ISO 8802-7:1991
(Main)Information technology — Local area networks — Part 7: Slotted ring access method and physical layer specification
Information technology — Local area networks — Part 7: Slotted ring access method and physical layer specification
Technologies de l'information — Réseaux locaux — Partie 7: Spécifications pour la méthode d'accès et la couche physique relatives à l'anneau à découpage temporel
General Information
Standards Content (Sample)
IS0
I N TE R NAT I O NA L
STANDARD 8802-7
First edition
1991-06-15
Information technology - Local area
networks -
Part 7:
Slotted ring access method and physical layer
spec if icat i on
Technologies de I'informaiion - Réseaux locaux
Partie 7: Spécifications pour la mBthode d'accds el la couche physique
relatives à l'anneau à d6coiipage temporel
Reference number
IS0 8802-7:1991(E)
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IS0 8802-E1991 (E)
Contents
Page
1 Scope . 1
2 Normative references . 2
3 Definitions . 2
4 Specification for the coding of bits and structure of slots and
mini-packets . . 5
4.1 Encoding of bits . 5
4.2 Slots . 5
4.3 Mini-packet structure . 6
5 Specification for configuration components . 7
5.1 Monitor . 7
5.2 Logging station . 7
5.3 Nodes . 8
5.4 Free-standing repeaters . 8
5.5 Slave power supplies . 8
5.6 Ring cables . 8
5.7 Ring connector sockets . 9
5.8 Continuity plugs . 9
5.9 Topology . 10
5.10 Total SR-LAN size . 10
5.11 Ring segment length . 10
O IS0 1991
Ali rlghts reserved . No part of this publication may be reproduced or utilized in any form
or by any means. electronic or mechanical. Including photocopying and rnlcrofllrn. without
permission In writing from the publisher .
international Organization for Standardization
Case Postale 56 CH-I211 Genève 20 Switzerland
Printed in Switzerland
ii
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IS0 8802-71 991 (E)
5.12 Combination of components . 11
6 Specification for free-standing repeaters . 11
6.1 Compliance . 11
6.2 Interfaces . 12
6.3 Power supply . .
6.4 Production of output . 14
6.5 Output in absence of input signal . 14
6.6 Output in presence of input signal . 14
7 Specification for nodes . 15
7.1 Compliance . . 15
7.2 Interfaces . 16
7.3 Operator controls . < . 19
7.4 Power supply . . 19
7.5 Output in absence of input signai .
7.6 Output in presence of input signal .
7.7 Slot structure . . 20
7.8 Input and output mini-packets . 21
7.9 Error procedures . . 22
7.10 Reception of mini-packets . 23
7.11 Transmission of mini-packets . . 27
7.12 Node/DTE interface lines . . 29
7.13 Reading from node .
7.14 Writing to node . 37
7.15 Enabling and disabling . 39
8 Specification for monitor . 41
8.1 Compliance .
8.2 Connectors
8.3 Operator controls .
8.4 Power supply
8.5 Modes of operation .
8.6 Production of output . 44
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IS0 8802-7:1991(E)
8.7 Start/restart mode . 45
I
8.8 Run mode . 46
8.9 Resynchronization mode . 48
8.10 Error reporting procedures . 48
9 Specification for logging station . I. 49
I
9.1 Compliance .
I
9.2 Interfaces and controls . . 50
9.3 Power supply . . 50
9.4 Production of output . . 51
Output in absence of input signal . 51
9.5
9.6 Output in presence of input signal .
9.7 Slot structure . . 51
9.8 Processing of mini-packets . 51
9.9 Errors detected by logging station . . 52
9.10 Error recording . 52
I
9.11 Analysis of messages . I. 53
~
9.12 Combination with monitor .
9.13 Combination with node . :. 53
10 Specification for slave power supplies .
10.1 Compliance .
10.2 Power rating . . 54
10.3 a.c. component of output . 54
10.4 Control of output . 54
Connection to ring .
