Redundancy Code
DLL Data Link Layer
EMS Element Management System
FCS Frame Check Sequence
HE Head-end Element
HEX Hexadecimal
HFC Hybrid Fibre Coax
HMS Hybrid Management Sub-Layer (defined in the standard)
I/G Individual / group address bit
IEEE Institute of Electrical and Electronics Engineers
IP Internet Protocol
ISO International Organization for Standardization
LSB Least Significant Bit
MAC Media Access Control
MIB Management Information Base
MSB Most Significant Bit
NE Network Element
OSI Open System Interconnection
OSP Outside Plant
OUI Organizationally Unique Identifier
PDU Protocol Data Unit
PHY Physical
POSIX Portable Operating System Interface
RF Radio Frequency
RFC Request for Comment
RSVD Reserved
SNMP Simple Network Management Protocol
TOD Time-of-Day
UART Universal Asynchronous Receiver/Transmitter
UDP User Data-gram Protocol
– 10 – 60728-7-2  IEC:2003(E)
4 Reference architecture forward and return channel specifications
The reference architecture for the series of specifications is illustrated in Figure 1.
Fiber Node
RF
RF Optical RF
Laser
RECEIVER
TRANSMITER Receiver
Splitter
RF Amplifier Chain
Headend
Status
Status
Monitoring
*
Monitoring
Diplexer
Device
Equipment
Optical RF
RF RF
Laser
Receiver Combiner
RECEIVER TRANSMITER
B C A
* The diplexer filter may be included as part of the network element to which the
transponder interfaces, or it may be added separately by the network operator.
IEC  2293/03
Figure 1 – Reference architecture diagram
All quantities relating to forward channel transmission or reverse channel reception are
measured at point A in Figure 1. All quantities relating to forward channel reception or reverse
channel transmission are measured at point B for two-port devices and point C for single-port
devices as shown in Figure 1.
5 Media access control layer specification
5.1 Overview
This clause describes the MAC protocol. Some of the MAC protocol features include:
• support for transaction-based message exchange over the HFC forward and return RF
channels. Transactions can be initiated by either the HE or the NE; support for transport of
multiple network PDU types over the HFC forward and return RF channels including, but
not limited to, IP over serial and SNMP over serial;
• extensions provided to support future transport of other network PDU types;
• efficient use of HFC forward and return RF spectrum through central HE management of
NE transmission opportunities.
5.1.1 Definitions and conventions
5.1.1.1 Separate forward and return channels
The one-way communication channel from the HE to a managed OSP NE is referred to as the
forward channel. The one-way communication channel from a managed OSP NE to the HE is
referred to as the return channel. Both the forward and the return channels are placed on
specific centre frequencies. The forward and return channels’ centre frequencies are different.
Since the NEs only listen to the forward channel, they cannot listen to return channel
transmissions from other NEs. This channel separation is a result of the sub-band split
between the forward and return portions of the typical HFC plant spectrum.

60728-7-2  IEC:2003(E) – 11 –
5.1.1.2 Single forward and return path channels per MAC layer domain
To keep management of carrier frequencies simple, each status monitoring system has a
single forward channel and a single return channel. This does not preclude the use of multiple
monitoring systems, each with its own individual forward and return RF channels.
A MAC layer domain consists of a single forward RF channel and a single return RF channel
over which a single MAC layer bandwidth allocation and management protocol operates. It
includes a centralized HE and multiple compliant transponders interfacing to managed OSP
NEs. The centralized HE may support multiple HMS-based status monitoring systems, i.e.
multiple MAC layer domains. Each OSP NE must only access a single forward channel and its
associated single return channel, i.e. it must only operate within a single HMS MAC layer
domain.
5.1.1.3 Network element (NE) term usage
The HMS MAC layer supports bandwidth management and reliable communications between
a HE and multiple compliant transponders that interface to managed OSP NEs. Throughout
this standard, the terms “compliant transponder”, “transponder”, and “NE” are used
interchangeably when describing the MAC processes that support the exchange of data or
other information between two or more entities at the DLL.
5.1.1.4 Packet
A packet is a unit of data exchanged between the HE and any of a number of managed OSP
NEs at the DLL. Packets are strings of bytes that can be sent contiguously or be separated by
periods of silence. Document Outside Plant Status Monitoring – Physical (PHY) Layer
Specification describes specific byte transmission modes that must be implemented in both
forward and return channels. A MAC packet consists of a MAC header, a variable-length
payload, and a frame check sequence (see 5.3).
