Another example of the use of the 'm' line use for audio media over
AAL1 is:
m=audio $ AAL1/ATMF 0 8
This indicates that any AAL1 VCC may be used. If it exists already,
then its selection is subject to glare rules. The audio media on
this VCC is encapsulated according to ATMF's af-vtoa-78 [7]. The
encodings to be used are either PCMU or PCMA, in preferential order.
The 'm' for AAL5-based audio media can be constructed as follows:
m=audio <virtualConnectionId> AAL5/ATMF <payloadType#1>
<payloadType#2>...<payloadType #n>
An example 'm' line use for audio media over AAL5 is:
m=audio PORT-2/VPI-6/$ AAL5/ITU 9 15
implies that any VCI on VPI= 6 of trunk port #2 may be used. The
identities of the terms in the virtual connection ID are implicit in
the application context. The audio media on this VCC is encapsulated
according to ITU-T H.222.1 [51]. The encodings to be used are either
ITU-T G.722 or ITU-T G.728 (LD-CELP), in preferential order.
The 'm' for AAL5-based H.323 Annex C audio [45] can be constructed as
follows:
m=audio <virtualConnectionId> RTP/AVP <payloadType#1>
<payloadType#2>...<payloadType #n>
For example:
m=audio PORT-9/VPI-3/VCI-$ RTP/AVP 2 96
a=rtpmap:96 X-G727-32
a=aalType:AAL5
a=aalApp:itu_h323c - -
implies that any VCI on VPI= 3 of trunk port #9 may be used. This VC
encapsulates RTP packets directly on AAL5 per [45]. The 'rtpmap'
(rather than the 'atmmap') attribute is used to dynamically map the
payload type of 96 into the codec name X-G727-32 (Table 2). This
name represents 32 kbps EADPCM.
The 'm' line for AAL5-based video media can be constructed as
follows:
m=video <virtualConnectionId> AAL5/ITU <payloadType#1>
<payloadType#2>...<payloadType #n>
In this case, the use of AAL5/ITU as the transport points to H.222.1
as the controlling standard [51]. An example 'm' line use for video
media is:
m=video PORT-9/VPI-3/VCI-$ AAL5/ITU 33
This indicates that any VCI on VPI= 3 of trunk port #9 may be used.
The video media on this VCC is encapsulated according to ITU-T
H.222.1 [51]. The encoding scheme is an MPEG 2 transport stream
("MP2T" in Table 1). This is statically mapped per [31] to a payload
type of 33.
Using the parameter definitions in the previous subsections, the
media information line for AAL2-based audio media can be constructed
as follows:
m=<media> <virtualConnectionId> <transport#1> <format list#1>
<transport#2> <format list#2> ... <transport#M> <format list#M>
where <format list#i> has the form <profile#i_1>...<profile#i_N>
Unlike the 'm' line for AAL1 or AAL5 applications, the 'm' line for
AAL2 applications can have multiple <transport> parameters, each
followed by a <format list>. This is because it is possible to
consider definitions from multiple sources (ATMF, ITU and non-
standard documents) when selecting AAL2 profile to be bound to a
connection.
In most applications, the ordering of profiles implies a preference
(preferred profiles before less favored ones). Therefore, there can
be multiple instances of the same <transport> value in the same 'm'
line.
An example 'm' line use for audio media over AAL2 is:
m=audio VCCI-27/CID-19 AAL2/ITU 7 AAL2/custom 100 AAL2/ITU 1
This indicates the use of CID #19 on VCCI #27 to carry audio. It
provides a preferential list of profiles for this connection: profile
AAL2/ITU 7 defined in [13], AAL2/custom 100 defined in an
application-specific or interoperability document and profile
AAL2/ITU 1 defined in [13].
Another example of the use of the 'm' line use for audio media over
AAL2 is:
m=audio VCCI-$/CID-$ AAL2/ATMF 6 8
This indicates that any AAL2 CID may be used, subject to any
applicable glare avoidance/reduction rules. The profiles that can be
bound to this connection are AAL2/ATMF 6 defined in af-vtoa-0113.000
[44] and AAL2/ATMF 8 defined in af-vmoa-0145.000 [52]. These sources
use non-overlapping profile number ranges. The profiles they define
fall under the <transport> category "AAL2/ATMF". This application
does not order profiles preferentially. This rule is known a priori.
It is not embedded in the 'm' line.
Another example of the use of the 'm' line use for audio media over
AAL2 is:
m=audio VCCI-20/CID-$ AAL2/xyzCorporation 11
AAL2 VCCs in this application are single-CID VCCs. Therefore, it is
possible to wildcard the CID. The single-CID VCC with VCCI=20 is
selected. The AAL2 profile to be used is AAL2/xyzCorporation 11
defined by xyzCorporation.
In some applications, an "-" can be used in lieu of:
- <format list>
- <transport> and <format list>
This implies that these parameters are irrelevant or are known by
other means (such as defaults). For example:
m=audio VCCI-234 - -
a=aalType:AAL1
indicates the use of VCCI=234 with AAL1 adaptation and unspecified
encoding.
In another example application, the 'aal2sscs3662' attribute can
indicate <faxDemod> = "on" and any other competing options as "off",
and the <aalType> attribute can indicate AAL2. Thus:
m=audio VCCI-123/CID-5 - -
a=aalType:AAL2
a=aal2sscs3662:audio off off on off on off off off - - -
Besides indicating an audio medium, a VCCI of 123 and a CID of 5, the
'm' line indicates an unspecified profile. The media attribute lines
indicate an adaptation layer of AAL2, and the use of the audio SAP
[13] to carry demodulated facsimile.
