RFC 3108
Conventions for the use of the Session Description Protocol (SDP) for ATM Bearer Connections
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 RFC 2327 [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, RFC 2327 [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 RFC 2327 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 RFC 2327. 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 Emulated Loop Control Protocol (ELCP) [52]. The Application ID is defined by the ATM forum [61]. The SSCS used is per ITU I.366.2 [13]. a=aalApp:itu_i3662 A03E A If leading zeros are not dropped, this can be represented as: a=aalApp:itu_i3662 00A03E 0000000A Since application identifiers have been specified only in the context of the AAL2 SSCS defined in ITU I.366.2 [13],the <appClass> can be set to '-' without ambiguity. The aalApp media attribute line can be reduced to: a=aalApp:- A03E A or a=aalApp:- 00A03E 0000000A5.6.2.2 The 'cbrRate' attribute
When present, the 'cbrRate' attribute is used to represent the CBR rate octet defined in Table 4-6 of ITU Q.2931 [15]. The format of this media attribute line is: a=cbrRate: <cbrRate> Here, <cbrRate> is represented as exactly two hex digits. The "0x" prefix is omitted since this parameter is always represented in hex. Values currently defined by the ITU are:
+------------+-----------------------------------------------+
| VALUE | MEANING |
| (hex) | |
+------------+-----------------------------------------------+
| 01 | 64 kbps |
+------------+-----------------------------------------------+
| 04 | 1544 kbps |
+------------+-----------------------------------------------+
| 05 | 6312 kbps |
+------------+-----------------------------------------------+
| 06 | 32064 kbps |
+------------+-----------------------------------------------+
| 07 | 44736 kbps |
+------------+-----------------------------------------------+
| 08 | 97728 kbps |
+------------+-----------------------------------------------+
| 10 | 2048 kbps |
+------------+-----------------------------------------------+
| 11 | 8448 kbps |
+------------+-----------------------------------------------+
| 12 | 34368 kbps |
+------------+-----------------------------------------------+
| 13 | 139264 kbps |
+------------+-----------------------------------------------+
| 40 | n x 64 kbps |
+------------+-----------------------------------------------+
| 41 | n x 8 kbps |
+------------+-----------------------------------------------+
It is preferable that the cbrRate attribute be omitted rather than
set to an unspecified value of "-", since it conveys no information
in the latter case.
5.6.2.3 The 'sbc' attribute
The 'sbc' media attribute line denotes the subchannel count and is
meaningful only in the case of n x 64 clear channel communication. A
clear n x 64 channel can use AAL1 (ATM forum af-vtoa-78) or AAL2
adaptation (ITU I.366.2). Although no such standard definition
exists, it is also possible to use AAL5 for this purpose. An n x 64
clear channel is represented by the encoding names of "X-CCD" and
"X-CCD-CAS" in Table 2.
The format of the 'sbc' media attribute line is as follows:
a=sbc:<sbc>
Here, <sbc> can be expressed as a decimal or hex integer. This attribute indicates the number of DS0s in a T1 or E1 frame that are aggregated for transmitting clear channel data. For T1-based applications, it can take on integral values in the inclusive range [1...24]. For E1-based applications, it can take on integral values in the inclusive range [1...31]. When omitted, other means are to be used to determine the subchannel count. Use of the 'sbc' attribute provides a direct way to indicate the number of 64 kbps subchannels bundled into an n x 64 clear channel. An alternate mechanism to indicate this exists within the SDP bandwidth information, or 'b', line [1]. In this case, instead of specifying the number of subchannels, the aggregate bandwidth in kbps is specified. The syntax of the 'b' line, copied verbatim from [1], is as follows: b=<modifier>:<bandwidth-value> In the case of n x 64 clear channels, the <modifier> is assigned a text string value of "AS", indicating that the 'b' line is application-specific. The <bandwidth-value> parameter, which is a decimal number indicating the bandwidth in kbps, is limited to one of the following values in the n x 64 clear channel application context: 64, 128, 192, 256, 320, 384, 448, 512, 576, 640, 704, 768, 832, 896, 960, 1024, 1088, 1152, 1216, 1280, 1344, 1408, 1472, 1600, 1664, 1728, 1792, 1856, 1920, 1984 Thus, for n x 64 circuit mode data service, a=sbc:6 is equivalent to b=AS:384 The media attribute line a=sbc:2 is equivalent to b=AS:128
5.6.2.4 The 'clkrec' attribute
When present, the 'clkrec' attribute is used to indicate the clock recovery method. This attribute is meaningful in the case of AAL1 unstructured data transfer (UDT). The format of the 'clkrec' media attribute line is as follows: a=clkrec:<clkrec> The <clkrec> field can take on the following string values: "NULL", "SRTS" or "ADAPTIVE". SRTS and adaptive clock recovery are defined in ITU I.363.1 [10]. "NULL" indicates that the stream (e.g., T1/E1) encapsulated in ATM is synchronous to the ATM network or is retimed, before AAL1 encapsulation, via slip buffers.5.6.2.5 The 'fec' attribute