5.1.1.5 Most significant byte
Unless otherwise specified, it is assumed throughout this standard that the left-most entry in
any numeric value is the most significant, i.e. for the address represented as 12-34-56-78-9A-
BC the left-most entry ‘12’ is the most significant value.
5.1.1.6 Byte number representation
Throughout this standard, bits labelled ‘0’ are the least significant bits (LSBs) and bits
labelled ‘7’ are the most significant bits (MSBs). The bits in a given byte will be described with
bit 7 (MSB) at the left and bit 0 (LSB) at the right. This convention has been adopted for
presentation purposes only and has no effect on the actual bit transmission order. Bit
transmission order details are provided in 5.2 of this standard.
5.1.1.7 Reserved bits
A number of bits are indicated with the word “Reserved” or the abbreviation “RSVD” in the
various MAC packets described in this standard. Any receiving NE must ignore these bits
when implementing this version of the MAC protocol.
5.2 MAC packet transport
5.2.1 Byte transmission format
Bytes transmitted over both forward and return channels are ten bits in length. They contain
one start bit, eight bits of data, and one stop bit. The start bit has the binary value ‘0’, and the
stop bit has the binary value ‘1.’

– 12 – 60728-7-2  IEC:2003(E)
5.2.2 Byte transmission order
Fields consisting of multiple bytes, i.e. a MAC address will have the most significant byte
transmitted first. Any exceptions to this rule will be specifically noted in this standard as
necessary.
5.2.3 Bit transmission order
The LSB of a single byte, bit 0, is always transmitted first following the start bit. The MSB of a
single byte, bit 7, is always transmitted last followed by the stop bit. The transmission order is
summarized in Figure 2.
IEC  2294/03
Figure 2 – Bit transmission order
NOTE In the NCTEA S-006, eleven bits data format is used as follows:
– start bit(0);
– LSB bit(0) ~ MSB bit(7);
– parity bit;
– stop bit(1).
5.2.4 Transmission timing
5.2.4.1 Forward channel packets
5.2.4.1.1 Timing
Forward channel packets shall be transmitted in such a manner that
a) no two bytes within a packet are separated by more than 3 ms, and
b) the entire packet must be transmitted within 120 % of the shortest time for that frame. The
shortest time is defined as the time for transmission of the packet with no gaps between
bytes.
5.2.4.2 Return channel packets
5.2.4.2.1 Front porch
NE transmission of the first byte of a message shall begin within a window of two and five
byte times after the transmitter power reaches 90 % of its final value. Until the first byte is
transmitted, the frequency will rest on the ‘mark’ frequency. This is standard Universal
Asynchronous Receiver/Transmitter (UART) transmission. The front porch ensures that the
receiving UART can be cleared of all framing errors prior to the start of reception of valid
data.
5.2.4.2.2 Timing
Return channel packets must be transmitted in such a manner that no two bytes within a
packet are separated by more than 260 μs (1 byte time). All bits within a single byte shall be
immediately contiguous; there shall be no gaps at bit boundaries within a byte.

60728-7-2  IEC:2003(E) – 13 –
5.3 MAC packet structure
MAC packets consist of a MAC header, a variable-length payload, and a two-byte frame check
sequence. Packet structure and sizes are identical for both forward and return channel
packets. MAC packet structure is illustrated in Figure 3.
Start End
Synch Control Address Sequence Length Payload FCS
IEC  2295/03
Figure 3 – MAC packet structure
All MAC packets must have the general format as described in Table 2.
Table 2 – Generic MAC packet structure
Field name Length (bits) Subclause
Synch 8 5.3.1
Control 8 5.3.2
Address 48 5.3.3
Sequence 8 5.3.4
Length 16 5.3.5
Payload N 5.3.6
FCS 16 5.3.7
5.3.1 Synch
The synch field consists of a single byte and identifies the start of the MAC layer packet. It
shall be set to 0xA5.
5.3.2 Control
The control field consists of a single byte and defines the type and format of the payload field.
The bit definition of the control byte is shown in Figure 4. The control field also serves, in
conjunction with the synch, length and FCS fields, as a packet delimiter as described in 5.4.
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
RSVD3 RSVD2 RSVD1 RSVD0 Protocol
IEC  2296/03
Figure 4 – MAC header control byte – bit definition
5.3.2.1 Protocol (bits 3:0)
The four-bit protocol field indicates the type of protocol to be used to interpret the payload
field of the MAC layer packet. In addition, the protocol field allows the message service
handler to pass messages with alternative protocol values to other upper layer processes
without having to unravel the entire message. The value represented by the protocol field
shall be as assigned in Table 3.

