Question 22/VII - Routing principles for public data networks
(continuation of Question 32/VII studied in 1985-88)

Considering

(a) that Recommendation X.110 specifies the international routing
principles and routing plan for public data networks (PDNs);

(b) that Recommendation X.353 specifies the routing principles for
interconnecting public maritime mobile satellite data transmission
systems with public data networks;

(c) that PDNs are expanding rapidly on a world-wide basis;

(d) that interconnection of PDNs with other public networks is envisaged
or has been effected and the following cases of international
interworking will be required:

- between public networks of the same type through PDNs,

- between a PSPDN and a CSPDN,

- between a PDN of any type and a PSTN,

- between a PDN of any type and an ISDN,

- between a PDN of any type and the Telex network,

- between a PDN of any type and a public mobile satellite system,

- between a PDN of any type and a public land mobile system;

(e) that Recommendation X.122 specifies the short term numbering plan
interworking between PSDNs and ISDNs or PSTNs;

(f) that long term numbering plan interworking between PDNs and networks
of different types is required;

the following points should be studied:

1. What additional routing principles, routing plans or routing
algorithms should be specified internationally in Recommendations X.110,
X.353 or in possible new Recommendations, particularly, for the cases of
interworking between two public networks, including dissimilar networks?

Note - Possible routing principles for interworking between public
networks are given in Annex 1 in the form of text for a draft
Recommendation as a basis for further study.

2. What routing strategy should be specified for the case where a
network receives information on "network congestion" or "network
failure" of the adjacent networks?

3. What routing information other than DCC/DNIC should be transferred to
the adjacent network, possibly by use of new network utility fields in
PSPDNs or new additional signals in CSPDNs. Possibilities to be studied
include:

a) location of cross-over points to be used;

b) ISDN bearer service requested, (e.g., packet or circuit mode);

c) type of circuit switched IWF to be used (e.g., based on T.70 or
X.25);

d) type of terminal interface (e.g., digital on an ISDN, or analogue on
a PSTN or integrated ISDN/PSTN).

4. What routing algorithms should be specified so as to respond to
users' requirements related to "Quality of Service" (QOS), such as
transfer delay, for the case where interface/network protocols and
signalling systems provide methods for such user requirements and the
principles for defining and allocating QOS parameters are agreed to.

ANNEX 1

(to Question 22/VII)

Draft Recommendation X.11y

Routing principles applicable to public data networks for the
establishment of data calls between networks of different types

The CCITT,

considering

(a) that Recommendation X.1 defines the international user classes of
service in public data networks (PDNs) and ISDN;

(b) that Recommendation X.2 defines the international user services and
facilities in PDN and ISDN;

(c) that Recommendation X.10 defines the different categories of access
of data terminal equipments (DTEs) to the different data transmission
services provided by PDNs and ISDN;

(d) that Recommendations X.20, X.20bis, X.21, X.21bis, X.25, X.32, X.28
and X.29 specify the detailed procedures applicable to different types
of DTE/DCE interfaces on PDNs;

(e) that Recommendations X.30 and X.31 specify the support of X.21,
X.21bis and X.25 terminals on ISDNs;

(f) that Recommendations X.61, X.70, X.71 and X.75 specify the detailed
procedures applicable to call control between two PDNs of the same type;

(g) that Recommendation X.300 specifies the general principles for
interworking between PDNs and between PDNs and other networks;

(h) that Recommendation X.121 specifies the international numbering plan
for PDNs;

(i) that Recommendations I.330 and I.331 (E.164) respectively specify
ISDN numbering and addressing principles and the numbering plan for the
ISDN era;

(j) that Recommendation X.122 specifies the methods to be used for
numbering plan interworking;

(k) that Recommendation X.110 already specifies the international
routing principles and routing plan for PDNs;

(l) that Recommendation X.353 already specifies the routing principles
for interconnecting the maritime satellite data transmission system with
PDNs;

(m) that Recommendation E.172 specifies call routing in the ISDN era;

(n) the need that DTEs can communicate through different types of
networks;

(o) the need for routing principles for the establishment of data calls
between public networks of different types;

(unanimously) declares the view

that the routing principles applicable to public data networks for the
establishment of data calls between networks of different types be in
accordance with principles specified in this Recommendation.