10.5
10.6 Combination with other devices . 54
Specification for media access control procedures for data
11
terminal equipment . 55
11.1 Notation . 55
11.2 Media access control service . 55
11.3 Media Access Control Unitdata Status Reporting Service . 59
11.4 Transmission frame format . 61
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IS0 8802-Rl991 (E)
11.5 Procedure for transmission of frames . 62
11.6 Method of transmitting logical mini-packets . 65
11.7 Determination of transmission result . 66
11.8 Reception of logical mini-packets . 67
11.9 Mapping of logical mini-packets on to frames . 70
11.10 Processing . 70
11.11 Issue of MA-UNITDATA indication . 72
11.12 Functions at the node interface . 73
11.13 Node interface lines . 73
11.14 Read registers . 75
11.15 Write registers . 78
12 Specification for implementation requirements for media access
control in general purpose terminal equipment . 81
12.1 Compliance . 81
12.2 Testing . 81
12.3 MAC service access points and addresses . 81
12.4 Implementation of MAC service at MAC-SAPS . 82
12.5 Node class compatibility . 82
12.6 Action on MA-UNITDATA request . 83
12.7 Frame format . 84
12.8 Frame transmission . 84
12.9 Transmission of mini-packets . 85
12.10 Determination of response . 86
12.11 Reception of mini-packets . 86
12.12 Mapping of mini-packets on to frames . 86
12.13 Processing of frames . 86
12.14 Issue of MA-UNITDATA indication . 87
12.15 Management functions . 88
12.16 Node interface connector . 88
12.17 Implementation of node interface fundions . 88
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IS0 8802-7:1991(E)
12.18 Technical documentation . 88
Annexes
Ring segment length test . 90
A
A.l Principle . 90
A.2 Equipment . 90
A.3 Test procedure . 90
B Sensitivity test . 93
B.1 Equipment . 93
8.2 Procedure .
C Closed loop transfer characteristics test .
C.l Principle . . 94
C.2 Apparatus . . . 94
Procedure . . 94
C.3
D Illustration of power and signal arrangements .
E Repeater classes .
E.l Distinction between classes . 97
Class I: repeaters with first order phase locked loops . 97
E.2
E.3 Class II: repeaters . 97
E.4 Class III: repeaters with dominant second order phase locked
loops . .
F Compatibility of nodes .
F.l Compatibility with other nodes .
F.2 Compatibility with monitor .
F.3 Error message logging function . 98
F.4 Compatibility with DTEs . 98
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IS0 8802-7:1991 (E)
Foreword
IS0 (the International Organization for Stanclardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work
of preparing International Standards is norinally carried out through IS0
technical committees. Each member body interested in a subject for
which a technical committee has been esta'blished has the right to be
represented on that committee. Internatiohai organizations, govern-
mental and non-governmental, in liaison with ISO, also take part in the
work. IS0 collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting PiJblication as an Inter-
naiional Standard requit-es approval by at least 75 % of the member
bodies casting a vote.
International Standard IS0 8802-7 was prepaired by Technical Committee
ISO/TC 97, lnforrnation processing systems
This part of IS0 8802 is part of a series of documents that form a single
standard for Local Area Networks. The standard embraces four alter-
native media access technologies and a common logical link control
(LLC) protocol (IS0 8802-2)
OS1 REFERENCE MODEL
-
r
-*I
I
LO6ICAL LINK CONTROL IS0 8802-2
DATA LINK
I
I
MAC - Servlce Intertuce
LAYER
PHYSICAL LAYER
The combination of the Logical Link Control (IS0 8802-2) and any one
of the four media access documents form the Data-Link and Physical
layers of the IS0 7498 (Open System Interconnection Basic Reference
Model) The Cour media access technologies are
- IS0 8802-3. a bus utilizing CSMA/CD as ihe access method;
- IS0 8802-4. a bus utilizing token passing as the access method;
- IS0 8802-5, a ring utilizing token passing as the access method:
- IS0 8802-7, a ring utilizing slotted ring as the access method.
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IS0 8802-7~1991 (E)
All four media access technologies provide the Media Access Control
(MAC) Sublayer Interface Service, as specified in IS0 8802-2. It is
through this sewice interface that the LLC sublayer and higher layer
entities communicate across the physical media.
It should be noted that the exact relationship of layers described in this
standard to the layers defined in the OS1 Reference Model is for further
study.