The media information line for "data" media has one of the following
the following formats:
m=data <virtualConnectionId> - -
m=data - - -
The data could be circuit emulation data carried over AAL1 or AAL2,
or packet data carried over AAL5. Media attribute lines, rather than
the 'm' line, are used to indicate the adaptation type for the data
media. Examples of the representation of data media are listed
below.
m=data PORT-7/VPI-6/VCI-$ - -
a=aalApp:AAL5_SSCOP- -
implies that any VCI on VPI= 6 of trunk port #7 may be used. This VC
uses SSCOP on AAL5 to transport data.
m=data PORT-7/VPI-6/VCI-50 - -
a=aalType:AAL1_SDT
a=sbc:6
implies that VCI 50 on VPI 6 on port 7 uses structured AAL1 to
transfer 6 x 64 kbps circuit emulation data. This may be alternately
represented as:
m=data PORT-7/VPI-6/VCI-50 - -
b=AS:384
a=aalType:AAL1_SDT
The following lines:
m=data VCCI-123/CID-5 - -
a=aalType:AAL2
a=sbc:2
imply that CID 5 of VCCI 123 is used to transfer 2 x 64 kbps circuit
emulation data.
In the AAL1 context, it is also permissible to represent circuit mode
data as an "audio" codec. If this is done, the codec types used are
X-CCD or X-CCD-CAS. These encoding names are dynamically mapped into
payload types through the 'atmmap' attribute. For example:
m=audio VCCI-27 AAL1/AVP 98
a=atmmap:98 X-CCD
a=sbc:6
implies that AAL1 VCCI=27 is used for 6 x 64 transmission.
In the AAL2 context, the X-CCD codec can be assigned a profile type
and number. Even though it is not possible to construct a profile
table as described in ITU I.366.2 for this "codec", it is preferable
to adopt the common AAL2 profile convention in its case. An example
AAL2 profile mapping for the X-CCD codec could be as follows:
PROFILE TYPE PROFILE NUMBER "CODEC" (ONLY ONE)
"custom" 200 X-CCD
The profile does not identify the number of subchannels ('n' in
nx64). This is known by other means such as the 'sbc' media
attribute line.
For example, the media information line:
m=audio $ AAL2/custom 200
a=sbc:6
implies 384 kbps circuit emulation using AAL2 adaptation.
It is not necessary to define a profile with the X-CCD-CAS codec,
since this method of CAS transport [7] is not used in AAL2
applications.
5.6 The Media Attribute Lines
In an SDP line sequence, the media information line 'm' is followed
by one or more media attribute or 'a' lines. Media attribute lines
are per the format below:
a=<attribute>:<value>
or
a=<value>
In general, media attribute lines are optional except when needed to
qualify the media information line. This qualification is necessary
when the "m" line for an AAL1 or AAL5 session specifies a payload
type that needs to be dynamically mapped. The 'atmmap' media
attribute line defined below is used for this purpose.
In attribute lines, subparameters that are meant to be left
unspecified are set to a "-". These are generally inapplicable or,
if applicable, are known by other means such as provisioning. In
some cases, a media attribute line with all parameters set to "-"
carries no information and should be preferably omitted. In other
cases, such as the 'lij' media attribute line, the very presence of
the media attribute line conveys meaning.
There are no restrictions placed by RFC2327 [1] regarding the order
of 'a' lines with respect to other 'a' lines. However, these lines
must not contradict each other or the other SDP lines.
Inconsistencies are not to be ignored and should be flagged as
errors. Repeated media attribute lines can carry additional
information. These should not be inconsistent with each other.
Applications will selectively use the optional media attribute lines
listed below. This is meant to be an exhaustive list for describing
the general attributes of ATM bearer networks.
The base specification for SDP, RFC2327 [1], allows the definition f
new attributes. In keeping with this spirit, some of the attributes
defined in this document can also be used in SDP descriptions of IP
nd other non-ATM sessions. For example, the 'vsel', 'dsel' and
'fsel' attributes defined below refer generically to codec-s. These
can be bed for service-specific codec negotiation and assignment in
non-ATM s well as ATM applications.
SDP media attributes defined in this document for use in the ATM
context are classified as:
* ATM bearer connection attributes (Section 5.6.1)
* AAL attributes (Section 5.6.2)
* Service attributes (Section 5.6.3).
* Miscellaneous media attributes, that cannot be classified as
ATM, AAL or service attributes (Section 5.6.4).
In addition to these, the SDP attributes defined in [1] can also be
used in the ATM context. Examples are:
* The attributes defined in RFC2327 which allow indication of
the direction in which a session is active. These are
a=sendonly, a=recvonly, a=sendrecv, a=inactive.
* The 'Ptime' attribute defined in RFC2327. It indicates the
packet period. It is not recommended that this attribute be
used in ATM applications since packet period information is
provided with other parameters (e.g., the profile type and
number in the 'm' line, and the 'vsel', 'dsel' and 'fsel'
attributes). Also, for AAL1 applications, 'ptime' is not
applicable and should be flagged as an error. If used in AAL2
and AAL5 applications, 'ptime' should be consistent with the
rest of the SDP description.
* The 'fmtp' attribute used to designate format-specific
parameters.
5.6.1 ATM bearer connection attributes
The following is a summary list of the SDP media attributes that can
be used to describe ATM bearer connections. These are detailed in
subsequent subsections.
