Service characteristics

4.1 General

The data link layer provides services to layer 3 and to the layer 2 m anagem ent entity and utilizes the services provided by the physical layer and layer managem ent. A formal description of the data link layer service provided to layer 3 and layer m anagem ent is given in § 4.2 and § 4.3, respectively. The layer m anagem ent service provided to the data link layer is given in § 4.4.

Note - C om m unication between different layers in the OSI reference model makes use of primitives which are passed across the layer boundaries. The data link layer primitives defined in this Recom m endation represent, in an abstract way, the logical exchange o f inform ation and control between the data link layer and adjacent layers. They do not specify nor constrain im plem entations.

4.2 Services provided to layer 3

The specification o f the interactions with layer 3, (primitives) provides a description o f the services that the data link layer, plus the physical layer, offer to layer 3, as viewed from layer 3.

Two forms o f inform ation transfer service are associated with layer 3. The first is based on unacknow ­ ledged inform ation transfer at the data link layer while the second service is based on acknowledged inform ation transfer at the data link layer.

Layer 3 message units are handled according to their respective layer 2 priority (see § 5.2).

4.2.1 Unacknowledged information transfer service

Note — In this case the inform ation transfer is not acknowledged at the data link layer. Acknowledge­

ment procedures may be provided at higher layers.

Fascicle VI. 10 — Rec. Q.920 ¹¹

The inform ation transfer is via broadcast or point-to-point data link connections.

The characteristics of the unacknow ledged inform ation transfer service are summarized in the following:

a) provision o f a data link connection between layer 3 entities for unacknow ledged inform ation transfer

o f layer 3 message units; /

b) identification o f data link connection endpoints; and

c) no verification o f message arrival within the peer data link layer entity.

The primitives associated with the unacknow ledged inform ation transfer service are:

D L -U N IT D A T A -R E Q U E S T /IN D IC A T IO N

The D L -U N IT D A TA -REQ U EST prim itive is used to request th at a message unit be sent using the procedures for unacknow ledged inform ation transfer service. The D L -U N IT D A T A -IN D IC A T IO N prim itive indicates the arrival o f a message unit received by m eans o f an unacknow ledged inform ation transfer service.

4.2.2 Acknowledged information transfer service

One mode o f operation is defined, multiple frame.

The characteristics o f the acknowledged inform ation transfer service are summarized in the following:

a) provision o f a data link connection between layer 3 entities for acknowledged inform ation transfer o f layer 3 message units;

b) identification o f data link connection endpoints;

c) sequence integrity o f d a ta link layer message units in the absence o f m alfunctions;

d) notification to the peer entity in the case o f errors, for example, loss o f sequence;

e) notification to the m anagem ent entity o f unrecoverable errors detected by the data link layer; and f) flow control.

The primitives associated with the acknowledged inform ation transfer services are:

i) Data transfer

D L -D A T A -R E Q U E S T /IN D IC A T IO N

The D L-D A TA -REQ U EST primitive is used to request that a message unit be sent using the procedures for the acknowledged inform ation transfer service. The D L-D A TA -IN D IC A T IO N prim i­

tive indicates the arrival o f a message unit received by m eans o f the acknowledged inform ation transfer service.

ii) Establishment o f multiple fra m e operation

D L -E S T A B L IS H -R E Q U E S T /IN D IC A T IO N /C O N F IR M

These primitives are used, respectively, to request, indicate and confirm the establishm ent o f m ultiple frame operation between two service access points.

iii) Termination o f multiple fra m e operation

D L -R E L E A S E -R E Q U E S T /IN D IC A T IO N /C O N F IR M

These primitives are used, respectively, to request, indicate and confirm an attem pt to term inate m ultiple fram e operation between two service access points.

f

4.3 Services provided to layer m anagement

Only the unacknow ledged inform ation transfer service is provided to layer m anagem ent in order th at the data link layer m anagem ent can com m unicate with its peer layer managem ent.

Note — In this case the inform ation transfer is not acknowledged at the data link layer. Acknowledge­

ment procedures may be provided by layer managem ent.

The inform ation transfer is via broadcast connections, but in principle inform ation transfer can also be via point-to-point connections [no application for data transfer via point-to-point connections has been identified or included in Recom m endation Q.921(L441)].

12 Fascicle VI. 10 — Rec. Q.920

The characteristics o f the unacknow ledged inform ation transfer service are sum m arized in the following:

a) provision of a data link connection between layer m anagem ent entities for unacknow ledged inform a­

tion transfer o f data units;

b) identification of data link connection endpoints; and

c) no verification of message arrival within the peer data link layer entity.

The primitives associated with the unacknow ledged inform ation transfer service provided for layer m anagem ent are:

M D L-U N IT D A T A -R E Q U E S T /IN D IC A T IO N

The M D L -U N IT D A TA -REQ U EST prim itive is used to request that a message unit be sent using the procedure for unacknow ledged inform ation transfer service for layer managem ent. The M D L -U N IT D A TA -IN D IC A T IO N prim itive indicates the arrival of a message unit received by m eans o f the unacknowledged inform ation transfer service to layer m anagement.

4.4 Administrative services

The characteristics of the adm inistrative services currently recognized are sum m arized in the following:

a) assignment, checking, and removal of TEI values; and

b) data link connection param eter passing (an optional service perform ed on a per connection basis).