When present, the 'fec' attribute is used to indicate the use of forward error correction. Currently, there exists a forward error correction method defined for AAL1 in ITU I.363.1 [10]. The format of the 'fec' media attribute line is as follows: a=fec:<fecEnable> The <fecEnable> flag indicates the presence of absence of Forward Error Correction. It can take on the string values of "NULL", "LOSS_SENSITIVE" and "DELAY_SENSITIVE". An "NULL" value implies disabling this capability. FEC can be enabled differently for delay-sensitive and loss-sensitive connections.5.6.2.6 The 'prtfl' attribute
When present, the 'prtfl' attribute is used to indicate the fill level of cells. When this attribute is absent, then other means (such as provisionable defaults) are used to determine the presence and level of partial fill. This attribute indicates the number of non-pad payload octets, not including any AAL SAR or convergence sublayer octets. For example, in some AAL1 applications that use partially filled cells with padding at the end, this attribute indicates the number of leading payload octets not including any AAL overhead. The format of the 'prtfl' media attribute line is as follows: a=prtfl:<partialFill> Here, <partialFill> can be expressed as a decimal or a hex integer.
In general, permitted values are integers in the range 1 - 48 inclusive. However, this upper bound is different for different adaptations since the AAL overhead, if any, is different. If the specified partial fill is greater than or equal to the maximum fill, then complete fill is used. Using a 'partial' fill of 48 always disables partial fill. In the AAL1 context, this media attribute line applies uniformly to both P and non-P cells. In AAL1 applications that do not distinguish between P and non-P cells, a value of 47 indicates complete fill (i.e., the absence of partial fill). In AAL1 applications that distinguish between P and non-P cells, a value of 46 indicates no padding in P-cells and a padding of one in non-P cells. If partial fill is enabled (i.e there is padding in at least some cells), then AAL1 structures must not be split across cell boundaries. These shall fit in any cell. Hence, their size shall be less than or equal to the partial fill size. Further, the partial fill size is preferably an integer multiple of the structure size. If not, then the partial fill size stated in the SDP description shall be truncated to an integer multiple (e.g., a partial fill size of 40 is truncated to 36 to support six 6 x 64 channels).5.6.2.7 The 'structure' attribute
This attribute applies to AAL1 connections only. When present, the ' structure' attribute is used to indicate the presence or absence of structured data transfer (SDT), and the size in octets of the SDT blocks. The format of the 'structure' media attribute line is as follows: a=structure: <structureEnable> <blksz> where the <structureEnable> flag indicates the presence of absence of SDT. It can take on the values of "on" or "off". An "on" value implies AAL1 structured data transfer (SDT), while an "off" value implies AAL1 unstructured data transfer (UDT). The block size field, <blksz>, is an optional 16-bit field [15] that can be represented in decimal or hex. It is set to a "-" when not applicable, as in the case of unstructured data transfer (UDT). For SDT, it can be set to a "-" when <blksz> is known by other means. For instance, af-vtoa-78 [7] fixes the structure size for n x 64 service, with or without CAS. The theoretical maximum value of <blksz> is 65,535, although most services use much less.
5.6.2.8 The 'cpsSDUsize' attribute
When present, the 'cpsSDUsize' attribute is used to indicate the maximum size of the CPCS SDU payload. There can be several ' cpsSDUsize' lines in an SDP description. The format of this media attribute line is as follows: a=cpsSDUsize:<directionFlag><cpcs> 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 <cpcs> fields is a 16-bit integer that can be represented in decimal or in hex. The meaning and values of these fields are as follows: Application Field Meaning Values AAL5 <cpcs> Maximum CPCS-SDU size 1- 65,535 AAL2 <cpcs> Maximum CPCS-SDU size 45 or 645.6.2.9 The 'aal2CPS' attribute
When present, the 'aal2CPS' attribute is used to describe parameters associated with the AAL2 CPS layer. The format of the 'aal2CPS' media attribute line is as follows: a=aal2CPS:<cidLowerLimit><cidUpperLimit><timerCU> <simplifiedCPS> Each of these fields can be set to a "-" when the intention is to not specify them in an SDP descriptor. The <cidLowerLimit> and <cidUpperLimit> can be assigned integer values between 8 and 255 [11], with the limitation that <cidUpperLimit> be greater than or equal to <cidLowerLimit>. For instance, for POTS applications based on [52], <cidLowerLimit> and <cidUpperLimit> can have values of 16 and 223 respectively. The <timerCU> integer represents the "combined use" timerCU defined in ITU I.363.2. This timer is represented as an integer number of microseconds. It is represented as the decimal integer equivalent of 32 bits.
