SIST ETS 300 278 E1:2003
(Main)Network Aspects (NA); Support of existing services with guaranteed constant bit rate and specified transfer delay on Metropolitan Area Network (MAN)
Network Aspects (NA); Support of existing services with guaranteed constant bit rate and specified transfer delay on Metropolitan Area Network (MAN)
DE/NA-053301
Omrežni vidiki (NA) – Podpora obstoječim storitvam z zagotovljeno konstantno bitno hitrostjo in določeno prenosno zakasnitvijo v velemestnih omrežjih (MAN)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2003
2PUHåQLYLGLNL 1$ ±3RGSRUDREVWRMHþLPVWRULWYDP]]DJRWRYOMHQRNRQVWDQWQR
ELWQRKLWURVWMRLQGRORþHQRSUHQRVQR]DNDVQLWYLMRYYHOHPHVWQLKRPUHåMLK 0$1
Network Aspects (NA); Support of existing services with guaranteed constant bit rate and
specified transfer delay on Metropolitan Area Network (MAN)
Ta slovenski standard je istoveten z: ETS 300 278 Edition 1
ICS:
35.110 Omreževanje Networking
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EUROPEAN ETS 300 278
TELECOMMUNICATION March 1994
STANDARD
Source: ETSI TC-NA Reference: DE/NA-053301
ICS: 33.080
Service, MAN
Key words:
Network Aspects (NA);
Support of existing services with guaranteed constant bit rate
and specified transfer delay on Metropolitan Area Network (MAN)
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
F-06921 Sophia Antipolis CEDEX - FRANCE
Postal address:
650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Office address:
c=fr, a=atlas, p=etsi, s=secretariat - secretariat@etsi.fr
X.400: Internet:
Tel.: +33 92 94 42 00 - Fax: +33 93 65 47 16
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1994. All rights reserved.
New presentation - see History box
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ETS 300 278: March 1994
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.
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ETS 300 278: March 1994
Contents
Foreword.5
1 Scope .7
2 Normative references .7
3 Definitions. 7
4 Symbols and abbreviations.9
5 Basic principles . 10
6 DQDB layer service model. 10
7 Functional architecture. 11
7.1 Access method. 11
7.2 Slot generation function at the HOB. 11
7.2.1 Frequency and periodicity . 12
7.2.2 Variability. 12
7.2.3 Basic data rates. 13
7.2.4 Compound data rates. 13
7.3 Functions at a DQDB node supporting a CBR interface. 14
7.3.1 Overview . 14
7.3.2 Common functions block . 14
7.3.3 PA-functions block. 15
7.3.3.1 PA-transmit functions. 15
7.3.3.1.1 Transmit interactions between PA
functions block and common functions
block. 15
7.3.3.1.2 PA segment header validation . 15
7.3.3.2 PA receive functions . 15
7.3.3.2.1 Receive interactions between PA
functions block and common functions
block. 15
7.3.3.2.2 PA segment header validation . 16
7.3.3.2.3 CBR convergence function selection . 16
7.3.4 CBR convergence functions block. 16
7.3.4.1 CBR convergence transmit function. 16
7.3.4.2 CBR convergence receive function . 16
7.3.5 CBR interfaces. 17
8 Protocol data unit format. 17
8.1 PA slot. 17
8.2 PA segment. 17
8.2.1 PA segment header fields . 17
8.2.1.1 VCI. 18
8.2.1.2 Payload_Type. 18
8.2.1.3 Segment_Priority. 18
8.2.1.4 Segment HCS . 18
8.2.2 PA segment payload . 18
9 Protocol performance constraints . 19
9.1 Delay constraints . 19
9.1.1 Delay constraints for voice connections. 19
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ETS 300 278: March 1994
9.2 Jitter constraints.19
9.3 Synchronisation constraints .19
9.4 Reconfiguration constraints .19
10 DQDB layer management interface in support of CBR service .20
Annex A (informative): Applications .24
A.1 Identified applications .24
A.1.1 Interconnection of CBR terminals.24
A.1.2 Video applications .24
A.1.3 Voice applications .24
A.1.4 Multi-application support .24
A.1.5 Multimedia applications.24
A.2 Delay considerations.25
A.2.1 Delay in network components.25
A.2.1.1 Transmission delay.25
A.2.1.2 Delay in the CBR Access Unit (CAU).25
A.2.1.3 Delay in a DQDB-DQDB bridge. 26
A.2.1.4 DQDB-ATM gateway.27
A.2.2 End-to-end delays for CBR connections - some examples .27
A.2.3 Access delay for CBR connections at 2 048 kbit/s .28
A.3 Feasibility of bandwidth allocation.29
A.3.1 Frame groups .29
A.3.2 Valid connections in a frame group .30