– 14 – 60728-7-2  IEC:2003(E)
Table 3 – Protocol field values
Description Binary value
MAC management message 0000
SNMP over serial (see note below) 0001
IP over serial 0010
SNMP trap over serial 0011
a
Available for future use 0100 to 1111
NOTE This is SNMPv1 as defined by RFC 1157. However, the UDP and IP protocols
are not used for this implementation. Thus, all references by RFC 1157 to UDP are not
relevant. Subclause 3.2.4 of RFC 1157 explains how the SNMP mechanisms are
suitable over different transport protocols. Clauses 4 and 4.1 of RFC 1157 explain this
further. In fact, in 4.1, the RFC states: “Other transport services may be used to support
the SNMP.”
a
Protocol 0101 is not allowed to prevent accidental creation of a synch byte (0xA5).
5.3.2.2 RSVDx (bits 7:4)
The bits identified as RSVD are reserved for future use. They must be set to 0.
5.3.3 Address
The Address field consists of six bytes. It is used to address devices on a unicast, multicast,
or broadcast basis. The address field follows the IEEE Organizationally Unique Identifier
(OUI) Std 802 usage for a Universal address. For clarity, this standard conforms to the
address documentation suggested by the IEEE as follows:
a) each byte is represented as a two digit hexadecimal numeral (with no radix identification)
using leading zeroes where the first (left-most) digit of the pair is the most significant.
b) each byte is separated by hyphens, with the most significant byte in the left-most position.
An example address is 00-AA-BB-00-43-21.
A universal address is a sequence of six bytes. The first three take the values of the three
bytes of the OUI in order. The last three bytes are administered by the assignee. The binary
representation of an address is formed by taking each byte in order and expressing it as a
sequence of eight bits, LSB to MSB, left to right. For example, the OUI AC-DE-48 could be
used to generate the address
AC-DE-48-00-00-80
Whose binary representation is:
First byte of Second byte of Third byte of
OUI OUI OUI
0011 0101 0111 1011 0001 0010 0000 0000 0000 0000 0000 0001
I/G Address bit
LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB
CA E D 8 4 0 0 0 0 0 8
The first (left-most) bit in the binary representation of the MAC address is the I/G
(Individual/Group) Address Bit. This bit is the LSB of the most significant byte. When set to 0
as shown above, it indicates an individual address. It may be set to 1 in an address allocated
by the assignee to indicate that that address is a group address. For example, the same OUI
above could be used to generate the group address.

60728-7-2  IEC:2003(E) – 15 –
AD-DE-48-00-00-80
First octet of Second octet Third octet of
OUI of OUI OUI
1011 0101 0111 1011 0001 0010 0000 0000 0000 0000 0000 0001
I/G address bit
LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB LSB MSB
DA E D 8 4 0 0 0 0 0 8
The address shall be transmitted the most significant byte first and the least significant byte
last.
5.3.3.1 Unicast
The Unicast address is the unique address assigned to a particular NE. An NE transmitting a
message places its unicast address in the Address field, most significant byte first. This
address is completely unique across all manufacturers. By definition, the I/G bit is set to 0.
Each vendor shall obtain an address prefix, or OUI, from the IEEE and assign a unique
address using this prefix to each HMS-compliant transponder at time of manufacture. This is
the Unicast address for that NE. This standard places no restriction on the number of OUIs a
single manufacturer may obtain as the IEEE governs that. An OUI assignment allows the
assignee to generate approximately 16 million addresses by varying the last three octets.
5.3.3.2 Broadcast
A message with a broadcast address is intended for all NEs that receive it. All NEs must
support the broadcast address. The broadcast address is FF-FF-FF-FF-FF-FF.
5.3.3.3 Multicast
The multicast address follows the IEEE Standard 802 for indicating a group Address, i.e. the
I/G address bit is set to 1.
A multicast address defines a group to which zero, one, or more than one NE has been
assigned by a higher level management system. An NE maintains a list of multicast addresses
to which it will respond. An NE is a member of a particular multicast group if at least one of its
provisioned multicast addresses matches that particular multicast address. The assignment
and usage of multicast addresses is out of the scope of this standard. However, examples
might be fault isolation, frequency changes, and firmware downloads.
All NEs must support a minimum of four (4) multicast addresses not counting the broadcast
address. This standard places no maximum limit on the number of multicast address groups
an NE may support.
