Essay/Term paper: What is isdn?

Essay, term paper, research paper:  Technology

What is ISDN?

ISDN, which stands for integrated services digital network, is a system of
digitizing phone networks which has been in the works for over a decade. This
system allows audio, video, and text data to be transmitted simultaneously
across the world using end-to-end digital connectivity.

The original telephone system used analog signals to transmit a signal across
telephone wires. The voice was carried by modulating an electric current with a
waveform from a microphone. The receiving end would then vibrate a speaker coil
for the sound to travel back to the ear through the air. Most telephones today
still use this method. Computers, however, are digital machines. All information
stored on them is represented by a bit, representing a zero or a one. Multiple
bits are used to represent characters, which then can represent words, numbers,
programs, etc. The analog signals are just varying voltages sent across the
wires over time. Digital signals are represented and transmitted by pulses with
a limited number of discrete voltage levels. [Hopkins]

The modem was certainly a big breakthrough in computer technology. It allowed
computers to communicate with each other by converting their digital
communications into an analog format to travel through the public phone network.
However, there is a limit to the amount of information that a common analog
telephone line can hold. Currently, it is about 28.8 kbit/s. [Hopkins] ISDN
allows multiple digital channels to be operated simultaneously through the same
regular phone jack in a home or office. The change comes about when the
telephone company's switches are upgraded to handle digital calls. Therefore,
the same wiring can be used, but a different signal is transmitted across the
line. [Hopkins] Previously, it was necessary to have a phone line for each
device you wished to use simultaneously. For example, one line each for the
phone, fax, computer, and live video conference. Transferring a file to someone
while talking on the phone, and seeing their live picture on a video screen
would require several expensive phone lines. [Griffiths] Using multiplexing (a
method of combining separate data signals together on one channel such that they
may be decoded again at the destination), it is possible to combine many
different digital data sources and have the information routed to the proper
destination. Since the line is digital, it is easier to keep the noise and
interference out while combining these signals. [Griffiths] ISDN technically
refers to a specific set of services provided through a limited and standardized
set of interfaces. This architecture provides a number of integrated services
currently provided by separate networks.

ISDN adds capabilities not found in standard phone service. The main feature is
that instead of the phone company sending a ring voltage signal to ring the bell
in your phone, it sends a digital package that tells who is calling (if
available), what type of call it is (data/voice), and what number was dialed (if
multiple numbers are used for a single line). ISDN phone equipment is then
capable of making intelligent decisions on how to answer the call. In the case
of a data call, baud rate and protocol information is also sent, making the
connection instantaneous. [Griffiths] ISDN Concepts:

With ISDN, voice and data are carried by bearer channels (B channels) occupying
a bandwidth of 64 kbit/s each. A delta channel (D channel) handles signalling at
16 kbit/s or 64 kbit/s. H channels are provided for user information at higher
bit rates. [Stallings] There are two types of ISDN service: Basic Rate ISDN
(BRI) and Primary Rate ISDN (PRI).

BRI: consists of two 64 kbit/s B channels and one 16 kbit/s D channel for a
total of 144 kbit/s. The basic service is intended to meet the needs of most
individual users. PRI: intended for users with greater capacity requirements.
Typically the channel structure is 23 B channels plus one 64 kbit/s D channel
for a total of 1.544 Mbit/s. H channels can also be implemented: H0=384 kbit/s,
H11=1536 kbit/s, H12=1920 kbit/s. [Stallings]

In this paper, I will concentrate on defining the specifics of Basic Rate ISDN
for local loop transmission. I will provide an in depth view of ISDN as it
relates to layer 1 to 3 of the seven layer OSI model. I will also provide the
specification for communication at the S/T customer interface.