A.3.3 Bandwidth allocation algorithms.31
Annex B (normative): Protocol implementation conformance statement .32
B.1 Introduction.32
B.1.1 Instructions for completing this PICS proforma .32
B.1.2 Definitions.33
B.1.2.1 Status column notation.33
B.1.2.2 Support column notation.33
B.2 Identification of the Implementation.33
B.3 Identification of the protocol .34
B.4 Global statement of conformance .34
B.5 Major capabilities and features . 35
B.5.1 Properties of the PLCP.35
B.5.2 Slot generation function at the HOB .35
B.5.3 Functions at a DQDB node supporting a CBR interface .36
B.5.4 Protocol Data Unit (PDU) format.37
B.5.5 DQDB layer management service in support of CBR connection type.37
Annex C (informative): Bibliography.38
History .39
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ETS 300 278: March 1994
Foreword
This European Telecommunication Standard (ETS) has been prepared by the Network Aspects (NA)
Technical Committee of the European Telecommunications Standards Institute (ETSI).
The need has been identified for the support of applications that require guaranteed constant bandwidth
and specified transfer delay on a Distributed Queue Dual Bus (DQDB) subnetwork.
Therefore, the DQDB protocol has to be enhanced for the provision of Constant Bit Rate (CBR) connection
types.
This final draft ETS addresses the use of pre-arbitrated functions as defined in IEEE Standard 802.6 [1] to
support CBR connection types.
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Blank page
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ETS 300 278: March 1994
1 Scope
This European Telecommunication Standard (ETS) details enhancements to the Distributed Queue Dual
Bus (DQDB) access method as defined in IEEE Standard 802.6 [1] in order to provide Constant Bit Rate
(CBR) connection types for the support of existing services (guaranteed constant bandwidth, specified
transfer delay) on MANs, using semi-permanent connections in the range of n x 64 kbit/s to 2 Mbit/s bit
rate capability. Therefore, signalling protocol specifications are outside the scope of this ETS.
This ETS does not cover the broadband specific aspects and the interworking of MANs with broadband
networks. For the broadband related aspects the CBR services are defined in CCITT Recommendations
I.362 and I.363 and the protocol reference model to be used in the Asynchronous Transfer Mode (ATM)
based networks to support these services is included in CCITT Recommendation I.321.
Annex B provides the Protocol Implementation Conformance Statement (PICS) proforma for this ETS, in
compliance with the relevant requirements, and in accordance with the relevant guidance, given in ISO/IEC
9646-2.
2 Normative references
This ETS incorporates, by dated or undated reference, provisions from other publications. These normative
references are cited at the appropriate places in the text and the publications are listed below. For dated
references, subsequent amendments to or revisions of these publications apply to this ETS only when
incorporated in it by amendment or revision. For undated references the latest edition of the publication
referred to applies.
[1] IEEE Standard 802.6 (1990): "Distributed Queue Dual Bus (DQDB) Subnetwork
of a Metropolitan Area Network (MAN)".
[2] CCITT Recommendation G.101 (1988): "The transmission plan".
[3] CCITT Recommendation G.114 (1988): "Mean one-way propagation time".
[4] CCITT Recommendation G.131 (1988): "Stability and Echo (General
characteristics of the 4-wire chain formed by the international circuits and
national extension circuits)".
[5] CCITT Recommendation G.823 (1988): "The Control of jitter and wander within
digital networks which are based on the 2 048 kbit/s hierarchy".
3 Definitions
For the purposes of this ETS, the definitions defined in IEEE Standard 802.6 [1] apply.
In addition, this Clause contains those definitions that are considered to be essential for the understanding
of this ETS.
Access delay: the time which an octet spends in the transmit queue of a CBR access unit before it can
gain access to the bus.
Basic data rate: a data rate out of the following list:
- 64 kbit/s;
- 192 kbit/s;
- 384 kbit/s;
- 768 kbit/s;
- 1 536 kbit/s;
- 2 048 kbit/s.
