Last update: 04.12.1995 trosa
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The layer 2 Recommendation (Q.921/I.441) describes the high level
data link (HDLC) procedures commonly referred to as the Link Access
Procedure for a D channel or LAP D. The objective of layer 2 is
to provide a secure, error-free connection between two end-points
connected by a physical medium. Layer 3 Call control information
is carried in the information elements of layer 2 frames and must
be delivered in sequence and without error. Layer 2 also has responsibility
for detecting and retransmitting lost frames. In LAP-D is also
included a feature that make it possible to multiplex frames by
having separate addresses at layer 2 allowing many link access
protocols to exist on the same physical connection. It is this
feature that allows up to eight terminals to share the signaling
channel in the passive bus arrangement. Each layer 2 connection
is a separate link access protocol and the termination points
for the link access protocols are within the terminals at one
end and at the periphery of the exchange at the other. Multiple
layer 3 entities is also supported. Discrimination between the
data link connections is by means of a data link connection identifier
(DLCI) contained in each frame.
Layer 2 operates as a series of frame exchanges between the two
communicating, or peer, entities. A fixed pattern, called flag,
is used to indicate both the beginning and end of a frame. Two
octets are needed for the Q.921 address to carry a Service Access
Point Identifier (SAPI), a terminal identifier (TEI) and a command/response
bit. The control field is one or two octets depending on the frame
type and carries information that identifies the frame and Q.921
sequence numbers used for link control. The information element
is only present in frames that carry layer 3 information and the
Frame Check Sequence (FCS) is used for error detection. There
is a procedure to avoid imitation of the flag by the data octets.
This is achieved by examining the serial stream between flags
and inserting an extra 0 after any run of five 1 bits. The receiving
Q.921 entity discards a 0 bit if it is preceded by five 1s.
Q.921 multiplexing is achieved by employing a separate Q.921 address
for each link access protocol in the system. To carry the link
access protocol identity the address is two octets long and identifies
the intended receiver of a command frame and the transmitter of
a response frame. The address has only local significance and
is known only to the two end-points using the link access protocol.
The address can not be used by the network for routing purposes
and no information about its value will be held outside the Q.921
entity. The Service Access Point Identifier (SAPI) is used to
identify the service that the signaling frame is intended for.
The value of the SAPI is fixed for a given service. The Terminal
Endpoint Identifier (TEI) takes a range of values that are associated
with terminals on the customer's line. As it is important that
no two TEIs are the same, the network has a special TEI management
entity which allocates TEI on request and ensures their correct
use. Non-automatic TEIs are selected by the user and therefore
the responsibility for their allocation is the users. The global
TEI is permanently allocated and is also refereed to as the broadcast
TEI. The broadcast TEI is used to broadcast a message to all terminals
with a given SAPI.
A request for service from the customer results in layer 3 requesting
a service from Q.921 by sending a DL_ESTABLISH primitive. Q.921
cannot offer a service unless layer 1 is available and so the
appropriate request is made to layer 1. Before Q.921 is ready
to offer its services to layer 3 it must establish the multiple
frames connection to its peer entity at the network side. Before
the multiple frames connection has been established the only frames
that may be transmitted are unnumbered frames. The establishment
procedure requires one end-point to transmit a Set Asynchronous
Balanced Mode Extended (SABME) and the far end to acknowledge
it with an Unnumbered Acknowledgment (UA). Once the multiple frames
connection is established Q.921 is able to carry layer 3 information.
In this state Q.921s frame protection mechanisms is in use. Once
established the link access protocol operates an acknowledged
service in which every information frame must be responded to
by the peer entity. Providing there are no errors all that would
be observed on the bus would be the exchange of I frames and RR
responses. Q.921 however is able to maintain the correct flow
of information, in the face of many different error types.
It is not very likely that a frame will disappear completely but
it is possible for frames to be corrupted by noise at the physical
line. Corrupted frames will be received with invalid Frame Check
Sequence (FCS) values and consequently discarded. This is done
by a shift register where all bits are initially preset to 1.
At the end of the protected bits the shift register contains the
remainder from the division. The 1's complement of the remainder
is the FCS. At the receiver the same process is gone through,
but this time the FCS is included in the division process. In
the absence of transmission errors the remainder should always
be 0001 1101 0000 1111. A timer is started every time a command
frame is transmitted and is stopped when the appropriate response
is received. This single timer is thus able to protect both the
command and response frame as the loss of either will cause it
to expire. When the timer expires Q.921 transmits a command frame
with the poll bit set. This frame force the peer to transmit a
response that indicates the value held by the state variables.
It is possible to tell from the value carried by the response
frame whether or not the original frame was received. If the first
frame was received, the solicited response frame will be the same
as the lost response frame and is an acceptable acknowledgment.
However, if the original frame was lost, the solicited response
will not be an appropriate acknowledgment and the Q.921 entity
will know that a retransmission is required. Q.921 will retransmit
a frame three times an after that it will try to re-establish
the multiple frames connection. The Receiver Not Ready (RNR) frame
is used to inhibit the peer Q.921 from transmitting I frames.
The FRaMe Reject frame (FRMR) may be received by a Q.921 entity
but may not be transmitted. After the detection of a frame reject
condition the data link is reset.
Q.921 goes to unnumbered frames state when the frames disconnect
(DISC) and UA are exchanged between peers. At this point the link
access protocol can no longer support the exchange of I frames
and supervisory frames. The Disconnect Mode (DM) frame is an unnumbered
acknowledgment and may be used in the same way as a UA frame.
It is used as a response to a SABME if the Q.921 entity is unable
to establish the link access protocol, and a response to a DISC
if the link access protocol already has been disconnected.
The Q.921/I.441/LAP-D protocol
Addressing
Operation
Error control
Returning to unnumbered frames state
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