CONTENTS

1. Introduction

2. Definition of the interworking routing problem

2.1 Context

2.2 Call route elements

2.3 Routing process

3.General routing principles for the establishment of data calls between
different types of networks

3.1 Interworking routing plan

3.2 Location, selection and number of cross-over points

4.Specific routing principles for different interworking conditions

4.1Routing principles for interworking at the network layer between
CSPDN and PSPDN

4.2Routing principles for switched access through a PSTN or CSPDN to a
PSPDN by a packet-mode DTE

4.3Routing principles for interworking at the network layer between CCSN
and PSPDN

4.4Routing principles for interworking via a non-OSI adapter between
PSTN and PSPDN

4.5Routing principles for interworking at the network layer between
PSPDN and private networks

4.6Routing principles for interworking between ISDN and PDNs

4.6.2Routing principles for interworking between CSPDN and ISDN where a
circuit switched bearer is requested

4.6.3Routing principles for interworking between PSPDN and ISDN where a
packet switched bearer is requested

4.6.3.1 Terminal with ISDN virtual circuit bearer service

4.6.3.2Terminal with ISDN transparent circuit switched service (for
further study)

4.6.4Routing principles for interworking between a PSPDN and an ISDN
where a circuit switched bearer is requested

4.6.5Routing principles for interworking between a CSPDN and an ISDN
that supports the packet switched virtual call scenario

4.7Routing principles for interworking between two ISDNs for the
provision of data transmission services

1. Introduction

1.1 In Recommendation X.300 it is recognized that there may be a demand
to interconnect different types of public networks; such as, public data
networks (PDNs) and integrated services digital networks (ISDNs) in
order for a DTE or one type of network to communicate with a DTE on
another type of network.

1.2 Recommendation X.300 includes within its scope the definition of the
principles and detailed arrangements for the interworking of different
networks in order to provide a data transmission service.

1.3 Implicit in the scope of Recommendation X.300, is the requirement
that a network of one type be able to effectively route calls to the
interfaces at which interworking with other types of networks takes
place, to allow the establishment of network - connections between
network-service users (within the end-systems) on different types of
networks.

1.4 The scope of this Recommendation is to define the general and
specific routing principles applicable to public data networks for the
establishment of network connections between DTEs on different types of
networks.

Note - The viewpoint taken in this Recommendation is that of the public
data network (PDN). Routing principles internal to other types of
network e.g., ISDN, are not included in the scope.

1.4.1 This Recommendation applies to the routing of calls which cross
international boundaries. Its application to calls within a single
country is a national matter.

1.4.2 Interworking involving more than two networks is included in the
scope of this Recommendation.

1.5 Recommendation X.110 applies to the routing of calls within a public
data network of a single type, even in the case where this network
interworks with a network of another type for the establishment of calls
requiring such interworking. Thus Recommendation X.11y is intended to
cover the specific routing principles applying to the interworking, and
does not supersede Recommendation X.110.

Note - Changes to Recommendation X.110 itself may be required to reflect
internal PDN routing requirements arising from interworking.

2. Definition of the interworking routing problem

2.1 Context

This Recommendation addresses the general problem of defining how
networks should determine the route to be taken by a data call in a
public network when both the calling and called DTEs are not connected
to the same type of network, or when a network of a different type from
those to which the calling and called DTEs are connected is used as a
transit network.

Note 1 - An interworking function specific to the type of interworking
may be required between two different types of networks. In certain
cases; e.g., port access, this interworking or access function may not
be directly involved in establishment of end-to-end network connections.

Note 2 - A call path may be either a circuit switched bearer or a
logical channel on a packet bearer, depending on the type of network
and/or bearer service requested.

2.2 Call route elements

2.2.1 Figure 1-2/X.11y illustrates five typical call routes involving
international interworking. Other possibilities exist, however, these
examples serve to indicate the typical elements which make up the route
of call involving interworking.

2.2.2 Examples a), b), and c) depict interworking from a network of one
type to that of a second type. Examples d) and e), depict interworking
where two networks of one type are connected via a transit route through
a network of a second type.

2.3 Routing process

2.3.1 For the routing of a call, the following types of information may
need to be considered at each node in all the networks involved:

a) numbering plan of called DTE;

b) called DTE address;

c) numbering plan of calling DTE;

d) calling DTE address;

e) incoming route;

f) type of bearer requested by calling party;

g) type of bearer requested by called party;

h) service requests (e.g., supplementary services in ISDN, and user
facilities/utilities in PDN). Examples are RPOA selection, and Quality
of Service indication;

i) information on prior routing history of call; e.g., number of
satellite hops, networks already transited.

Note - All the above information elements may not be available in all
types of networks, nor at all cross-over points between different types
of networks.