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ETS 300 278: March 1994
The use of the term "basic data rate" implies that the Pre-Arbitrated (PA) slot generation process for a
CBR connection at a basic data rate is based on the frequency parameter pairs (M, N) where M=1 and
N=2, 4, 8, 16, 48 or M=2 and N=3.
CBR Access Unit (CAU): a DQDB node supporting a CBR interface.
CBR Convergence Functions (CCF) block: consists of the CCF transmit functions block and the CCF
receive functions block.
CBR Service User (CSU): sends and receives octets at a constant bit rate across a CBR interface of a
DQDB subnetwork.
CCF receive functions block: receives octets from the PA functions block and stores them in a buffer. It
forwards the octets to the CBR service user at precisely regular intervals according to the rate of the CBR
service.
CCF transmit functions block: receives service octets from the CBR interface. It stores these octets in
a buffer until they are requested by the PA functions block. Then they are passed on to the PA functions
block.
Common Functions (CF) block: provides functions which are needed by some or all of the other
functional blocks in the local DQDB layer subsystem. It provides the DQDB layer function of relaying the
slot octets and management information octets between the service access points to the local physical
layer subsystem.
Composite slot-switching: the service octets of a CBR connection may gain access to an offset in the
payload of a PA slot on one of the busses if, and only if, the pair virtual channel identifier of the PA slot,
offset is contained in a list of such pairs associated at connection set-up with the CBR connection on that
bus.
Compound data rate: is a (necessarily unique) sum of two or more basic data rates. The use of the term
"compound data rate" implies that the PA slot generation process for a CBR connection at a compound
data rate consists in generating the PA slots for the basic data rates of which it is the sum.
Constant Bit Rate (CBR): the time characteristic of an event or signal recurring at known periodic time
intervals, i.e. guaranteed constant bandwidth, specified transfer delay.
NOTE: The term for this characteristic which is used in IEEE Standard 802.6 [1] is "CBR".
CBR Service Data Unit (CSDU): are presented by the CSU at the specified rate for the CBR connection.
Similarly, the CBR service periodically delivers a CSDU to a CSU at the rate specified for the connection.
Dedicated slot-switching: the service octets of a CBR connection may gain access to an offset in the
payload of a PA slot on one of the busses if, and only if, the Virtual Channel Identifier (VCI) of the PA slot
has been associated at connection set-up with the CBR connection on that bus.
Delay end-to-end, one way: the time it takes for a CBR service octet to be transferred between two
corresponding terminal equipments.
Frame: refers to the 125 μs transmission frame of the Physical Layer Convergence Procedure (PLCP).
Frequency: is a pair of positive integer numbers (M, N). It implies that the PA slot generation for a CBR
connection is required to maintain the frequency of M slots every N frames.
PA functions block: controls the transfer in PA segment payloads of CBR service octets received from
the CBR convergence functions block. For this purpose it maintains one transmit table and one receive
table.
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ETS 300 278: March 1994
Receive table: is part of the PA receive functions block. It associates a CBR connection end-point
identifier with the bus and with the VCI of the PA slots on which the service octets of the connection are
received.
Slot number: slots in a frame are numbered by starting with the first slot fully contained in the frame and
numbering from 1 to the last slot fully contained in that frame.
Transmit table: is part of the PA transmit functions block. It associates a CBR connection end-point
identifier with the bus and with the VCI of the PA slots on which the service octets of the connection are
transmitted.
Variability: is the maximum difference in the slot number between the k-th and the (k+M)-th PA slots
allocated to a connection with slot generation frequency = (M, N).
4 Symbols and abbreviations
For the purposes of this ETS, the symbols and abbreviations defined in IEEE Standard 802.6 [1] apply.
In addition, those symbols and abbreviations that are essential for the understanding of this ETS are listed:
CAU CBR Access Unit
CBR Constant Bit Rate
CCF CBR Convergence Functions
CF Common Functions
CRC Cyclic Redundancy Check
CSDU CBR Service Data Unit
CSU CBR Service User
DQDB Distributed Queue Dual Bus
HCS Header Check Sequence
HOB Head Of Bus
ISPBX Integrated Services Private Branch Exchange
LM-ACTION Layer Management Action
MAC Media Access Control
MSS MAN Switching System
NMP Network Management Process
PA Pre-Arbitrated
PBX Private Branch Exchange
PDU Protocol Data Unit
PICS Protocol Implementation Conformance Statement
PLCP Physical Layer Convergence Procedure
QA Queued Arbitrated
SDH Synchronous Digital Hierarchy
VCI Virtual Channel Identifier
V default Variability
def
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ETS 300 278: March 1994
5 Basic principles
The CBR capability shall provide for the following data rates defined by CCITT:
- 64 kbit/s;
- 384 kbit/s;
- 1 536 kbit/s;
- 2 048 kbit/s.