Basic Rate ISDN:

Basic Rate Interface (BRI) - The BRI is the fundamental building block of an
ISDN network. It is composed of a single 16 kbit/s "D-channel" which is used for
call setup and control and two 64 kbit/s "B-channels". The B-channels can be
used to carry voice and both circuit mode and packet mode data traffic. The D-
channel may also be used to carry X.25 packet traffic if the network supports
that option. [Griffiths]

Basic Rate Interface D Channel - In the analog world, a telephone call is
controlled in-band. Tones and voltages are sent across lines for signalling
conditions. ISDN does away with this. The D channel becomes the vehicle for
signalling. This signalling is called common channel since a separate channel
for signalling is used by two or more bearer channels. [Hopkins]

User - Network protocols define how users interact with ISDN networks. Between
the user equipment and network equipment is a set of defined interfaces. The U
interface is between the central office and the customer premise. This
interface carries information on the twisted pair of wires between the customer
and the central office. At the S/T interface located at the customer location,
two pairs of wires (one for transmitting, one for receiving) are used. The
intermediate device between the U and the S/T interface is known as an NT1. The
NT1 is a hybrid that converts from four wire to two wire and also transforms the
2B+D signal into a different bit stream format. [Griffiths]

ISDN and the OSI Model - The OSI (Open Systems Interconnect) seven layer
protocol was developed to promote interoperability in the data world. ISDN,
which followed OSI, was designed to be a network technology inhabiting the lower
three layers of the OSI model. Consequently, an OSI end system that implements
an OSI seven layer stack can contain ISDN at the lower layers. Also, services
such as TCP/IP (Internet Transmission Control Protocol) can use the ISDN network.
[Griffiths]

Layer 1 of User-Network Interface:

Layer 1 protocols provide the details that describe how the signals (electrical
or optical) are encoded onto the physical medium. These protocols describe how
the user data and signalling bits are transformed into line signals, then back
again into user data bits. The ISDN layer 1 protocol supports the functions
outlined below. [ITU-T, I.430] ( B Channel Transmission ( D Channel Transmission
( D Channel Access Procedure

B Channel Transmission - Layer 1 must support for each direction of transmission,
two independent 64 kbit/s B channels. The B channels contain user data which is
switched by the network to provide the end-to-end transmission source. There is
no error correction provided by the network on these channels. [ITU-T, I.430]

D Channel Transmission - Layer 1 must support for each direction of transmission,
a 16 kbit/s channel for the signalling information. In some networks user
packet data may also be supported on the D channel. [ITU-T, I.430]

D Channel Access Procedure - This procedure ensures that in the case of two or
more terminals, on a point to multipoint configuration, attempting to access the
D Channel simultaneously, one terminal will always successfully complete the
transmission of information. [ITU-T, I.430]

Binary Organization of Layer 1 frame - The structures of Layer 1 frames across
the interface are different in each direction of transmission. Both structures
are shown in figure 1 below. [Griffiths]

A frame is 48 bits long and lasts 250(s. The bit rate is therefore 192 kbit/s
and each bit is approximately 5.2(s long. Figure 1 also shows that there is a
2-bit offset between transmit and receive frames. This is the delay between
frame start at the receiver of a terminal and the frame start of the transmitted
signal. [Griffiths] Figure 1 also illustrates that the line coding used is AMI
(Alternate Marks Inversion); a logical 1 is transmitted as zero volts and a
logical 0 as a positive or negative pulse. Note that this convention is the
inverse of that used on line transmission systems. The nominal pulse amplitude
is 750mV. [Griffiths] A frame contains several L bits. These are balance bits
to prevent a build up of DC on the line. For the direction TE to NT, where each
B-channel may come from a different terminal, each terminal's output contains an
L bit to form a balanced block. [ITU-T, I.430] Examining the frame in the NT to
TE direction, the first bits of the frame are the F/L pair, which is used in the
frame alignment procedure. The start of a new frame is signalled by the F/L
pair violating the AMI rules. Once a violation has occurred there must be a
second violation to restore correct polarity before the next frame. This takes
place with the first mark after the F/L pair. The FA bit ensures this second
violation occurs should there not be a mark in the B1, B2, D, E, or A channels.
The E channel is an echo channel in which D-channel bits arriving at the NT are
echoed back to the TEs. There is a 10 bit offset between the D channel leaving
a terminal, traveling to the NT and being echoed back in the E channel. [ITU-T,
I.430] The A bit is used in the activation procedure to indicate to the
terminals that the system is in synchronization. Next is a byte of the B2
channel, a bit of the E channel and a bit of the D channel, followed by an M bit.
This is used for multiframing. The M bit identifies some FA bits which can be
stolen to provide a management channel. [ITU-T, I.430] The B1, B2, D, and E
channels are then repeated along with the S bit which is a spare bit. [ITU-T,
I.430]