* The 'eecid' attribute. This stands for 'end-to-end connection
identifier'. It provides a means of correlating service-level
connections with underlying ATM bearer connections. In the
Q.1901 [36] context, the eecid is synonymous with the bnc-id
(backbone network connection identifier).
* The 'aalType' attribute. This is used to indicate the nature
of the ATM adaptation layer (AAL).
* The 'capability' attribute, which indicates the ATM transfer
capability (ITU nomenclature), synonymous with the ATM Service
Category (ATMF nomenclature).
* The 'qosClass' attribute, which indicates the QoS class of the
ATM bearer connection.
* The 'bcob' attribute, which indicates the broadband connection
oriented bearer class, and whether end-to-end timing is
required.
* The 'stc' attribute, which indicates susceptibility to
clipping.
* The 'upcc' attribute, which indicates the user plane connection
configuration.
* The 'atmQOSparms' attribute, which is used to describe certain
key ATM QoS parameters.
* The 'atmTrfcDesc' attribute, which is used to describe ATM
traffic descriptor parameters.
* The 'abrParms' attribute, which is used to describe ABR-
specific parameters. These parameters are per the UNI 4.0
signaling specification [5].
* The 'abrSetup' attribute, which is used to indicate the ABR
parameters needed during call/connection establishment.
* The 'bearerType' attribute, which is used to indicate whether
the underlying bearer is an ATM PVC/SPVC, an ATM SVC, or a
subchannel within an existing ATM SVC/PVC/SPVC.
* The 'lij' attribute, which is used to indicate the presence of
a connection that uses the Leaf-initiated-join capability
described in UNI 4.0 [5], and to optionally describe parameters
associated with this capability.
* The 'anycast' attribute, which is used to indicate the
applicability of the anycast function described in UNI 4.0 [5],
and to optionally qualify it with certain parameters.
* The 'cache' attribute, which is used to enable SVC caching and
to specify an inactivity timer for SVC release.
* The 'bearerSigIE' attribute, which can be used to represent ITU
Q-series information elements in bit-map form. This is useful
in describing parameters that are not closely coupled to the
ATM and AAL layers. Examples are the B-HLI and B-LLI IEs
specified in ITU Q.2931 [15], and the user-to-user information
element described in ITU Q.2957 [48].
5.6.1.1 The 'eecid' attribute
The 'eecid' attribute is synonymous with the 4-byte 'bnc-id'
parameter used by T1SI, the ATM forum and the ITU (Q.1901)
standardization effort. The term 'eecid' stands for 'end-to-end
connection identifier', while 'bnc-id' stands for 'backbone network
connection identifier'. The name "backbone" is slightly misleading
since it refers to the entire ATM network including the ATM edge and
ATM core networks. In Q.1901 terminology, an ATM "backbone" connects
TDM or analog edges.
While the term 'bnc-id' might be used in the bearer signaling plane
and in an ISUP (Q.1901) call control plane, SDP session descriptors
use the neutral term 'eecid'. This provides a common SDP baseline
for applications that use ISUP (Q.1901) and applications that use
SIP/SIP+.
Section 5.6.6 depicts the use of the eecid in call establishment
procedures. In these procedures, the eecid is used to correlate
service-level calls with SVC set-up requests.
In the forward SVC establishment model, the call-terminating gateway
selects an eecid and transmits it via SDP to the call-originating
gateway. The call originating gateway transmits this eecid to the
call terminating gateway via the bearer set-up message (SVC set-up or
Q.2630.1 establish request).
In the backward SVC establishment model, the call-originating gateway
selects an eecid and transmits it via SDP to the call-terminating
gateway. The call terminating gateway transmits this eecid to the
call originating gateway via the bearer set-up message (SVC set-up or
Q.2630.1 establish request).
The value of the eecid attribute values needs to be unique within the
node terminating the SVC set-up but not across multiple nodes.
Hence, the SVC-terminating gateway has complete control over using
and releasing values of this parameter. The eecid attribute is used
to correlate, one-to-one, received bearer set-up requests with
service-level call control signaling.
Within an SDP session description, the eecid attribute is used as
follows:
a=eecid:<eecid>
where <eecid> consists of up to 8 hex digits (equivalent to 4
octets). Since this is always represented in hex, the "0x" prefix
shall not be used.
Within the text representation of the <eecid> parameter, hex digits
to the left are more significant than hex digits to the right
(Section 2.2).
This SDP document does not specify how the eecid (synonymous with
bnc-id) is to be communicated through bearer signaling (Q.931, UNI,
PNNI, AINI, IISP, proprietary signaling equivalent, Q.2630.1). This
is a task of these bearer signaling protocols. However, the
following informative statements are made to convey a sense of the
interoperability that is a goal of current standardization efforts:
- ITU Q.2941.3 and the ATMF each recommend the use of the GIT IE for
carrying the eecid (synonymous with bnc-id) in the set-up message
of ATM signaling protocols (Q.2931, UNI 4.0, PNNI, AINI, IISP).
The coding for carrying the eecid (bnc-id) in the GIT IE is
defined in ITU Q.2941.3 and accepted by the ATM forum.
- Another alternate method is to use the called party subaddress IE.
In some networks, this might be considered a protocol violation
and is not the recommended means of carrying the eecid (bnc-id).
The GIT IE is the preferred method of transporting the eecid
(bnc-id) in ATM signaling messages.