2.3.2 The above information is used as an input to a routing decision
process, which for the purposes of interworking, must in general
determine the following:

a) in the calling and transit networks, the type of network of the
called DTE;

b) in the calling and transit networks the location of the cross- over
points, and the routes to take to these points;

c) in the calling and transit networks, the appropriate type of
interworking units to be applied.

These determinations are to be made within the constraints imposed by
customer service requests, avoidance of looping, and barring of non-
permitted calls.

3.General routing principles for the establishment of data calls between
different types of networks

3.1 Interworking routing plan

3.1.1 The international routing of calls involving the connection of
networks of different types will be in accordance with an interworking
routing plan which is the responsibility of the administrations
concerned, and is subject to bilateral agreement.

3.1.2 The interworking routing plan will identify the agreed points of
interconnection of the different networks, and the location of required
interworking functions (IWFs).

3.2 Location, selection and number of cross-over points

3.2.1 The location at which interworking between two different types of
networks takes place, is termed the cross-over point. An interworking
function may be required at this point between the two networks.

3.2.2 The following principles, which govern the selection of a cross-
over point with respect to call origination, are for further study:

Principle 1

The originating network normally routes the call to an IWF in the
originating country;

Principle 2

Where an originating country has no IWF within its boundaries, it can
choose a route to access an IWF within the destination country.

Principle 3

The originating country may choose to route a call to access an IWF on
the international boundary between the source and a transit or
destination country, or between a transit and another transit or
destination country.

3.2.3 A cross-over point may be located within the national segment of a
route, or at an international boundary.

3.2.4 The preferred location, with respect to the boundary, of an
interworking unit between any two given types of networks interworking
across an international boundary is a function of the types of networks,
and is specified for each case, in section 4 of this Recommendation.

3.2.5 Where interworking between two types of networks takes place at
the network layer as defined in Recommendation X.300, the selection of
the cross- over point will be made by the Administrations concerned,
according to the routing plan and not by the users.

3.2.6 Where interworking between two types of networks takes place by
port access, as defined in Recommendation X.300 the user may be required
to select the cross-over point.

3.2.7 The number of cross-over points through which a call is routed
should be as low as possible; in most cases one. However, where it is
necessary to transit a network of one type when interworking between
networks of another type, two cross-over points are allowed.

Note - Given, that except in the case of PSPDNs, there is no signalling
information available on the route history of a call, it is for further
study how the number of cross-over points can be controlled.

4. Specific routing principles for different interworking conditions

This section defines the specific routing principles applicable to
different interworking conditions, involving the interworking of
different types of networks, where these networks interwork to provide a
data transmission service. Table 4-1/X.11y summarizes the different
interworking conditions considered, and indicates for each case whether
or not the specific routing principles are presently covered or are for
further study in this or other XSeries Recommendations (X.110, X.353).
Where covered by this Recommendation, a reference to the pertinent
sections is given.

4.1Routing principles for interworking at the network layer between
CSPDN and PSPDN

Section 8.2 of Recommendation X.300 presents three variations of this
interworking case:

a) where interworking takes place across an international boundary, and
only one PSPDN and one CSPDN are involved (Figure 81/X.300);

b) where a transit PSPDN is involved, and interworking takes place
internally in a country (Figure 82/X.300);

c) where a transit PSPDN is involved internationally, with interworking
at an international boundary (Figure 83/X.300).

Note - The case where the transit network is a CSPDN is also allowed by
exception.

4.1.1 In all three cases, the determination of the type of called
network and that this type is in fact different from the calling network
can be made from the DNIC of the called network.

4.1.1.1 The responsibilities for determining that interworking is
involved can lie with either the originating network, transit networks,
or both. The assignment of these responsibilities is through bilateral
agreement.

4.1.2 Where interworking takes place across an international boundary,
the Interworking Unit shall normally be located in the same country as
the CSPDN; however, the administrations involved may agree exceptionally
to placement of the IWU in the country where the PSPDN is located.

4.1.3 An originating or transit network which, for a given call, is not
able to determine that interworking is involved should route the call
per Recommendation X.110.

4.1.4 A network which receives a call and determines that interworking
is involved can:

a) immediately route the call to cross-over point, e.g., a cross- over
point connected to the originating node, or node at which the call is
received;

b) route the call through the network to a remote cross-over point; or

c) route the call to a transit network of the same type, where the
choice of transit network is consistent with the need for this call to
eventually arrive at a cross-over point.