IS0 8802 consists of the following parts, under the general title Informa-
tion technology - Local area networks:
- Part I: General introduction
- Part 2: Logical /ink control
- Part 3: Carrier sense multiple access with collision detection
(CSMAICD) access method and physical layer specifications
- Part 4: Token-passing bus access method and physical layer spec-
ifications
- Part 5: Token ring access method and physical layer specifications
- Part 7: Slotted ring access mefhod and physical layer specification
Annexes A, E3 and C form an integral pari of this part of IS0 8802. An-
nexes D, E and F are for information only.
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IS0 8802-7:1991(E)
I nt roduct ion
0.1 A slotted ring local area network
This section gives a general description of the implementation of a
slotted ring local area network.
0.2 Principles
At the lowest level the communication link comprises a closed ring of
cable and active repeaters. The delay in signal propagation through the
cable and repeaters means that the systenn may be considered as a
continuously circulating shift register. Each 100 m of cable causes a
delay of 450 ns and so may be thought of as storage of 4,5 bits for a
10 MHz signalling frequency.
The circulating storage contains a fixed number of bits for any particular
configuration of the network; in order to provide communication a regu-
lar structure is imposed on these bits by a Monitor. This consists of a
all the same length, with a single
small number of concatenated slots,
gap of a few bits to mark a complete cycle. The start of each slot is in-
dicated by a leader bit which is always one. The gap contains only zero
bits. Very simple algorithms may be used to synchronize with this
structure.
The signalling frequency is nominally 10 MHz but may vary slightly to
produce an integral number of bits in circulation. To allow for adjust-
ment of the amount of circulating storage a variable size shift register
is used to provide additional “padding”. The variable padding is set to
permit an integral number of slots and a small gap.
The repeaters regenerate the signals on to each section of the ring, al-
lowing rings up to several kilometres to be constructed, and providing
for different cable types to suit the environment of each part of the net-
work The repeaters are also the points of attachment to the network for
communicating devices. They demodulate signals from the ring and
present them to a station which may alter them before they are modu-
lated on to the ring again.
An active system of this kind lends itself to the implementation of a
number of low level error detection and correction techniques that can
be used to quickly localize faulty devices.
Since the operation of the repeaters is essential to ring integrity they
are powered along the ring cable and are thus independent of the sta-
tion and attached device.
0.3 Access
Communication takes place between stations which synchronize to the
slot structure. The unit of transmitted data between stations is known
as a mini-packet and occupies exactly one slot. Each mini-packet is in-
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IS0 8802-7:1991(E)
dividually addressed, carrying eight bit destination and source ad-
dresses, 2, 4, 6, or 8 bytes of user data are carried, and in addition two
“type” bits that provide for flexible user identification or framing of the
data. The first bit following the slot leader bit is a fulliempty marker,
used to control access to the slots. Following the user data and type bits
are two response bits.
A station wishing to transmit waits until an empty slot arrives; it then
marks it as full, inserts the addresses and data, and initializes the re-
sponse bits. The transmitter is only allowed one mini-packet in flight at
the time and it counts passing slots to determine when the mini-packet
it transmitted returns. The transmitter marks the returning slot empty
and copies the response bits. By requiring that the returning slot be
emptied, round-robin scheduling is introduced in which, however heavily
loaded the ring, each slot passes around the stations giving them an
opportunity to transmit within a determined time.
The response bits are used to carry back low level flow control infor-
mation to the transmitter. The receiver may be physically or logically
absent from the ring and in this case the transmitter will see that the
mini-packet has been ignored. The receiver may also indicate that al-
though he is present he is not listening; a “source selector” inay be set
to “anyone”, “no-one” or “n”. When it is set to no-one or a value of n
different to that of the transmitter address, the response bits indicate
“not selected” to the transmitter. Thus the receiver is able to listen to
all transmitters (anyone) and multiplex the incoming mini-packet indi-
vidually or may concentrate on one transmitter (n = transmitter ad-
dress), allowing very simple implementation of block protocols. The
receiver can also generate a “busy” response when unable to process
the mini-packets as fast as the transmitter tries to send them. A trans-
mitter seeing the busy response sends the data again in a new slot. A
mini-packet that has been successfully received is marked as
” .