- The establish request (ERQ) message of the Q.2630.1 [37] signaling
protocol can use the SUGR (Served User Generated Reference) IE to
transport the eecid (bnc-id).
The node assigning the eecid can release and re-use it when it
receives a Q.2931 [15] set-up message or a Q.2630.1 [37] establish
request message containing the eecid.
However, in both cases (backward and forward models), it is
recommended that this eecid be retained until the connection
terminates. Since the eecid space is large enough, it is not
necessary to release it as soon as possible.
5.6.1.2 The 'aalType' attribute
When present, the 'aalType' attribute is used to indicate the ATM
adaptation layer. If this information is redundant with the 'm'
line, it can be omitted. The format of the 'aalType' media attribute
line is as follows:
a=aalType: <aalType>
Here, <aalType> can take on the following string values: "AAL1",
"AAL1_SDT", "AAL1_UDT", "AAL2", "AAL3/4", "AAL5" and
"USER_DEFINED_AAL". Note that "AAL3/4" and "USER DEFINED AAL" are
not addressed in this document.
5.6.1.3 The 'capability' attribute
When present, the 'capability' attribute indicates the ATM Transfer
Capability described in ITU I.371 [28], equivalent to the ATM Service
Category described in the UNI 4.1 Traffic Management specification
[6].
The 'capability' media attribute line is structured in one of the
following ways:
a=capability:<asc> <subtype>
a=capability:<atc> <subtype>
Possible values of the <asc> are enumerated below:
"CBR", "nrt-VBR", "rt-VBR", "UBR", "ABR", "GFR"
Possible values of the <atc> are enumerated below:
"DBR","SBR","ABT/IT","ABT/DT","ABR"
Some applications might use non-standard <atc> and <asc> values not
listed above. Equipment designers will need to agree on the meaning
and implications of non-standard transfer capabilities / service
capabilities.
The <subtype> field essentially serves as a subscript to the <asc>
and <atc> fields. In general, it can take on any integer value, or
the "-" value indicating that it does not apply or that the
underlying data is to be known by other means, such as provisioning.
For an <asc> value of CBR and an <atc> value of DBR, the <subtype>
field can be assigned values from Table 4-6 of ITU Q.2931 [15].
These are:
<asc>/<atc> <subtype> Meaning
"CBR"/"DBR" 1 Voiceband signal transport
(ITU G.711, G.722, I.363)
"CBR"/"DBR" 2 Circuit transport (ITU I.363)
"CBR"/"DBR" 4 High-quality audio signal transport
(ITU I.363)
"CBR"/"DBR" 5 Video signal transport (ITU I.363)
Note that [15] does not define a <subtype> value of 3.
For other values of the <asc> and <atc> parameters, the following
values can be assigned to the <subtype> field, based on [6] and [28].
<asc>/<atc> <subtype> Meaning
nrt-VBR 1 nrt-VBR.1
nrt-VBR 2 nrt-VBR.2
nrt-VBR 3 nrt-VBR.3
rt-VBR 1 rt-VBR.1
rt-VBR 2 rt-VBR.2
rt-VBR 3 rt-VBR.3
UBR 1 UBR.1
UBR 2 UBR.2
GFR 1 GFR.1
GFR 2 GRR.2
SBR 1 SBR1
SBR 2 SBR2
SBR 3 SBR3
It is beyond the scope of this specification to examine the
equivalence of some of the ATMF and ITU definitions. These need to
be recognized from the ATMF and ITU source specifications and
exploited, as much as possible, to simplify ATM node design.
When the bearer connection is a single AAL2 CID connection within a
multiplexed AAL2 VC, the 'capability' attribute does not apply.
5.6.1.4 The 'qosClass' attribute
When present, the 'qosClass' attribute indicates the QoS class
specified in ITU I.2965.1 [34].
The 'qosClass' media attribute line is structured as follows:
a=qosClass:<qosClass>
Here, <qosClass> is an integer in the range 0 - 5.
<qosClass> Meaning
0 Default QoS
1 Stringent
2 Tolerant
3 Bi-level
4 Unbounded
5 Stringent bi-level
5.6.1.5 The 'bcob' attribute
When present, the 'bcob' attribute represents the broadband
connection oriented bearer class defined in [5], [15] and [33]. It
can also be used to indicate whether end-to-end timing is required.
The 'bcob' media attribute line is structured as follows:
a=bcob:<bcob> <eetim>
Here, <bcob> is the decimal or hex representation of a 5-bit field.
The following values are currently defined:
<bcob> Meaning
0x01 BCOB-A
0x03 BCOB-C
0x05 Frame relaying bearer service
0x10 BCOB-X
0x18 BCOB-VP (transparent VP service)
The <eetim> parameter can be assigned a value of "on" or "off"
depending on whether end-to-end timing is required or not (Table 4-8
of [15]).
Either of these parameters can be left unspecified by setting it to a
"-". A 'bcob' media attribute line with all parameters set to "-"
carries no information and should be omitted.
5.6.1.6 The 'stc' attribute
When present, the 'stc' attribute represents susceptibility to
clipping. The 'stc' media attribute line is structured as follows:
a=stc:<stc>
Here, <stc> is the decimal equivalent of a 2-bit field. Currently,
all values are unused and reserved with the following exceptions:
<stc> value Binary Equivalent Meaning
0 00 Not susceptible to clipping
1 01 Susceptible to clipping
5.6.1.7 The 'upcc' attribute
When present, the 'upcc' attribute represents the user plane
connection configuration. The 'upcc' media attribute line is
structured as follows:
a=upcc:<upcc>
Here, <upcc> is the decimal equivalent of a 2-bit field. Currently,
all values are unused and reserved with the following exceptions:
<upcc> value Binary Equivalent Meaning
0 00 Point to point
1 01 Point to multipoint
5.6.1.8 The 'atmQOSparms' attribute
When present, the 'atmQOSparms' attribute is used to describe certain
key ATM QoS parameters.