4.1.5 Since the specific type of interworking function required may
depend on the type of terminal on the CSPDN, there is a need for further
study to determine how an IWF appropriate to the type of CSPDN terminal
can be chosen.

4.2Routing principles for switched access through a PSTN or CSPDN to a
PSPDN by a packet-mode DTE

Section 8.3 of Recommendation X.300 describes this case, and the two
sub-cases (CSPDN and PSTN access) are illustrated in Figure 84/X.300.

4.2.1 For the case where calls are incoming to the PSPDN, the routing of
the calls in the PSPDN is in accordance with Recommendation X.110.

4.2.2 For the case of outgoing calls to a CSPDN from a PSPDN the
responsibility for determining that interworking is involved and for
providing the interworking will normally fall to a PSPDN in the called
country. This internal interworking will be transparent to interworking
conditions between the PSPDN and other networks across an international
boundary; thus international routing shall be per Recommendation X.110.

Note - Study is required of whether the switched access may be provided
from networks other than the PSPDN in the called country. If it can, how
are these networks to discriminate between case 4.2 (switched access)
and 4.1 (network layer interworking)?

4.2.3 For the case of outgoing calls to a PSTN the format of the called
address will be a function of the time T.

4.2.3.1 Pre time T, an internationally standardized escape code "9" may
be used to indicate that the digits which follow are in E.163 format;
i.e., the called address may have the format 9 + TCC + NSN. It is also
possible that the called address will be in normal X.121 format. Such
will be the case when the PSTN has been assigned a DNIC as a national
option, or the PSTN terminal is registered on a PSPDN and assigned an
X.121 address.

4.2.3.2 Post time T, (or earlier by bilateral agreement) the format of
the called address will be indicated by the NAPI in X.25, X.28 and X.75
signalling. For calls to both PSTN and ISDN the NAPI must indicate E.164
format.

Note - It is for further study how, post time T, it shall be indicated
whether a call is for a terminal accessing a PSPDN per Recommendation
X.32, or a terminal accessing an ISDN per Recommendation X.31.

4.2.3.3 In both the pre and post time T cases, where the called address
is in normal X.121 format (DNIC + NTN) then the PSTN/PSPDN interworking
will be transparent to interworking conditions across an international
boundary; thus international routing shall be per Recommendation X.110.

4.2.3.4 Pre time T, in the case where the called address is in the
format 9 + TCC + NSN, the originating and transit PSPDNs may apply one
of the following routing strategies:

a) a network may choose to implement full E.163 digit analysis and route
on the basis of the E.163 number following the escape code "9";

b) a network may treat the 9 + TCC as a pseudo DNIC, and route using the
same digit analysis algorithm it applies to X.121 format addresses;

c) a network may choose to route entirely on the basis of the presence
of the escape code "9". In this case, the network will route the call to
a predetermined interworking interface with the PSTN, at which point a
switched access will be set up to the called DTE, using X.32 procedures
appropriate for an unregistered DTE.

Note - Since the DTE may very well be registered with a PSPDN in the
called country, the service implications of this type of routing require
further study.

4.2.3.5 The post time T routing procedure to be used is for further
study. (This procedure will be dependent on the solution chosen, for
post time T discrimination between PSTN and ISDN).

4.3Routing principles for interworking at the network layer between CCSN
and PSPDN

Section 8.4 of Recommendation X.300 describes this case and illustrates
it in Figure 8-5/X.300.

4.3.1 The routing principles applicable to this case are for further
study.

4.4Routing principles for interworking via a non-OSI adapter between
PSTN and PSPDN

Section 8.5 of Recommendation X.300 describes this case, and illustrates
two sub-cases:

a) direct interworking, with a possible arrangement illustrated in
Figure 8-6/X.300;

b) port access method, with a possible arrangement illustrated in Figure
8-7/X.300.

4.4.1 In both cases access is outgoing from PSTN to PSPDN only. Since
for both cases a) and b) above the interworking arrangements between the
PSTN and the PSPDN are transparent to interworking conditions across an
international boundary, international routing in the PSPDN shall be per
Recommendation X.110.

4.5Routing principles for interworking at the network layer between
PSPDN and private networks

Section 8.6 of Recommendation X.300 describes this case, and it is
illustrated in Figure 8-8/X.300.

4.5.1 The need for additional routing principles beyond those of
Recommendation X.110 is for further study.

4.6Routing principles for interworking between ISDN and PDNs

4.6.1 Recommendation X.300 describes a set of ISDN/PSPDN interworking
cases for both interworking at the network layer and port access. The
routing principles applicable to these cases are defined in the
following sections.