Layer 1 D Channel Contention Procedure - This procedure ensures that, even in
the case of two or more terminals attempting to access the D channel
simultaneously, one terminal will always successfully complete the transmission
of information by first gaining control of the D channel and then retransmitting
its information. The procedure relies on the fact that the information to be
transmitted consists of layer 2 frames delimited by flags consisting of the
binary pattern 01111110. Layer 2 applies a zero bit insertion algorithm to
prevent flag imitation by a layer 2 frame. The interframe time fill consists of
binary 1s which are represented by zero volts. The zero volt line signal is
generated by the TE transmitter going high impedance. This means a binary 0
from a parallel terminal will overwrite as binary 1. Detection of collision is
done by the terminal monitoring the E channel (D channel echoed from the NT).
[ITU-T, I.430]

To access the D channel a terminal looks for the interframe time fill by
counting the number of consecutive binary 1s in the D channel. Should a binary
0 be received the count is reset. When the number of consecutive 1s reaches a
predetermined value (which is greater than the number of consecutive 1s possible
in a frame because of the zero bit insertion algorithm) the counter is reset and
the terminal may access the D channel. When a terminal has just completed
transmitting a frame the value of the count needed to be reached before another
frame may be transmitted is incremented by 1. This gives other terminals a
chance to access the channel. Hence an access and priority mechanism is
established. [ITU-T, I.430] There is still the possibility of collision between
two terminals of the same priority. This is detected and resolved by each
terminal comparing its last transmitted bit with the next E bit. If they are
the same the terminal continues to transmit. If, however, they are different
the terminal detecting the difference ceases transmission immediately and
returns to the D channel monitoring state leaving the other terminal to continue
transmission. [ITU-T, I.430]

Layer 1 Activation/Deactivation Procedure - This procedure permits activation of
the interface from both the terminal and network side, but deactivation only
from the network side. This is because of the multi-terminal capability of the
interface. Activation and deactivation information is conveyed across the
interface by the use of line signals called

'Info signals'. [ITU-T, I.430] Info 0 is the absence of any line signal; this is
the idle state with neither terminals nor the NT working. [ITU-T, I.430] Info 1
is flags transmitted from a terminal to the NT to request activation. Note this
signal is not synchronized to the network. [ITU-T, I.430] Info 2 is transmitted
from the NT to the TEs to request their activation or to indicate that the NT
has activated as a response to receiving an Info 1. An Info 2 consists of Layer
1 frames with a high density of binary zeros in the data channels which permits
fast synchronization of the terminals. [ITU-T, I.430] Info 2 and Info 4 are
frames containing operational data transmitted from the TE and NT
respectively.[ITU-T, I.430] The principal activation sequence is commenced when
a terminal transmits an Info 1. The NT activates the local transmission system
which indicates to the exchange that the customer is activating. The NT1
responds to the terminals with an Info 2 to which the TEs synchronize. The TEs
respond with an Info 3 containing operational data and the NT is then in a
position to send Info 4 frames. Note that all terminals activate in parallel;
it is not possible to have just one terminal activated in a multi-terminal
configuration. The network activates the bus by the exchange activating the
local network transmission system. Deactivation occurs when the exchange
deactivates the local network transmission system. [ITU-T, I.430]

Layer 2 of User-Network Interface:

The Layer 2 recommendation 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
endpoints connected by a physical medium. Layer 3 call control information is
carried in the information elements of Layer 2 frames and it must be delivered
in sequence and without error. Layer 2 also has the responsibility for
detecting and retransmitting lost frames.