The 'atmQOSparms' media attribute line is structured as follows:
a=atmQOSparms:<directionFlag><cdvType><acdv><ccdv><eetd><cmtd><aclr>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <cdvType> parameter can take on the string values of "PP" and
"2P". These refer to the peak-to-peak and two-point CDV as defined
in UNI 4.0 [5] and ITU Q.2965.2 [35] respectively.
The CDV parameters, <acdv> and <ccdv>, refer to the acceptable and
cumulative CDVs respectively. These are expressed in units of
microseconds and represented as the decimal equivalent of a 24-bit
field. These use the cell loss ratio, <aclr>, as the "alpha"
quantiles defined in the ATMF TM 4.1 specification [6] and in ITU
I.356 [47].
The transit delay parameters, <eetd> and <cmtd>, refer to the end-
to-end and cumulative transit delays respectively in milliseconds.
These are represented as the decimal equivalents of 16-bit fields.
These parameters are defined in Q.2965.2 [35], UNI 4.0 [5] and Q.2931
[15].
The <aclr> parameter refers to forward and backward acceptable cell
loss ratios. This is the ratio between the number of cells lost and
the number of cells transmitted. It is expressed as the decimal
equivalent of an 8-bit field. This field expresses an order of
magnitude n, where n is an integer in the range 1-15. The Cell Loss
Ratio takes on the value 10 raised to the power of minus n.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
There can be several 'atmQOSparms' lines in an SDP description.
An example use of these attributes for an rt-VBR, single-CID AAL2
voice VC is:
a=atmQOSparms:f PP 8125 3455 32000 - 11
a=atmQOSparms:b PP 4675 2155 18000 - 12
This implies a forward acceptable peak-to-peak CDV of 8.125 ms, a
backward acceptable peak-to-peak CDV of 4.675 ms, forward cumulative
peak-to-peak CDV of 3.455 ms, a backward cumulative peak-to-peak CDV
of 2.155 ms, a forward end-to-end transit delay of 32 ms, a backward
end-to-end transit delay of 18 ms, an unspecified forward cumulative
transit delay, an unspecified backward cumulative transit delay, a
forward cell loss ratio of 10 raised to minus 11 and a backward cell
loss ratio of 10 to the minus 12.
An example of specifying the same parameters for the forward and
backward directions is:
a=atmQOSparms:fb PP 8125 3455 32000 - 11
This implies a forward and backward acceptable peak-to-peak CDV of
8.125 ms, a forward and backward cumulative peak-to-peak CDV of 3.455
ms, a forward and backward end-to-end transit delay of 32 ms, an
unspecified cumulative transit delay in the forward and backward
directions, and a cell loss ratio of 10 raised to minus 11 in the
forward and backward directions.
5.6.1.9 The 'atmTrfcDesc' attribute
When present, the 'atmTrfcDesc' attribute is used to indicate ATM
traffic descriptor parameters. There can be several 'atmTrfcDesc'
lines in an SDP description.
The 'atmTrfcDesc' media attribute line is structured as follows:
a=atmTrfcDesc:<directionFlag><clpLvl>
<pcr><scr><mbs><cdvt><mcr><mfs><fd><te>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
The <clpLvl> (CLP level) parameter indicates whether the rates and
bursts described in these media attribute lines apply to CLP values
of 0 or (0+1). It can take on the following string values: "0",
"0+1" and "-". If rates and bursts for both <clpLvl> values are to
be described, then it is necessary to use two separate media
attribute lines for each direction in the same session descriptor.
If the <clpLvl> parameter is set to "-", then it implies that the CLP
parameter is known by other means such as default, MIB provisioning
etc.
The meaning, units and applicability of the remaining parameters are
per [6] and [28]:
PARAMETER MEANING UNITS APPLICABILITY
<pcr> PCR Cells/ CBR, rt-VBR, nrt-VBR,
second ABR, UBR, GFR;
CLP=0,0+1
<scr> SCR Cells/ rt-VBR, nrt-VBR;
second CLP=0,0+1
<mbs> MBS Cells rt-VBR, nrt-VBR,
GFR;
CLP=0,0+1
<cdvt> CDVT Microsec. CBR, rt-VBR, nrt-VBR,
ABR, UBR, GFR;
CLP=0,0+1
<mcr> MCR Cells/ ABR,GFR;
second CLP=0+1
<mfs> MFS Cells GFR;
CLP=0,0+1
<fd> Frame "on"/"off" CBR, rt-VBR, nrt-VBR,
Discard ABR, UBR, GFR;
Allowed CLP=0+1
<te> CLP "on"/"off" CBR, rt-VBR, nrt-VBR,
tagging ABR, UBR, GFR;
Enabled CLP=0
<fd> indicates that frame discard is permitted. It can take on the
string values of "on" or "off". Note that, in the GFR case, frame
discard is always enabled. Hence, this subparameter can be set to
"-" in the case of GFR. Since the <fd> parameter is independent of
CLP, it is meaningful in the case when <clpLvl> = "0+1". It should
be set to "-" for the case when <clpLvl> = "0".