4.6.2Routing principles for interworking between CSPDN and ISDN where a
circuit switched bearer is requested

This type of interworking is described in Section 8.7.2 of
Recommendation X.300. Figure 8-9/X.300 presents two possible scenarios
for further study.

4.6.2.1 Specific routing principles for this case will be required;
however, the specification must await the outcome of the further studies
with respect to Recommendation X.300.

Note - The address analysis requirements should be similar to those for
PSPDN/ISDN interworking at the network layer, where a circuit switched
bearer is requested, as specified in Section 4.6.4.

4.6.3Routing principles for interworking between PSPDN and ISDN where a
packet switched bearer is requested

Section 8.7.3 of Recommendation X.300 describes the case where the
packet terminal on the ISDN has an ISDN virtual circuit bearer service.

This case is illustrated in Figure 8-10/X.300 and the routing principles
specified below in 4.6.3.1.

4.6.3.1 Terminal with ISDN virtual circuit bearer service

4.6.3.1.1 Since the ISDN is assumed to directly support X.75, no
interworking function is required for this type of interworking at the
cross-over point.

4.6.3.1.2 The location of the cross-over point with respect to an
international boundary is not described in X.300; it can be at any point
in the connection, internally within a national segment or across an
international boundary.

4.6.3.1.3 For incoming calls to the PSPDN the called address will always
be in X.121 format, and the PSPDN can route on the basis of this
address.

4.6.3.1.4 For outgoing calls from the PSPDN the format of the called
address will be a function of the time T.

4.6.3.1.5 Pre time T an escape code "0" at the DTE/DCE interface and
within the PSPDN may indicate that the address which follows is in E.164
format; i.e., the called address may have the format 0 + TCC + NDC +
NSN, with a pre time T length of not greater than 14 digits, including
the escape code.

4.6.3.1.6 Pre time T PSPDNs may choose to apply one of the following
routing strategies:

a) route based on full E.164 digit analysis;

b) treat the 0 + TCC as a pseudo DNIC;

c) escape immediately to an ISDN.

Note - The implications of this method require further study.

4.6.3.1.7 Post time T (or earlier by bilateral agreement) the format of
the called address will be indicated by the NAPI.

4.6.3.1.8 Post time T the called address of calls incoming to a PSPDN
will be in X.121 format, and the PSPDN can route on the basis of this
address.

4.6.3.1.9 Post time T the called address for calls outing from the PSPDN
will be in E.164 format. The PSPDN will be required to route on the
basis of E.164 digit analysis. The maximum number of digits which may
need to be analysed is for further study.

4.6.3.1.10 In both the pre and post time T cases for calls outgoing from
the PSPDN it will be necessary for the PSPDN to select a cross-over
point on the basis of the type of bearer service requested by the ISDN
terminal (packet switched in this case), plus the type of interworking
(network layer in this case). Further study is required on how this
selection is to be made.

4.6.3.2 Terminal with ISDN transparent circuit switched service

(For further study)

4.6.4Routing principles for interworking between a PSPDN and an ISDN
where a circuit switched bearer is requested

Recommendation X.300 describes two sub-cases of this type of
interworking:

i) interworking at the network layer as illustrated in Figure 811/X.300;

ii) interworking by post access, as illustrated in Figure 812/X.300.

4.6.4.1 In both the above cases, the pre and post time T called address
analysis requirements for routing are the same as in Section 4.6.3
above.

4.6.4.2 The same requirement specified in 4.6.3.1.10 above, concerning
the selection of the cross-over point applies in this case, with the
additional need to select an IWF appropriate to the type of called
circuit switched DTE.

4.6.5Routing principles for interworking between a CSPDN and an ISDN
that supports the packet switch virtual call scenario

This case is described in Section 8.7.5 of Recommendation X.300, and
illustrated in Figure 8-13/X.300.

4.6.5.1 For incoming calls to the CSPDN the called address will be in
X.121 format, thus routing of the call in the CSPDN will be based on
X.121 addresses.

4.6.5.2 For outgoing calls from the CSPDN the address analysis
requirements are the same as for PSPDN - ISDN interworking in Section
4.6.3.1.

4.6.5.3 The method to be used by the CSPDN to determine that a packet
switched bearer is requested by the ISDN terminal, is for further study.
This information is required to select the proper IWF.

4.7Routing principles for interworking between two ISDNs for the
provision of data transmission services

The routing principles applicable to this type of interworking are for
further study.