LAP D was based originally on LAP B of the X.25 Layer 2 recommendation. However,
certain features of LAP D give it significant advantages. The most striking
difference is the possibility of frame multiplexing by having separate addresses
at Layer 2 allowing many LAPs to exist on the same physical connection. It is
this feature that allows up to eight terminals to share the signalling channel
in the passive bus arrangement. [ITU-T, Q.920] Each Layer 2 connection is a
separate LAP and the termination points for the LAPs are within the terminals at
one end and at the periphery of the exchange at the other. Layer 2 operates as
a series of frame exchanges between the two communicating, or peer entities.
The frames consist of a sequence of eight bit elements and the elements in the
sequence define their meaning as shown in Figure 2 below. [ITU-T, Q.920]

A fixed pattern called a flag is used to indicate both the beginning and end of
a frame. Two octets are needed for the Layer 2 address and carry a service
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 the Layer 2 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. A detailed breakdown of the individual elements is given in Figures
3 and 4 below. [ITU-T, Q.920] What cannot be shown in the diagrams is the
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 Layer 2 entity discards a 0 bit if it is
preceded by five 1's. [ITU-T, Q.920]

Layer 2 Addressing - Layer 2 multiplexing is achieved by employing a separate
Layer 2 address for each LAP in the system. To carry the LAP 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 LAP. No use can
be made of the address by the network for routing purposes and no information
about its value will be held outside the Layer 2 entity. [ITU-T, Q.921]

The Layer 2 address is constructed as shown in Figure 3. The Service Access
Identifier (SAPI) is used to identify the service intended for the signalling
frame. An extension of the use of the D channel is to use it for access to a
packet service as well as for signalling. Consider the case of digital
telephones sharing a passive bus with packet terminals. The two terminal types
will be accessing different services and possibly different networks. It is
possible to identify the service being invoked by using a different SAPI for
each service. This gives the network the option of handling the signalling
associated with different services in separate modules. In a multi-network ISDN
it allows Layer 2 routing to the appropriate network. The value of the SAPI is
fixed for a given service. [ITU-T, Q.921] The Terminal Endpoint Identifier (TEI)
takes a range of values that are associated with terminals on the customer's
line. In the simplest case each terminal will have a single unique TEI value.
The combination of TEI and SAPI identify the LAP and provide a unique Layer 2
address. A terminal will use its Layer 2 address in all transmitted frames and
only frames received carrying the correct address will be processed. [ITU-T,
Q.921] In practice a frame originating from telephony call control has a SAPI
that identifies the frame as 'telephony' and all telephone equipment examine
this frame. Only the terminal whose TEI agrees with that carried by the frame
will pass it to the Layer 2 and Layer 3 entities for processing. There is also
a SAPI identified in standards for user data packet communication. [ITU-T,
Q.921] Since 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. The values that TEIs can take fall into the ranges:

0-63 Non-Automatic Assignment TEIs
64-126 Automatic Assignment TEIs
127 Global TEI [ITU-T, Q.921]

Non-Automatic TEIs are selected by the user; their allocation is the
responsibility of the user. Automatic TEIs are selected by the network; their
allocation is the responsibility of the network. The global TEI is permanently
allocated and is referred to as the broadcast TEI. [ITU-T, Q.921] Terminals
which use TEIs in the range of 0-63 need not negotiate with the network before
establishing a Layer 2 connection. Terminals which use TEIs in the range 64-126
cannot establish a Layer 2 connection until they have requested a TEI from the
network. In this case it is the responsibility of the network not to allocate
the same TEI more than once at any given time. The global TEI is used to
broadcast information to all terminals within a given SAPI; for example a
broadcast message to all telephones, offering an incoming telephone call. [ITU-T,
Q.921]

Layer 2 Operation - The function of Layer 2 is to deliver Layer 3 frames, across
a Layer 1 interface, error free and in sequence. It is necessary for a Layer 2
entity to interface both Layer 1 and Layer 3. To highlight the operation of
Layer 2 we will consider the operation of a terminal as it attempts to signal
with the network. [ITU-T, Q.921]