<te> (tag enable) indicates that CLP tagging is allowed. These can
take on the string values of "on" or "off". Since the <te> parameter
applies only to cells with a CLP of 0, it is meaningful in the case
when <clpLvl> = "0". It should be set to "-" for the case when
<clpLvl> = "0+1".
An example use of these media attribute lines for an rt-VBR, single-
CID AAL2 voice VC is:
a=atmTrfcDesc:f 0+1 200 100 20 - - - on -
a=atmTrfcDesc:f 0 200 80 15 - - - - off
a=atmTrfcDesc:b 0+1 200 100 20 - - - on -
a=atmTrfcDesc:b 0 200 80 15 - - - - off
This implies a forward and backward PCR of 200 cells per second all
cells regardless of CLP, forward and backward PCR of 200 cells per
second for cells with CLP=0, a forward and backward SCR of 100 cells
per second for all cells regardless of CLP, a forward and backward
SCR of 80 cells per second for cells with CLP=0, a forward and
backward MBS of 20 cells for all cells regardless of CLP, a forward
and backward MBS of 15 cells for cells with CLP=0, an unspecified
CDVT which can be known by other means, and an MCR and MFS which are
unspecified because they are inapplicable. Frame discard is enabled
in both the forward and backward directions. Tagging is not enabled
in either direction.
The <pcr>, <scr>, <mbs>, <cdvt>, <mcr> and <mfs> are represented as
decimal integers, with range as defined in Section 6. See section
2.2 regarding the omission of leading zeros in decimal
representations.
5.6.1.10 The 'abrParms' attribute
When present, the 'abrParms' attribute is used to indicate the '
additional' ABR parameters specified in the UNI 4.0 signaling
specification [5]. There can be several 'abrParms' lines in an SDP
description.
The 'abrParms' media attribute line is structured as follows:
a=abrParms:<directionFlag><nrm><trm><cdf><adtf>
The <directionFlag> can be assigned the following string values: "f",
"b" and "fb". "f" and "b" indicate the forward and backward
directions respectively. "fb" refers to both directions (forward and
backward). Conventions for the forward and backward directions are
per section 2.3.
The <directionFlag> is always specified. Except for the
<directionFlag>, the remaining parameters can be set to "-" to
indicate that they are not specified, inapplicable or implied.
However, there must be some specified parameters for the line to be
useful in an SDP description.
These parameters are mapped into the ABR service parameters in [6] in
the manner described below. These parameters can be represented in
SDP as decimal integers, with fractions permitted for some. Details
of the meaning, units and applicability of these parameters are in
[5] and [6].
In SDP, these parameters are represented as the decimal or hex
equivalent of the binary fields mentioned below.
+-----------+----------------------------------+-----------------------+
| PARAMETER | MEANING | FIELD SIZE |
+-----------+----------------------------------+-----------------------+
| <nrm> | Maximum number of cells per | 3 bits |
| | forward Resource Management cell | |
+-----------+----------------------------------+-----------------------+
| <trm> | Maximum time between | 3 bits |
| | forward Resource Management cells| |
+-----------+----------------------------------+-----------------------+
| <cdf> | Cutoff Decrease Factor | 3 bits |
+-----------+----------------------------------+-----------------------+
| <adtf> | Allowed Cell Rate Decrease | 10 bits |
| | Time Factor | |
+-----------+----------------------------------+-----------------------+
5.6.1.11 The 'abrSetup' attribute
When present, the 'abrSetup' attribute is used to indicate the ABR
parameters needed during call/connection establishment (Section
10.1.2.2 of the UNI 4.0 signaling specification [5]). This line is
structured as follows:
a=abrSetup:<ficr><bicr><ftbe><btbe><crmrtt><frif><brif><frdf><brdf>
These parameters are defined as follows:
+-----------+----------------------------------+-----------------------+
| PARAMETER | MEANING | REPRESENTATION |
+-----------+----------------------------------+-----------------------+
| <ficr> | Forward Initial Cell Rate | Decimal equivalent |
| | (Cells per second) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <bicr> | Backward Initial Cell Rate | Decimal equivalent |
| | (Cells per second) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <ftbe> | Forward transient buffer | Decimal equivalent |
| | exposure (Cells) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <btbe> | Backward transient buffer | Decimal equivalent |
| | exposure (Cells) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <crmrtt> | Cumulative RM round-trip time | Decimal equivalent |
| | (Microseconds) | of 24-bit field |
+-----------+----------------------------------+-----------------------+
| <frif> | Forward rate increase factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <brif> | Backward rate increase factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <frdf> | Forward rate decrease factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
| <brdf> | Backward rate decrease factor | Decimal integer |
| | (used to derive cell count) | 0 -15 |
+-----------+----------------------------------+-----------------------+
See Section 2.3 for a definition of the terms 'forward' and
'backward'.
If any of these parameters in the 'abrSetup' media attribute line is
not specified, is inapplicable or is implied, then it is set to h "-
".
5.6.1.12 The 'bearerType' attribute
When present, the 'bearerType' attribute is used to indicate whether
the underlying bearer is an ATM PVC/SPVC, an ATM SVC, or a subchannel
within an existing ATM SVC/PVC/SPVC. Additionally, for ATM SVCs and
AAL2 CID connections, the 'bearerType' attribute can be used to
indicate whether the media gateway initiates connection set-up via
bearer signaling (Q.2931-based or Q.2630.1 based). The format of the
'bearerType' media attribute line is as follows:
a=bearerType: <bearerType> <localInitiation>
The <bearerType> field can take on the following string values:
"PVC", "SVC", "CID", with semantics as defined above. Here, "PVC"
includes both the PVC and SPVC cases.