The <simplifiedCPS> parameter can be assigned the values "on" or "off". When it is "on", the AAL2 CPS simplification described in [52] is adopted. Under this simplification, each ATM cell contains exactly on AAL2 packet. If necessary, octets at the end of the cell are padded with zeros. Since the <timerCU> value in this context is always 0, it can be set to "-".5.6.2.10 The 'aal2CPSSDUrate' attribute
When present, the 'aal2CPSSDUrate' attribute is used to place an upper bound on the SDU bit rate for an AAL2 CID. This is useful for limiting the bandwidth used by a CID, specially if the CID is used for frame mode data defined in [13], or with the SSSAR defined in [12]. The format of this media attribute line is as follows: a=aal2CPSSDUrate: <fSDUrate><bSDUrate> The fSDUrate and bSDUrate are the maximum forward and backward SDU rates in bits/second. These are represented as decimal integers, with range as defined in Section 6. If any of these parameters in these media attribute lines is not specified, is inapplicable or is implied, then it is set to "-".5.6.2.11 The 'aal2sscs3661unassured' attribute
When present, the 'aal2sscs3661unassured' attribute is used to indicate the options that pertain to the unassured transmission SSCS defined in ITU I.366.1 [12]. This SSCS can be selected via the aalApp attribute defined below, or by virtue of the presence of the ' aal2sscs3661unassured' attribute. The format of this media attribute line is as follows: a=aal2sscs3661unassured: <ted> <rastimer> <fsssar> <bsssar> Each of these fields can be set to a "-" when the intention is to not specify them in an SDP descriptor. The <ted> flag indicates the presence or absence of transmission error detection as defined in I.366.1. It can be assigned the values of "on" or "off". An "on" value indicates presence of the capability. The <rastimer> subparameter indicates the SSSAR reassembly timer in microseconds. It is represented as the decimal equivalent of 32 bits.
The <fsssar> and <bsssar> fields are 24-bit integers that can be
represented in decimal or in hex. The meaning and values of the
<fsssar> and <bsssar> fields are as follows:
Field Meaning Values
<fsssar> Maximum SSSAR-SDU size 1- 65,568
forward direction
<bsssar> Maximum SSSAR-SDU size 1- 65,568
backward direction
If present, the SSTED (Service-Specific Transmission Error Detection)
sublayer is above the SSSAR (Service-Specific Segmentation and
Reassembly) sublayer [12]. Since the maximum size of the SSTED-SDUs
can be derived from the maximum SSSAR-SDU size, it need not be
specified separately.
5.6.2.12 The 'aal2sscs3661assured' attribute
When present, the 'aal2sscs3661assured' attribute is used to indicate
the options that pertain to the assured transmission SSCS defined in
ITU I.366.1 [12] on the basis of ITU Q.2110 [43]. This SSCS can be
selected via the aalApp attribute defined below, or by virtue of the
presence of the 'aal2sscs3661assured' attribute. The format of this
media attribute line is as follows:
a=aal2sscs3661assured: <rastimer> <fsssar> <bsssar> <fsscopsdu>
<bsscopsdu><fsscopuu> <bsscopuu>
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
The <rastimer> subparameter indicates the SSSAR reassembly timer in
microseconds. It is represented as the decimal equivalent of 32
bits.
The <fsssar> and <bsssar> fields are 24-bit integers that can be
represented in decimal or in hex. The <fsscopsdu>, <bsscopsdu>,
<fsscopuu> and <bsscopuu> fields are 16-bit integers that can be
represented in decimal or in hex. The meaning and values of these
fields is as follows:
Field Meaning Values
<fsssar> Maximum SSSAR-SDU size 1- 65,568
forward direction
<bsssar> Maximum SSSAR-SDU size 1- 65,568
backward direction
<fsscopsdu> Maximum SSCOP-SDU size 1- 65,528
forward direction
<bsscopsdu> Maximum SSCOP-SDU size 1- 65,528
backward direction
<fsscopuu> Maximum SSCOP-UU field 1- 65,524
size, forward direction
<bsscopuu> Maximum SSCOP-UU field 1- 65,524
size, backward direction
The SSTED (Service-Specific Transmission Error Detection) sublayer is
above the SSSAR (Service-Specific Segmentation and Reassembly)
sublayer [12]. The SSADT (Service-Specific Assured Data Transfer)
sublayer is above the SSTED sublayer. Since the maximum size of the
SSTED-SDUs and SSADT-SDUs can be derived from the maximum SSSAR-SDU
size, they need not be specified separately.