It is the action to establish a call that causes protocol exchange between
terminal and network. If there has been no previous communication it is
necessary to activate the interface in a controlled way. A request for service
from the customer results in Layer 3 requesting a service from Layer 2. Layer 2
cannot offer a service unless Layer 1 is available and so a request is made to
Layer 1. Layer 1 then initiates its start-up procedure and the physical link
becomes available for Layer 2 frames. Before Layer 2 is ready to offer its
services to Layer 3 it must initiate the Layer 2 start-up procedure known as
'establishing a LAP'. [ITU-T, Q.921] LAP establishment is achieved by the
exchange of Layer 2 frames between the Layer 2 handler in the terminal and the
corresponding Layer 2 handler in the network. The purpose of this exchange is
to align the state variables that will be used to ensure the correct sequencing
of information frames. Before the LAP 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). [ITU-T,
Q.921] Once the LAP is established Layer 2 is able to carry the Layer 3
information and is said to be the 'multiple frame established state'. In this
state Layer 2 operates its frame protection mechanisms. Figure 5 below shows a
normal Layer 2 frame exchange. [ITU-T, Q.921]

Once established the LAP operates an acknowledged service in which every
information frame must be responded to by the peer entity. The most basic
response is the Receiver Ready (RR) response frame. Figure 5 shows the LAP
establishment and the subsequent I frame RR exchanges. The number of I frames
allowed to be outstanding without an acknowledgment is defined as the window
size and can vary between 1 and 127. For telephony signalling applications the
window size is 1 and after transmitting an I frame the Layer 2 entity will await
a response from the corresponding peer entity before attempting to transmit the
next I frame. Providing there are no errors all that would be observed on the
bus would be the exchange of I frames and RR responses. However Layer 2 is able
to maintain the correct flow of information in the face of many different error
types. [ITU-T, Q.921]

Layer 2 Error Control - It is unlikely that a frame will disappear completely
but it is possible for frames to be corrupted by noise at Layer 1. Corrupted
frames will be received with invalid Frame Check Sequence (FCS) values and
consequently discarded. [ITU-T, Q.920]

The frame check sequence is generated by dividing the bit sequence starting at
the address up to (but not including) the start of the frame check sequence by
the generator polynomial X16 + X12 + X5 + 1. In practical terms this is done by
a shift register as shown in figure 6. All registers are preset to 1 initially.
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 1101 0000 1111. [ITU-T, Q.920]

The method for recovering from a lost frame is based on the expiration of a
timer. 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 as the loss of either will cause
it to expire. [ITU-T, Q.920] When the timer expires it is not possible to tell
which of the two frames has been lost and the action taken is the same in both
cases. Upon the timer expiring, Layer 2 transmits a command with the poll bit
set. This frame forces 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. If however the original
frame was lost, the solicited response will not be an appropriate acknowledgment
and the Layer 2 entity will know that a retransmission is required.

It is possible for the same frame to be lost more than once and Layer 2 will
restransmit the frame three times. If after three transmissions of the frame
the correct response has not been received , Layer 2 will assume that the
connection has failed and will attempt to re-establish the LAP. [ITU-T, Q.921]

Another possible protocol error is the arrival of an I frame with an invalid
send sequence number N(S). This error is more likely to occur when the LAP is
operating with a window size greater than one. If, for example, the third frame
in the sequence of four is lost the receiving Layer 2 entity will know that a
frame has been lost from the discontinuity in the sequence numbers. The Layer 2
must not acknowledge the fourth frame as this will imply acknowledgment of the
lost third frame. The correction operation is to send a Reject (REJ) frame with
the receive sequence number N(R) equal to N(S) + 1 where N(S) is the send
variable of the last correctly received I frame, in this case I frame 2. This
does two things; first it acknowledges all the outstanding I frames up to and
including the second I frame, and secondly it causes the sending end to
retransmit all outstanding I frames starting with the lost third frame. [ITU-T,
Q.920] The receipt of a frame with an out of sequence, or invalid, N(R) does not
indicate a frame loss and cannot be corrected by retransmissions. It is
necessary in this case to re-establish the LAP to realign the state variables at
each end of the link. [ITU-T, Q.920] The Receiver Not Ready (RNR) frame is used
to inhibit the peer Layer 2 from transmitting I frames. The reasons for wanting
to do this are not detailed in the specification but it is possible to imagine a
situation where Layer 3 is only one of many functions to be serviced by a
microprocessor and a job of higher priority requires that no Layer 3 processing
is performed. [ITU-T, Q.920] Another frame specified in Layer 2 is the FRaMe
Reject frame (FRMR). This frame may be received by a Layer 2 entity but may not
be transmitted. It is included in the recommendation to preserve alignment
between LAP D and LAP B. After the detection of a frame reject condition the
data link is reset. [ITU-T, Q.920]