“accepted
Some refinements to this basic mechanism enhance its performance
and reliability.
Whenever a transmitter receives a response other than accepted it is
not permitted to transmit immediately but must wait for the ring struc-
ture to cycle aroiind. Subsequent unsuccessful transmission attempts
cause the transmitter to be backed off by a number of slots. This pre-
vents the ring being swamped with useless traffic. Further, the delay is
made traffic dependent by requiring that only empty slots are counted
for backoff purposes. The round-robin scheduling puts an upper limit on
the delay while the variable backoff produces a system in which effi-
ciency improves under load.
When a mini-packet returns, a transmitter is able to check for consist-
ency with the mini-packet that was sent out. If a discrepancy is fouiid,
then the response seen by the attached DTE is “transmit error”, over-
riding the response within the mini-packet.
A repeater and station together make up a Node that is common to all
DTE’s communicating on the network. Logic is required to interface the
Node to any particular DTE and this logic is called an Interface Unit.
0.4 Maintenance
In addition to the error detection used by the transmitter, facilities are
included to continually monitor the entire system.
Every slot includes a parity bit that is checked and maintained by all
nodes. A node that detects a slot with faulty parity corrects it and also
sends a fault message in the next empty slot to destination zero. The
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IS0 8802-7:1991 (E)
fault message contains the address of the sending node and SO indi-
cates the section of the ring where the fault occurred.
The mini-packet structure includes a bit, the Monitor Pass bit that is set
by a transmitter when it fills a slot. This bit is always cleared by the
Monitor on passing slots. If the Monitor detects a slot that has this bit
slot empty. It is, there-
cleared but is still marked full then it marks the
fore, impossible for a fault to cause a slot to become permanently full.
The Monitor is able to detect errors that interfere with the perrnanent
structure and rapidly reinstates the correct structure in a non-
destructive way. A burst of errors, perhaps caused by a power dip,
to reinitialize the network, resetting its basic fre-
cause the Monitor
quency.
A Monitor can also fill empty slots with random address and data as
they pass and check them as they return if they are still marked empty.
In this way the Monitor keeps the perforrnance of the ring under contin-
uous surveillance and can give warning of incipient faults.
0.5 Performance
The access control rules require that a transmitter has only one miiii-
packet in flight at a time, and also that a slot is emptied after it is used.
This specification also requires that the slot after the one emptied be
allowed to pass. Thus the maximum slot utilization that a transmitter can
achieve is one in every (n + 2) slots. Therefore, the maximum point to
point data transmission rate is
n,, = RJ(n -1- 2)
B, is the total system data bandwidth, and is a function of the bit rate
(clocking frequency) ‘If”, the number of slois on the ring “n”, the number
of data bits in each slot “I” and the number of bit periods in the
“8”:
gap
If 171 > 1 nodes all wish to transmit simultaneously then each Node is still
guaranteed access to one in every (rn 4- n) slots (provided of course it
is effectively shared
is not backed off by a slow receiver). The bandwidth
out with each Node achieving at least flsl(i?z + n) unless limited by the
maximum point to point bandwidth B, If 772 > n -t- 2 then this maximum
Ilpp is reduced to fl, the traffic limited Landwidth.
BS
n, =
rn (12 -+- 2)
Notice that n + 2 communicating pairs can achieve B without traffic
limiting of the bandwidth. Figure0.1 shows a typical ghed Ring net-
work configuration, and the relationship between OS1 Architecture,
Functional blocks and the Implemeiitatiori of the Slotted Ring LAN
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IS0 8802-7:1991 (E)
0J.E ]
STATION MONITOR
CONTINUITY
I NQDE 1
PLUG
1
4
I
DATA FLOW
PHYSICAL
SEGMENT
I
I ---.--I
LOGICAL
SEGMENT
TYPICAL S. R- LAN CONFIGURATION
OS1 ARCHITECTURE FUNCTIONS IMPLEMENTATION
7, 1
MAC FRAME
I 1 ASSEMBLER DISASSEMBLER 1-
NODE
MAC sub-lnyer TRANSMISSION CHECKS
SLOT ACCESS RING CABLE
ENCODE, DECODE,
TRANSMIT, RECEIVE
Figure 0.1 - Typical SR-LAN configuration and relationship between
OS1 architecture - Functions - Implementation
xii
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INTERNATIONAL STANDARD IS0 8802-7:1991(E)
Information technology - Local area networks -
Part 7:
Slotted ring access method and physica layer specification
a) when transmitting
1 Scape
1) in the construction of a frame in response to
a request from the user of the media access
This part of IS0 8802 specifies the requirements for
control service;