In the case when the underlying bearer is a PVC/SPVC, or a CID
assigned by the MGC rather than through bearer signaling, the
<localInitiation> flag can be omitted or set to "-". In the case
when bearer signaling is used, this flag can be omitted when it is
known by default or by other means whether the media gateway
initiates the connection set-up via bearer signaling. Only when this
is to be indicated explicitly that the <localInitiation> flag takes
on the values of "on" or "off". An "on" value indicates that the
media gateway is responsible for initiating connection set-up via
bearer signaling (SVC signaling or Q.2630.1 signaling), an "off"
value indicates otherwise.
5.6.1.13 The 'lij' attribute
When present, the 'lij' attribute is used to indicate the presence of
a connection that uses the Leaf-initiated-join capability described
in UNI 4.0 [5], and to optionally describe parameters associated with
this capability. The format of the 'lij' media attribute line is as
follows:
a=lij: <sci><lsn>
The <sci> (screening indication) is a 4-bit field expressed as a
decimal or hex integer. It is defined in the UNI 4.0 signaling
specification [5]. It is possible that the values of this field will
be defined later by the ATMF and/or ITU. Currently, all values are
reserved with the exception of 0, which indicates a 'Network Join
without Root Notification'.
The <lsn> (leaf sequence number) is a 32-bit field expressed as a
decimal or hex integer. Per the UNI 4.0 signaling specification [5],
it is used by a joining leaf to associate messages and responses
during LIJ (leaf initiated join) procedures.
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
5.6.1.14 The 'anycast' attribute
When present, the 'anycast' attribute line is used to indicate the
applicability of the anycast function described in UNI 4.0 [5].
Optional parameters to qualify this function are provided. The format
of the 'anycast' attribute is:
a=anycast: <atmGroupAddress> <cdStd> <conScpTyp> <conScpSel>
The <atmGroupAddress> is per Annex 5 of UNI 4.0 [5]. Within an SDP
descriptor, it can be represented in one of the formats (NSAP, E.164,
GWID/ALIAS) described elsewhere in this document.
The remaining subparameters mirror the connection scope selection
information element in UNI 4.0 [5]. Their meaning and representation
is as shown below:
PARAMETER MEANING REPRESENTATION
<cdStd> Coding standard for the Decimal or hex
connection scope selection IE equivalent of
Definition: UNI 4.0 [5] 2 bits
<conScpTyp> Type of connection scope Decimal or hex
Definition: UNI 4.0 [5] equivalent of
4 bits
<conScpSel> Connection scope selection Decimal or hex
Definition: UNI 4.0 [5] equivalent of
8 bits
Currently, all values of <cdStd> and <conScpTyp> are reserved with
the exception of <cdStd> = 3 (ATMF coding standard) and <conScpTyp> =
1 (connection scope type of 'organizational').
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
5.6.1.15 The 'cache' attribute
This attribute is used to enable SVC caching. This attribute has the
following format:
a=cache:<cacheEnable><cacheTimer>
The <cacheEnable> flag indicates whether caching is enabled or not,
corresponding to the string values of "on" and "off" respectively.
The <cacheTimer> indicates the period of inactivity following which
the SVC is to be released by sending an SVC release message into the
network. This is specified as the decimal or hex equivalent of a
32-bit field, indicating the timeout in seconds. As usual, leading
zeros can be omitted. For instance,
a=cache:on 7200
implies that the cached SVC is to be deleted if it is idle for 2
hours.
The <cacheTimer> can be set to "-" if it is inapplicable or implied.
5.6.1.16 The 'bearerSigIE' attribute
ATM signaling standards provide 'escape mechanisms' to represent,
signal and negotiate higher-layer parameters. Examples are the B-HLI
and B-LLI IEs specified in ITU Q.2931 [15], and the user-to-user
information element described in ITU Q.2957 [48].
The 'bearerSigIE'(bearer signaling information element) attribute is
defined to allow a similar escape mechanism that can be used with
these ATM SDP conventions. The format of this media attribute line
is as follows:
a=bearerSigIE: <bearerSigIEType> <bearerSigIELng> <bearerSigIEVal>
When an 'bearerSigIE' media attribute line is present, all its
subparameters are mandatory. The "0x" prefix is not used since these
are always represented in hex.
The <bearerSigIEType> is represented as exactly 2 hex digits. It is
the unique IE identifier as defined in the ITU Q-series standards.
Leading zeros are not omitted. Some pertinent values are 7E (User-
user IE per ITU Q.2957 [48]), 5F (B-LLI IE) and 5D (B-HLI IE). B-LLI
and B-HLI, which stand for Broadband Low-layer Information and
Broadband High-layer Information respectively, are defined in ITU
Q.2931 [15]. Both of these refer to layers above the ATM adaptation
layer.
The <bearerSigIELng> consists of 1-4 hex digits. It is the length of
the information element in octets. Leading zeros may be omitted.
The <bearerSigIEVal> is the value of the information element,
represented as a hexadecimal bit map. Although the size of this bit
map is network/ service dependent, setting an upper bound of 256
octets (512 hex digits) is adequate. Since this a bit map, leading
zeros should not be omitted. The number of hex digits in this bit map
is even.
5.6.2 ATM Adaptation Layer (AAL) attributes
The following is a summary list of the SDP media attributes that can
be used to describe the ATM Adaptation Layer (AAL). These are
detailed in subsequent subsections.