Disconnecting the LAP - After Layer 3 has released the call it informs Layer 2
that it no longer requires a service. Layer 2 then performs its own
disconnection procedures so that ultimately Layer 1 can disconnect and the
transmission systems associated with the local line and the customer's bus can
be deactivated. [ITU-T, Q.921]

Layer 2 disconnection is achieved when the frames disconnect (DISC) and UA are
exchanged between peers. At this point the LAP can no longer support the
exchange of I frames and supervisory frames. [ITU-T, Q.921] The last frame type
to be considered is the Disconnect Mode (DM) frame. This 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 Layer 2 entity is unable to establish the LAP, and a
response to a DISC if the Layer 2 entity has already disconnected the LAP. [ITU-
T, Q.921]

TEI Allocation - Because each terminal must operate using a unique TEI,
procedures have been defined in a Layer 2 management entity to control their use.
The TEI manager has the ability to allocate, remove, check, and verify TEIs
that are in use on the customer's bus. As the management entity is a separate
service point all messages associated with TEI management are transmitted with a
management SAPI. [ITU-T, Q.921]

TEI management procedures must operate regardless of the Layer 2 state and so
the unnumbered information frame (UI) is used for all management messages. The
UI frames have no Layer 2 response and protection of the frame content is
achieved by multiple transmissions of the frame.

In order to communicate with terminals which have not yet been allocated TEIs a
global TEI is used. All management frames are transmitted on a broadcast TEI
which is associated with a LAP that is always available. All terminals can
transmit and receive on the broadcast TEI as well as their own unique TEI. All
terminals on the customer's line will process all management frames. To ensure
that only one terminal acts upon a frame a unique reference number is passed
between the terminal and the network. This reference number is contained within
an element in the UI frame and is either a number randomly generated by the
terminal, or 0 is the TEI of the terminal, depending on the exact situation.
Figure 7 below shows the frame exchange required for a terminal to be allocated
a TEI and establish its data link connection. [ITU-T, Q.921]

Layer 3 of User-Network Interface:

This layer effects the establishment and control of connections. It is carried
in Layer 2 frames as can be seen in figure 8. [ITU-T, Q.930]

The first octet contains a protocol discriminator which gives the D channel the
capability of simultaneously supporting additional communications protocols in
the future. The bits shown in figure 8 are the standard for user-network call
control messages. [ITU-T, Q.930] The call reference value in the third octet is
used to identify the call with which a particular message is associated. Thus a
call can be identified independently of the communications channel on which it
is supported.

The message type coded in the fourth octet describes the intention of the
message (e.g. a SETUP message to request call establishment). These are listed
in Table 1 at the end of this paper. A number of other information elements may
be included following the message type code in the fourth octet. The exact
contents of a message are dependent on the message type. [ITU-T, Q.931]

The message sequence for call establishment is shown in figure 9. In order to
make an outgoing call request, a user must send all of the necessary call
information to the network. Furthermore, the user must specify the particular
bearer service required for the call (i.e. Speech, 64 kbit/s/s unrestricted, or
3.1 kHz Audio) and any terminal compatibility information which must be checked
at the destination. [ITU-T, Q.931]

The initial outgoing call request may be made in an en bloc or overlap manner.
Figure 9 illustrates the call establishment procedures. If overlap sending is
used then the SETUP message must contain the bearer service request but the
facility requests and called party number information may be segmented and
conveyed in a sequence of INFORMATION messages as shown. Furthermore if a
speech bearer service is requested and no call information is contained in the
SETUP message, then the network will return in-band dial tone to the user until
the first INFORMATION message has been received. [ITU-T, Q.931] Following the
receipt of sufficient information for call establishment , the network returns a
call PROCEEDING