The CBR capability provides for these basic data rates and combinations thereof, e.g. n x 64 kbit/s.
The PLCP of a DQDB subnetwork providing CBR capability shall have a 125 μs frame structure. In order
to allow for simple and efficient bandwidth allocation/management algorithms, the Head Of Bus (HOB)
node shall generate slots used for CBR services which fit as a whole into a single 125 μs frame.
Therefore, the use of a 1,5 Mbit/s or 2 Mbit/s transmission system for the DQDB subnetwork is excluded.
All other standardised PLCPs are supported.
6 DQDB layer service model
The CBR service provided by the DQDB layer is described abstractly by means of the service primitives
notation defined in ISO/TR 8509.
NOTE: The abstract description does not constrain an implementation in any way. For
example, an implementation might use a service data unit consisting of a number of
octets or only a single bit.
The primitives used to describe the service are the following:
- CSU-DATA request;
- CSU-DATA indication.
CSDUs to be sent via the CBR service are presented by the CSU at the specified rate for the constant bit
rate connection. Similarly, the CBR service periodically delivers a CSDU unit to a CSU at the rate specified
for the connection.
CSU-DATA request
Function:
- this primitive requests the transfer of a CSDU over an established CBR connection.
Semantics of the Service Primitive:
- CSU-DATA request;
- the CSDU parameter conveys a single CSDU.
When generated:
- this primitive is generated by an CSU whenever a CSDU is required to be transferred over a
connection.
Effect on receipt:
- the receipt of this primitive by the DQDB layer results in the DQDB layer attempting to transfer the
CSDU over the established connection. CSDUs are transferred in the same order in which they are
submitted by the CSU.
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CSU-DATA indication
Function:
- this primitive indicates the arrival of a CSDU over an established CBR connection.
Semantics of the service primitive:
- CSU-DATA indication (CSDU).
The CSDU parameter conveys a single CSDU.
When generated:
- this primitive is generated by the DQDB Layer to deliver a CSDU that has arrived over an
established CBR connection.
Effect on receipt:
- the effect of receipt of this primitive is dependent upon the CSU;
- the CSDUs shall be received by the receiving CSU in the same order in which they were sent by the
sending CSU.
7 Functional architecture
7.1 Access method
CBR service octets will be carried in the payload of PA slots. For the format of PA slots see Clause 8. In
particular, the payload of PA slots contains 48 octets. PA slots are generated by the slot generation
function at the HOB according to algorithms which are discussed in subclause 7.2.
Dedicated slot-switching shall be used for the nodes to gain access to CBR bandwidth in the payload of
the PA-slots on a DQDB bus.
In dedicated slot-switching, the octets of the payload of a PA-slot are associated with only one connection,
and there is a one-to-one correspondence between CBR connections and VCIs in the headers of PA-slots
on each bus.
NOTE: This is in contrast to "composite slot-switching" where different octet positions in one
PA slot may be associated with different connections. There is no contradiction
between the two access methods as dedicated slot-switching may be viewed as just a
particular way to use composite slot-switching.
7.2 Slot generation function at the HOB
In order to achieve interoperability of equipment it has to be specified how the PA slots associated with a
CBR connection (i.e. with a particular VCI) are generated by the HOB. The bandwidth provided by all PA
slots with a particular VCI defines a CBR connection on a DQDB subnetwork.
For a PLCP based on CCITT Recommendation G.703 at 34 Mbit/s or at 140 Mbit/s, all slots are fully
contained in a frame and may be used as PA slots.
For certain PLCPs, however, slots can cross frame boundaries. Furthermore, the number of slots which
are fully contained in a frame varies between Max and Max+1 where Max is an integer. In this case, in
order to simplify PA slot generation procedures, only the first Max complete slots in a frame may be used
as PA slots.