These services are considered to be conceptually provided by layer m anagem ent either on the user side or the netw ork side. The method o f describing these adm inistrative functions uses service primitives.

The primitives associated with these services are:

i) Assignment o f TE I value

M D L -A S S IG N -R E Q U E S T /IN D IC A T IO N

The M D L -A S S IG N -IN D IC A T IO N prim itive is used to indicate to layer m anagem ent the need for a TEI value. The M D L-A SSIG N -R EQ U EST prim itive is used to pass the TEI value from layer m anagem ent to the data link layer in order that the user data link layer entities can begin to com m unicate with the network data link layer entities.

ii) Removal o f TE I value M D L-REM O V E-REQ U EST

This prim itive is used to convey a layer m anagem ent function request for removal o f a TEI value that has been previously assigned via the M D L-A SSIG N primitives.

iii) Notification o f error

M D L -E R R O R -IN D IC A T IO N /R E S P O N S E

These primitives are used to report error situations between layer m anagem ent and the data link layer entities.

4.5 M odel o f the data link service

4.5.1 General

The ability o f the data link layer to execute a service request by layer 3 depends on the internal state o f the data link layer. For the layer 3 entity, the internal state of the data link layer is represented by the state o f that data link connection endpoint within a data link service access point which is used by this layer 3 entity to invoke a service.

Consequently, the data link service may be defined by means o f data link connection endpoint states, whereby the capabilities provided by the data link layer and the service primitives may be related to these states.

In order to allow a data link service user to invoke a service by making use o f primitives, the DL-primitives defined in R ecom m endation Q.921 (1.441) have to be related to: point-to-point data link connections (acknowledged or unacknow ledged transfer o f inform ation) a n d /o r broadcast data link connections (unacknow ­ ledged transfer o f inform ation) (see Table 1 /Q.920).

Fascicle VI.10 - Rec. Q.920 13

An unconfirm ed service is defined as, a service which does not result in an explicit confirm ation. A confirm ed service is defined as a service which results in an explicit confirm ation from the service-provider. There is not necessarily any relationship to a response from the peer service-user.

TABLE 1/Q .920

Applicability o f DL-Primitives to information transfer modes

G eneric name o f the DL-prim itive

PO IN T-TO -PO IN T

IN FO R M A T IO N T R A N SFER M O D E BROADCAST IN FO R M A T IO N T R A N SFER M O DE A C K N O W L ED G E D U N A C K N O W L E D G E D

ESTABLISH C O N F IR M E D SERV ICE

RELEASE C O N F IR M E D SERV ICE

DATA U N C O N F IR M E D

SERV ICE

U N IT DATA U N C O N F IR M E D

SERV ICE

U N C O N F IR M E D SERV ICE

4.5.2 Data link layer representation as seen by layer 3

4.5.2.1 Data link connection endpoint states

The states o f a data link connection endpoint may be derived from the internal states o f the data link layer entity supporting this type of a data link connection.

4.5.2.2 Broadcast data link layer connection services

A broadcast data link connection provides an unacknow ledged inform ation transfer service.

The broadcast data link connection endpoint is always in the inform ation transfer state.

4.5.2.3 Point-to-point data link connection endpoint services

A point-to-point data link connection provides both an unacknow ledged and acknowledged inform ation transfer service. W ithin each data link service access point, one or more than one data link connection endpoint may be present, each identified by a CES.

The acknowledged inform ation transfer service, in addition, implies the presence of the link establishm ent, link re-establishm ent and link release services.

The point-to-point data link connection endpoint states are:

— link connection released state;

— awaiting establish state;

— awaiting release state;

— link connection established state.

1 4 Fascicle VI.10 — Rec. Q.920

4.5.2.4 Sequences o f primitives at one point-to-point data link connection endpoint

The primitives provide the procedural means to specify conceptually how a data link service user can invoke a service.

This section defines the constraints on the sequence in which the primitives may occur. The sequences are related to the states at one point-to-point data link connection endpoint.

The possible overall sequences o f primitives at a point-to-point data link connection endpoint are defined in the state transition diagram , Figure 9/Q.920. The lin k co n n ectio n re le a se d and lin k c o n n ectio n e sta b lis h e d states are stable states whilst the a w a itin g esta b lish and a w a itin g relea se are transition states.

4.6 S ervices re q u ire d fr o m th e p h y s ic a l la yer

The services provided by the physical layer are described in detail in Recom m endation 1.430 [8] or 1.431 [9]. They are summarized in the following:

a) physical layer connection for the transparent transm ission of bits in the same order in which they are subm itted to the physical layer;

b) indication of the physical status of the D -channel; and

c) transm ission of data link layer message units according to their respective data link layer priority.

Some of th e'ab o v e services may be im plem ented in the m anagem ent entity on the user side or network side. The method o f describing these services is by means o f service primitives. The primitives between the data link layer and the physical layer are:

i) P H -D A T A -R E Q U E ST /IN D IC A T IO N

These primitives are used to request that a message unit be sent and to indicate the arrival o f a message unit.

ii) A c tiv a tio n

PH -A C T IV A T E -R E Q U E ST /IN D IC A T IO N

These primitives are used to request activation of the physical layer connection, and to indicate that the physical layer connection has been activated.

iii) D ea ctiv a tio n

PH -D E A C T IV A T E -R E Q U E ST /IN D IC A T IO N

This primitive is used to indicate that the physical layer connection has been deactivated.