* The 'aalApp' attribute, which is used to point to the
controlling standard for an application layer above the ATM
adaptation layer.
* The 'cbrRate' attribute, which represents the CBR rate octet
defined in Table 4-6 of ITU Q.2931 [15].
* The 'sbc' attribute, which denotes the subchannel count in the
case of n x 64 clear channel communication.
* The 'clkrec' attribute, which indicates the clock recovery
method for AAL1 unstructured data transfer (UDT).
* The 'fec' attribute, which indicates the use of forward error
correction.
* The 'prtfl' attribute, which indicates indicate the fill level
of partially filled cells.
* The 'structure' attribute, which is used to indicate the
presence or absence of AAL1 structured data transfer (SDT), and
the size of the SDT blocks.
* The 'cpsSDUsize' attribute, which is used to indicate the
maximum size of the CPCS SDU payload.
* The 'aal2CPS' attribute, which is used to indicate that an AAL2
CPS sublayer as defined in ITU I.363.2 [13] is associated with
the VCC referred to in the 'm' line. Optionally, it can be
used to indicate selected CPS options and parameter values for
this VCC.
* The 'aal2CPSSDUrate' attribute, which is used to place an upper
bound on the SDU bit rate for an AAL2 CID.
* The 'aal2sscs3661unassured' attribute, which is used to
indicate the presence of an AAL2 SSCS sublayer with unassured
transmission as defined in ITU I.366.1 [12]. Optionally, it
can be used to indicate selected options and parameter values
for this SSCS.
* The 'aal2sscs3661assured' attribute, which is used to indicate
the presence of an AAL2 SSCS sublayer with assured transmission
as defined in ITU I.366.1 [12]. Optionally, it can be used to
indicate selected options and parameter values for this SSCS.
* The 'aal2sscs3662' attribute, which is used to indicate the
presence of an AAL2 SSCS sublayer as defined in ITU I.366.2.
Optionally, it can be used to indicate selected options and
parameter values for this SSCS.
* The 'aal5sscop' attribute, which is used to indicate the
existence of an SSCOP protocol layer over an AAL5 CPS layer,
and the parameters which pertain to this SSCOP layer.
5.6.2.1 The 'aalApp' attribute
When present, the 'aalApp' attribute is used to point to the
controlling standard for an application layer above the ATM
adaptation layer. The format of the 'aalApp' media attribute line is
as follows:
a=aalApp: <appClass> <oui> <appId>
If any of the subparameters, <appClass>, <oui> or <appId>, is meant
to be left, unspecified, it is set to "-". However, an 'aalApp'
attribute line with all subparameters set to "-" carries no
information and should be omitted.
The <appClass>, or application class, field can take on the string
values listed below.
This list is not exhaustive. An "X-" prefix should be used with
<appClass> values not listed here.
<appClass> Meaning
"itu_h323c" Annex C of H.323 which specifies direct
RTP on AAL5 [45].
"af83" af-vtoa-0083.001, which specifies
variable size AAL5 PDUs with PCM voice
and a null SSCS [46].
"AAL5_SSCOP" SSCOP as defined in ITU Q.2110 [43]
running over an AAL5 CPS [21].
No information is provided regarding
any layers above SSCOP such as Service
Specific Coordination Function (SSCF)
layers.
"itu_i3661_unassured" SSCS with unassured transmission,
per ITU I.366.1 [12].
"itu_i3661_assured" SSCS with assured transmission,
per ITU I.366.1 [12]. This uses SSCOP [43].
"itu_i3662" SSCS per ITU I.366.2 [13].
"itu_i3651" Frame relay SSCS per ITU I.365.1 [39].
"itu_i3652" Service-specific coordination function,
as defined in ITU I.365.2, for Connection
Oriented Network Service (SSCF-CONS) [40].
This uses SSCOP [43].
"itu_i3653" Service-specific coordination function,
as defined in ITU I.365.3, for Connection
Oriented Transport Service (SSCF-COTS) [41].
This uses SSCOP [43].
"itu_i3654" HDLC Service-specific coordination function,
as defined in ITU I.365.4 [42].
"FRF5" Use of the FRF.5 frame relay standard [53],
which references ITU I.365.1 [39].
"FRF8" Use of the FRF.8.1 frame relay standard [54].
This implies a null SSCS and the mapping of
the frame relay header into the ATM header.
"FRF11" Use of the FRF.11 frame relay standard [55].
"itu_h2221" Use of the ITU standard H.222.1 for
audiovisual communication over AAL5 [51].
The <oui>, or Organizationally Unique Identifier, refers to the
organization responsible for defining the <appId>, or Application
Identifier. The <oui> is maintained by the IEEE. One of its uses is
in 802 MAC addresses. It is a three-octet field represented as one
to six hex digits. Since this is always represented in hex, the "0x"
prefix is not used. Leading zeros may be omitted.
The <appId> subparameter refers to the application ID, a hex number
consisting of up to 8 digits. Leading zeros may be omitted. The
"0x" prefix is not used, since the representation is always
hexadecimal. Currently, the only organization that has defined
application identifiers is the ATM forum. These have been defined in
the context of AAL2 ([44], [52], Section 5 of [61]). Within SDP,
these can be used with <appClass> = itu_i3662. The <oui> value for
the ATM forum is 0x00A03E.
In the following example, the aalApp media attribute line is used to
indicate 'Loop Emulation Service using CAS (POTS only) without the