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ETS 300 278: March 1994
For the PLCP for CCITT Recommendations G.707, G.708 and G.709 SDH based systems at 155 Mbit/s,
the payload of the PLCP frame is 2 340 = 44 x 53 + 8 octets. Here, the mapping of slots onto PLCP
frames has been defined such that slots are allowed to cross frame boundaries. Any given PLCP frame
will contain either 43 or 44 complete slots, i.e. Max = 43.
The slot generation process for a CBR connection is characterised at a minimum by the parameters of
"frequency" (see subclause 7.2.1) and "variability" (see subclause 7.2.2). The slot generation process is
further characterised by the attributes "basic" or "compound" (which are mutually exclusive). The term
"basic" means that the rate of the channel is a "basic data rate", i.e. a data rate associated with a
particularly simple slot generation process (see subclause 7.2.3). The term "compound" means that the
CBR connection is obtained by combining the bandwidth of two or more connections at basic data rates
(see subclause 7.2.4).
7.2.1 Frequency and periodicity
Frequency
The frequency parameter determines the average rate of the CBR channel. In general, CBR bandwidth
allocations are of the form "M slots every N frames". To make bandwidth allocation simple, M and N may
be constrained. PA slot generation by the HOB shall maintain the frequency of M slots every N frames for
each CBR connection.
NOTE: This definition requires two parameters, M and N, to specify frequency which have to
be included in the LM-ACTION invoke (PA VCI ADD HOB) service primitive (see Clause
10 below).
Periodicity
When frequency = (M, N) the HOB is required to generate the PA slots with a particular VCI periodically
with period N in the following sense: the k-th and the (k + N)-th PLCP frames (125 μs transmission frames)
shall contain the same number of PA slots with that particular VCI for k = 0,1,2,.
7.2.2 Variability
According to the periodicity requirement above the number of PA slots per frame associated with a
connection repeats every N frames. However, nothing is implied about the slot position within each frame,
hence variability is allowed there.
Slots in a frame can be numbered by starting with the first slot fully contained in the frame and numbering
from 1 to the last slot fully contained in that frame. Using this definition of slot number, variability V is
defined as the maximum difference in the slot number between the k-th and the (k + M)-th PA slots
allocated to a connection with slot generation frequency (M, N):
V = max { | S(k) - S(k+M) | }
where S(k) is the slot number of the k-th PA slot allocated to a connection and the max is taken over
k = 0,1,2,.
The minimum variability is V = 0. A default value, V , shall be chosen that all HOB stations will support
def
such that the possibility of CBR connections with no more variability than V can be assured. This default
def
value is chosen to be V = 0.
def
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ETS 300 278: March 1994
Minimum variability
For a PLCP based on CCITT Recommendation G.703 at 34 Mbit/s or at 140 Mbit/s, the k-th slot in each
PLCP frame appears with precisely the same offset (i.e. precisely the same number of octets from the
beginning of the frame). Therefore, when V = 0 for a connection, PA slot appearances for that connection
are precisely periodic with period N frames, ignoring PLCP induced jitter.
Minimum variability would be supported for any given CBR connection by allocating to that connection the
same slot number(s) in each frame that contains slots for that connection (not necessarily in every frame).
For the PLCP for CCITT Recommendations G.707, G.708 and G.709 SDH based systems at 155 Mbit/s,
the offset of the k-th complete slot within a PLCP frame varies. It can be shown that the offset of the k-th
complete slot will never be identical between adjacent frames and will occasionally change by as much as
45 octets. Between non-adjacent frames, the change in offset for a given slot number depends on the
distance between frames. In the general case, the offset of a given slot number will vary by up to 52
octets (i.e. one less than the length of a slot).
7.2.3 Basic data rates
A basic data rate is one of the data rates out of the list below. It is associated with a particularly simple
slot generation process based on simple frequency parameter pairs.
For M=1, N may take on the values 1, 2, 4, 8, 16 and 48. For M=2, N=1 or N=3. For all other values of M,
N is equal to 1. These frequency parameter pairs define the basic data rates given in table 1.