Multicast addresses are not assigned at manufacture. The network provider provisions
multicast addresses into the NE. The method for this provisioning is out of the scope of this
standard. To avoid accidental assignment to the wrong multicast address, all multicast
addresses held at the NE shall default to the broadcast address prior to provisioning.
5.3.4 Sequence
The MAC protocol is transaction-based, i.e. every originating message from a “requestor” has
a corresponding response from the “responder” regardless of which device originated the
message. The sequence field consists of a single byte and defines a message sequence
number to ensure message exchanges are synchronized. In order to handle possible loss of
messages in either the forward or the return channel, and to avoid duplication of messages at

– 16 – 60728-7-2  IEC:2003(E)
the application layer, all messages have a sequence number. The bit definition of the
sequence byte is shown in Figure 5.
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
SYN MSGSEQ
IEC  2297/03
Figure 5 – MAC header sequence byte – bit definition
5.3.4.1 MSGSEQ (Bits 6:0)
The 7-bit MSGSEQ field indicates the message sequence number. Sequence numbers are
generated by either the HE or NE. Sequence number generation, modification and elicited
behaviour must conform to the following rules:
a) HE-originated transactions will have bit 6 of the MSGSEQ field set to 1. Thus, the range for
HE-generated sequence numbers is 0x40 through 0x7F with wraparound to 0x40;
b) the HE shall generate and track sequence numbers per unicast address;
c) HE-generated messages directed to broadcast or multicast addresses, i.e. where the I/G
bit is set to 1, shall have a sequence number of 0 since this field is ignored by the NE for
multicast and broadcast messages. Upon receiving a valid broadcast or multicast message
from the HE, the NE shall not change the last received HE sequence number. The NE shall
always process broadcast or multicast messages regardless of sequence number;
d) NE-originated transactions will have bit 6 of the MSGSEQ field set to 0. Thus, the range for
the NE is 0x00 through 0x3F with wraparound to 0x00;
e) as a requestor, the NE can have only a single originating message requiring a response
outstanding at any time (see 6.8). Thus, the NE only needs to track the sequence number
for this single value;
f) the MSGSEQ field is incremented by the originator of the number. The sequence number
shall be incremented only:
1) when a response is received with a matching sequence number (excluding the SYN
bit, see 5.3.4.2), or
2) when a requestor’s maximum allowed number of MAC layer transmission retries has
been exceeded. The MSGSEQ shall not change if a message must be retransmitted at
the MAC layer, which occurs only when a response or acknowledgement has not been
received within a pre-determined timeout window, and the maximum allowed number of
MAC layer transmission retries for this message has not been exceeded. See clause 6
of this standard for additional details on MAC protocol operation;
g) the responding entity shall save the sequence number of the last received message
directed to its unicast address and perform one of the following:
1) if the MSGSEQ field is different from the last one seen, the responder should process
the message, form a response if one is required, and send it. The responding entity
shall take the value of the sequence number in the MSGSEQ field from the request
and place it in the MSGSEQ field of its response,
2) if the MSGSEQ field is the same as the last one seen, the responder knows that its
last response was not received and it should resend the response. The responder
should not process the message again. It must simply resend the previously
transmitted message. The responding entity shall take the value of the sequence
number in the MSGSEQ field from the request and place it in the MSGSEQ field of its
response;
h) if the responding entity has been reset, it shall process the first received message
directed to its unicast address regardless of the value in the MSGSEQ field.
NOTE A possible implementation refinement to ensure subsequent message exchanges are synchronized is to
initialize the “last sequence number” to an out-of-range value that a requestor cannot possibly support. This would
guarantee that the first message from a responder after a reset always has a unique sequence number associated
with it that forces the requestor to re-issue the original request thus ensuring message exchange re-
synchronization.
60728-7-2  IEC:2003(E) – 17 –
5.3.4.2 SYN (Bit 7)
The following rules govern the selection of the SYN bit value and elicited behaviour:
a) after a device (HE or NE) is reset, it shall set the SYN bit to 1 in every packet it originates
and sends to a given responder until the first correct response is received from that
responder. When the SYN bit is set to 1 by a requestor, the responder is not to verify the
MSGSEQ field in the message. The MSGSEQ field contained within the packet is to be
used by the requestor as the last received value. This will synchronize the responder to
the current requestor sequence number;
b) the SYN bit shall always be set to 0 when responding to a request. When the SYN bit is
set to 0 by a requestor, the responder is to verify the MSGSEQ field in the packet.
Clause 6 of this standard provides additional information on the use of the s
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