a local area network that utilizes the slotted ring
access method, and that operates at
2) in the transfer of the frame to the local area
10 Mbitskecond data rate.
network node for transmission as a series of
min i-packets;
3 defines the terms used in this part of
Clause
IS0 8802.
3) in determining whether the mini-packets have
been successfully transmitted to the destina-
Clause 4 specifies the coding of bits and structure
tion node;
of slots and mini-packets.
Ouse 5 specifies, in general terms, the compo-
4) in notifying the user of the media access
nenls of a configured sloited ring local area network control service whether the frame has been
(SR-LAN); in detail, the functional requirements for successfully transmitted.
ring cable, ring connector sockets and ring continu-
NOTE 1 This event does not necessarily imply re-
ity plugs; and the constraints on tho size of, and the
ceipt by the remote inedia access control service user.
combination of components in, a configured
SR-LAN.
b) when receiving
Clauses 6, 7, 8, 9 and 10 specify in detail the func-
tional requirements for the following components:
1) in collecting from the node the contents of
received mi n i- pac ket s ;
free standing repeaters;
nodes;
2) in assembling the contents into frames;
monitors;
logging stations;
3) in transferring the contents of the frames to
slave power supplies.
the user of the media access control service.
Each component is specified in terms of its
This clause specifies requirements to be met in each
functionality and measurable characteristics at the
instance of the communication of a frame to and
ring connector plug; and in the case of nodes
from a single media access control service access
(clause 7) the functions at the node/DTE interface
point (MAC-SAP), with conditions applying when
connector and the processing of mini-packets.
multiplexirig frames to and from one or more
MAC-SAPS.
Clause 11 specifies the procedures to be used by
the DTE at the time of communications:
1
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IS0 8802-7~1991 (E)
This clause also specifies a range of additional
3 Definitions
functions from communication with the local node
that are not required for the communication of
For the purposes of this part of IS0 8802, the follow-
frames, but which may be used for management
ing definitions apply.
purposes and to improve efficiency and error de-
tection.
3.1 slotted ring local area network (SR-LAN): A
network for data communication within an area
Clause 12 specifies the implementation requirement
where maximum distances are in the order of 10 km;
for the media access procedures specified in
the network is in the form of a ring connecting all
clause 11.
attached data terminal equipment, around which
slots for data circulate continuously.
The functionality is described in terms of two
classes: Basic class being the minimum require-
3.2 ring: The physical communication medium
ment, and Enhanced Class being a defined exten-
comprising ring cable, ring connector sockets and
sion to the basic functionality. Certain optional
I U g s.
continuity p
features are also specified.
Each clause includes the specification of the infor- 3.3 ring cable: Any cable used in the ring, com-
mation that must be provided by the supplier. prising two twisted pairs conveying data, plus
or more conductors used for control voltage re
Excluded from this part of IS0 8802 is
(VSi,) and screening.
a) internal design information used to meet these
NOTE 3 This may be either a dedicated cable or a set
requirements;
of conductors from any suitable multi-conductor cable
which may carry other traffic.
requirements relating to safety, for which the
b)
3.4 Pair A: The twisted pair in a ring cable, carrying
regulations and standards relevant to the envi-
the positive d.c. supply and one of the two data
ronment in which the equipment is installed ap-
signalling channels.
Ply.
3.5 Pair B: The twisted pair in a ring cable carrying
the negative d.c. supply arid the other data
sign a I I i rig channel.
2 Normative references
3.6 ring connector socket: A socket at which the
monitor, repeater, node, logging station or continuity
The following standards contain provisions which,
plug is connected to the ring.
through reference in this text, constitute provisions
of this part of IS0 8802. At the time of publication,
3.7 ring connector plug: A plug to mate with the
the editions indicated were val
...
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