Table 1
�˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¿
‡ M ‡ N ‡ basic data rate (in kbit/s) ‡
ˆ˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡ 1 ‡ 48 ‡ 64 ‡
‡ 1 ‡ 16 ‡ 192 ‡
‡ 1 ‡ 8 ‡ 384 ‡
‡ 1 ‡ 4 ‡ 768 ‡
‡ 1 ‡ 2 ‡ 1 536 ‡
‡ 2 ‡ 3 ‡ 2 048 ‡
�˜˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜�
Therefore, the smallest granularity of CBR bandwidth to be allocated is 64 kbit/s, sufficient for a voice
connection. A 64 kbit/s connection is supported by allocating one PA-slot every 48 frames to the VCI for
that connection.
1 slot x 48 octets x 8 bits
________________________
i.e. 64 kbit/s =
48 frames x 125 μs
When frequency = (1, N) (i.e. when one slot is generated every N frames) the default variability V = 0
implies that PA slots associated with a given connection are generated at precisely regular intervals. This
is not the case for frequency = (2, 3), equivalent to a data rate of 2 048 kbit/s (see also Annex A,
subclause A.2.3).
NOTE: Each of the data rates mentioned in Clause 5 above is a basic data rate.
7.2.4 Compound data rates
A compound data rate is a sum of basic data rates. Hence, a CBR connection at a compound data rate is
obtained by combining the bandwidth of two or more connections at basic data rates. As 64 kbit/s is a
basic data rate it is clear from this definition that any data rate of the form n x 64 kbit/s which is not a basic
data rate is a compound data rate. It is, however, desirable to reduce the number of basic data rates of
which the compound data rate is composed, as this reduces the maximum access delay.
Inspection of the list of data rates of subclause 7.2.3 shows that the representation of a compound data
rate of the form n x 64 kbit/s (n<=32) as a sum of basic data rates with a minimum number of summands
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ETS 300 278: March 1994
is unique. At most five addenda are needed. (Examples: 1 600 kbit/s = 1 536 + 64 kbit/s,
23 x 64 kbit/s = (12 + 6 + 3 + 1 + 1) x 64 kbit/s).
Compound data rates are useful by providing more flexibility in the definition of CBR connections. The finer
granularity of data rates of CBR connections can avoid a waste of bandwidth.
PA-slot Frequency in Support of compound data rates
Bandwidth for a compound data rate d = d1 + d2 + . + dn with basic data rates d1, d2, . , dn is
allocated by simultaneously allocating bandwidth for those basic data rates, i.e. the bandwidth allocation is
of the form "M1 slots every N1 frames plus M2 slots every N2 frames plus . plus Mn slots every Nn
frames". n different "LM-ACTION invoke (PA VCI ADD HOB)" messages (see Clause 10) are needed.
Bandwidth allocation for a connection with a compound data rate d = d1 + d2 + . + dn is considered
successfully completed when the HOB has confirmed that PA-slots with the specified VCI are generated
for n connections with rates d1, d2 and dn respectively. Hence, bandwidth allocation for compound data
rates concerns only the bandwidth management which is not subject to standardization. As far as the slot
generation function in the HOB and the communication between bandwidth manager and HOB are
concerned, there are only basic data rates.
As the representation of a compound data rate as the sum of basic data rates with a minimal number of
terms is unique for all data rates of the form n x 64 kbit/s (n < = 32) and as it is clear from subclause 7.2.3
how PA-slots for basic data rates are generated by the HOB there is also a clear understanding of how
PA-slots for these compound data rates are generated by the HOB. This allows the design of DQDB
nodes with interfaces for these compound data rates.
7.3 Functions at a DQDB node supporting a CBR interface
7.3.1 Overview
A CAU is a DQDB node supporting a CBR interface.
An overview of its functional blocks is given in figure 1.
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¿
‡ CCF ‡
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜·
�----------¿ ‡ PAF ‡
‡ empty ‡ ˆ˜˜˜˜˜˜˜˜˜˜·
�----------� ‡ CF ‡
�˜˜˜˜˜˜˜˜˜˜¿ ˆ˜˜˜˜˜˜˜˜˜˜·
‡ CBR ‡ ‡ PLCP ‡
‡Interface ‡ ‡ ‡
CSU �˜´˜˜˜˜˜´˜˜� �˜˜´˜˜˜˜´˜˜�
<˜˜˜˜˜˜˜˜˜˜˜˜� ‡ ‡ �˜˜˜˜˜˜˜˜˜> Bus A
˜˜˜˜˜˜˜˜>˜˜˜˜˜˜˜˜˜˜� �˜<˜˜˜˜˜˜˜˜˜˜˜˜˜ Bus B
CSU = CBR Service User.