The SSCOP protocol defined in [43] is used by the Assured Data
Transfer service defined in [12]. In the context of the ITU I.366.1
SSCS, it is possible to use the 'aal2sscs3661assured' attribute to
limit the maximum sizes of the SSCOP SDUs and UU (user-to-user)
fields in either direction. Note that it is necessary for the
parameters on the 'aal2sscs3661assured' media attribute line to be
consistent with each other.
5.6.2.13 The 'aal2sscs3662' attribute
When present, the 'aal2sscs3662' attribute is used to indicate the
options that pertain to the SSCS defined in ITU I.366.2 [13]. This
SSCS can be selected via the aalApp attribute defined below, or by
the presence of the 'aal2sscs3662' attribute.
The format of this media attribute line is as follows:
a=aal2sscs3662: <sap> <circuitMode> <frameMode> <faxDemod>
<cas> <dtmf> <mfall> <mfr1> <mfr2>
<PCMencoding> <fmaxFrame> <bmaxFrame>
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor. Additionally, the values of these
fields need to be consistent with each other. Inconsistencies should
be flagged as errors.
The <sap> field can take on the following string values: "AUDIO" and "MULTIRATE". These correspond to the audio and multirate Service Access Points (SAPs) defined in ITU I.366.2. For the multirate SAP, the following parameters on the aal2sscs3662 attribute line do not apply: <faxDemod>,<cas>, <dtmf>, <mfall>, <mfr1>, <mfr2> and <PCMencoding>. These are set to "-" for the multirate SAP. The <circuitMode> flag indicates whether the transport of circuit mode data is enabled or disabled, corresponding to the string values of "on" and "off" respectively. For the multirate SAP, it cannot have a value of "off". For the audio SAP, it can be assigned a value of "on", "off" or "-". Note that the <sbc> attribute, defined elsewhere in this document, can be used to specify the number of 64 kbps subchannels bundled into a circuit mode data channel. The <frameMode> flag indicates whether the transport of frame mode data is enabled or disabled, corresponding to the string values of "on" and "off" respectively. The <faxDemod> flag indicates whether facsimile demodulation and remodulation are enabled or disabled, corresponding to the string values of "on" and "off" respectively. The <cas> flag indicates whether the transport of Channel Associated Signaling (CAS) bits in AAL2 type 3 packets is enabled or disabled, corresponding to the string values of "on" and "off" respectively. The <dtmf> flag indicates whether the transport of DTMF dialled digits in AAL2 type 3 packets is enabled or disabled, corresponding to the string values of "on" and "off" respectively. The <mfall> flag indicates whether the transport of MF dialled digits in AAL2 type 3 packets is enabled or disabled, corresponding to the string values of "on" and "off" respectively. This flag enables MF dialled digits in a generic manner, without specifying type (e.g., R1, R2 etc.). The <mfr1> flag indicates whether the transport, in AAL2 type 3 packets, of MF dialled digits for signaling system R1 is enabled or disabled, corresponding to the string values of "on" and "off" respectively. The <mfr2> flag indicates whether the transport, in AAL2 type 3 packets, of MF dialled digits for signaling system R2 is enabled or disabled, corresponding to the string values of "on" and "off" respectively.
The <PCMencoding> field indicates whether PCM encoding, if used, is
based on the A-law or the Mu-law. This can be used to qualify the '
generic PCM' codec stated in some of the AAL2 profiles. The
<PCMencoding> field can take on the string values of "PCMA" and
"PCMU".
The <fmaxFrame> and <bmaxFrame> fields are 16-bit integers that can
be represented in decimal or in hex. The meaning and values of the
<fmaxFrame> and <bmaxFrame> fields are as follows:
Field Meaning Values
<fmaxFrame> Maximum length of a 1- 65,535
frame mode data unit,
forward direction
<bmaxFrame> Maximum length of a 1- 65,535
frame mode data unit,
backward direction
5.6.2.14 The 'aal5sscop' attribute
When present, the 'aal5sscop' attribute is used to indicate the
existence of an SSCOP [43] protocol layer over an AAL5 CPS layer
[21], and the parameters which pertain to this SSCOP layer. SSCOP
over AAL5 can also be selected via the aalApp attribute defined
below. The format of the 'aal5sscop' media attribute line is as
follows:
a=aal5sscop: <fsscopsdu> <bsscopsdu> <fsscopuu> <bsscopuu>
Each of these fields can be set to a "-" when the intention is to not
specify them in an SDP descriptor.
The representation, meaning and values of the <fsscopsdu>,
<bsscopsdu>, <fsscopuu> and <bsscopuu> fields are identical to those
for the 'aal2sscs3661assured' media attribute line (Section
5.6.2.12). Note that it is necessary for the parameters on the '
aal5sscop' media attribute line to be consistent with each other.
(page 58 continued on part 3)