CCF = CBR Convergence Functions.
PAF = Pre-Arbitrated Functions.
CF = Common Functions.
PLCP = Physical Layer Convergence Procedure.
Figure 1: Functions at a DQDB Node supporting a CBR interface
A CAU preserves the octet sequence integrity, i.e. the CF block delivers octets to the PLCP in the same
order in which they were received by the CCF block from the CSU, and the CCF block delivers octets to
the CSU in the same order in which they were received by the CF block from the PLCP layer.
7.3.2 Common functions block
As far as the provision of CBR service is concerned the role of the CF block shall be as specified in the
IEEE Standard 802.6 [1], section 5.4, except for the additions concerning the PA slot generation by the
HOB station which are dealt with in subclause 7.2.
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ETS 300 278: March 1994
7.3.3 PA-functions block
The PA functions block controls the transfer in PA segment payloads of CBR service octets received from
CCF blocks.
The PA functions block maintains two tables, a transmit table and a receive table. Each table associates
each connection end-point identifier and the corresponding CCF with the one pair (bus, VCI) characterising
the PA slots used to carry the octets on the connection.
For the maintenance of these tables the LM-ACTIONs invoke (OPEN CE CCF) and invoke (CLOSE CE
CCF) and the corresponding reply actions are used (see Clause 10).
7.3.3.1 PA-transmit functions
The PA-transmit functions block receives octets from the CCF block and maps these octets onto offset
positions in the payload of PA slots according to the entries of a transmit table maintained by the PA
functions block. The PA-transmit functions block examines the VCIs of PA-slots passing on the bus and
receives the corresponding offset signals. When the VCI is found to be valid the transmit table is searched
for a matching entry. When a matching entry has been found, for each of the following 48 offset signals, a
service octet is requested from the corresponding CCF block and is forwarded to the CF block.
7.3.3.1.1 Transmit interactions between PA functions block and common functions block
When the common functions block receives an ACF with the BUSY bit set to one and the SL_TYPE bit set
to one (i.e. the ACF of a PA slot), it then delivers a copy of the octets of the PA segment header to the PA
functions header to the PA functions block as they pass on the bus in the common functions block. It also
asserts the OFFSET_SIGNAL control to the offset of the PA segment payload as each octet passes on
the bus in the common functions block. If the PA functions block is to write an octet at the offset asserted
by OFFSET_SIGNAL, then it OR-writes it as the octet passes along the bus in the common functions
block.
7.3.3.1.2 PA segment header validation
Upon receipt of a PA segment header, the PA functions block validates the correctness of the PA segment
header using the Header Check Sequence (HCS) (see subclause 8.2.1.4), and then:
1) if the HCS is not valid, then the PA segment payload shall be ignored;
2) if the HCS is valid, then the value of VCI received in the PA segment header is used for processing
the associated PA segment payload.
7.3.3.2 PA receive functions
In the reverse direction, the PA receive functions block receives octets from the common functions block
and forwards them to the CCF blocks to which they are destined according to the entries in the receive
table.
The PA receive functions involve examining the VCI in the PA segment header of PA slots on
bus x (x = A or B) and determining whether it is currently associated with reception for a CCF block at the
node.
7.3.3.2.1 Receive interactions between PA functions block and common functions block
When the common functions block receives an ACF with the BUSY bit set to one and the SL_TYPE bit set
to one (i.e. the ACF of a PA slot), it then delivers a copy of the octets of the PA segment header to the PA
functions block as they pass on the bus in the common functions block. It also delivers a copy of each
octet of the PA segment payload to the PA functions block and asserts the OFFSET_SIGNAL control to
the offset of the PA segment payload as the octet passes on the bus in the common functions block.
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ETS 300 278: March 1994
7.3.3.2.2 PA segment header validation
Upon receipt of a PA segment header, the PA functions block validates the correctness of the PA segment
header using the HCS (see IEEE Standard 802.6 [1], section 8.2.1.4), and then:
1) if the HCS is not valid, then the PA segment payload shall be ignored;
2) if the HCS is valid, then the value of VCI received in the PA segment header is used for processing
the associated PA segment payload, as described in subclause 7.3.3.2.3.
7.3.3.2.3 CBR convergence function selection
If the VCI in the PA segment header is accepted as valid, according to subclause 7.3.3.2.2, then the PA
functions block stores the VCI and the bus on which it was received and compares it with the set of
receive [VCI, bus] values in the receive table associated with all CCF blocks at the node, and then:
1) if the comparison does not match the receive [VCI, bus] value for any CCF block at this node no
action is taken;
2) if the comparison does match the receive [VCI, bus] value for an CCF block at this node, the PA
function block delivers the octets in the payload of this segment to that CCF block.
7.3.4 CBR convergence functions block
There is one type of CCF for each CBR interface and there is one instance of an CCF block for each CBR
service user connected to the node.
The PA functions block will receive and transmit CBR service octets at a guaranteed average rate, but
they will not necessarily be evenly distributed. The irregularity in arrival will depend on the distribution of PA
slots generated by the slot marking function at each active HOB function. The function of a CCF is, if
necessary, to provide buffering to smooth any irregular arrival of CBR service octets from the PA functions
block to the arrival rate expected by the CSU.
7.3.4.1 CBR convergence transmit function
The CCF block receives CBR service octets across the CBR interface. It stores these octets in a buffer
until they are requested by the PA functions block. They are passed on to the PA functions block.
Before transmission begins the CCF has to additionally delay the octets queued for transmission so that
the transmit buffer does not become empty. The additional delay depends on the service rate and the
nature of the PA slot generation process for the CBR connection which is permitted by subclause 7.2 for
each data rate. Applicable delay values can be computed from the algorithms given in this ETS.
The CCF block receives CSU-DATA request primitives at a constant rate, according to the requirements of
the CSU.
When the CCF block receives an CSU-DATA request it places the CBR service octet received as the
CSDU in a transmit buffer, which is used to store the CSDUs in the order received from the CSU. When
the PA functions block asserts the SEND_SIGNAL to the CCF block, it delivers the octet at the head of the
transmit buffer to the PA functions block. The definition of a CCF will specify the actions taken if the
transmit buffer either underflows or overflows.
7.3.4.2 CBR convergence receive function
The PA functions block delivers CBR octets to the CCF block as they are received. On receipt of a CBR
service octet, the CCF places it in a receive buffer, which is used to store the octets in the order received
from the PA functions block.
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ETS 300 278: March 1994
The CCF block is required to generate CSU-DATA indication primitives according to the requirements of
the CSU. To support this function, the CCF block uses a 125 μs clock derived by the DQDB layer
subsystem from the Ph-TIMING-MARK indication primitives received at the node.
When the CCF block is required to send an CSU-DATA indication, it delivers the octet at the head of the
receive buffer as the CSDU in an CSU-DATA indication.
The CBR convergence receive function is required to generate CSU-DATA indications according to the
requirements of the CSU.
The definition of an CCF will specify the actions taken if the receive buffer either underflows or overflows.
7.3.5 CBR interfaces
The method for providing CBR bandwidth on a DQDB subnetwork described in the previous subclauses
allows for the specification of nodes with a variety of CBR interfaces. The specification of these interfaces
is outside the scope of this ETS.
8 Protocol data unit format
8.1 PA slot
A PA slot is used to transfer CBR service octets. For the format of slots see IEEE Standard 802.6 [1],
section 6.2.
8.2 PA segment
Each PA segment contains a header of 4 octets and a payload of 48 octets, as shown in figure 2.
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜´˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¿
‡ PA Segment header ‡ PA Segment payload ‡
ˆ˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜¯˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜·
‡ (4 octets) ‡ (48 octets) ‡
�˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜`˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜˜�
Figure 2: PA segment format
A PA segment is carried in a PA slot. A PA slot shall be generated by the slot marking function at the head
of the bus with the BUSY bit of the ACF set to 1, the SL_TYPE bit of the ACF set to 1, and all other bits of
the ACF set to 0. The BUSY bit and SL_TYPE bit of the PA slot should remain unchanged at all times, but
the REQ bits in the ACF may be operated on according to the rules of the distributed queue. The slot
marking function at the head of the bus shall also write the PA segment header (see subclause 8.2.1),
which is carried by the PA slot. The slot marking function at the head of the bus shall set every bit in the
PA segment payload to 0.
8.2.1 PA segment header fields
The PA segment header contains the fields shown in figure 3. The length of each field is shown in bits.
These fields are written by the slot marking function at the head of the bus and should remain the same as
the slot passes along the bus.
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