X.25 Protocol





This project was written by: Levi Ehud, Hadad Eli and
Epstein Amir as part of the course : Protocols and Computer Networks
given by Dr.Debby Koren at the Tel-Aviv University, Israel.
References


I n t r o d
u c t i o n




In the early 1970's there were many data communication networks(also known as
Public Networks), which were owned by private companies, organizations and
governments agencies. Since those public networks were quite different
internally, and the interconnection of networks was growing very fast, there was
a need for a common network interface protocol.
In 1976 X.25 was recommended as the desired protocol by the International
Consultative Committee for Telegraphy and Telephony (CCITT) called the
International Telecommunication Union (ITU) since 1993.
X.25 is a packet switched data network protocol which defines an
international recommendation for the exchange of data as well as control
information between a user device (host), called Data Terminal Equipment
(DTE) and a network node, called Data Circuit Terminating Equipment
(DCE).
X.25 utilizes a Connection-Oriented
service which insures that packets are transmitted in order.
X.25 comes with three levels based on the first three layers of the Open
Systems Interconnection(OSI) seven layers architecture as defined by the
International Standard Organization(ISO).
The levels are:

The
Physical Level describes the interface with the physical environment. It
is similar to the Physical Layer in the OSI model.
The Link
Level responsible for the reliable communication between the DTE and the
DCE. It is similar to the Data Link Layer in the OSI model.
The Packet
Level describes the data transfer protocol in the packet switched network.
It is similar to the Network Layer in the OSI model.

X.25 was originally approved in 1976 and subsequently revised in 1977, 1980,
1984, 1988 and 1992. It is currently (1996) one of the most widely used
interfaces for data communication networks.
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We will examine now each level in more detail:
The Physical Level




The physical level (level 1) deals with the electrical, mechanical,
procedural and functional interface between the DTE and the DCE. The
physical level is specified by the X.21, X.21-bis and the V.24 recommendation
for modems and interchange circuits.

X.21 is a CCITT recommendation for operation of digital circuits.
The X.21 interface operates over eight interchange circuits(i.e. signal
ground, DTE common return, transmit, receive, control, indication, signal
element timing and byte timing) their functions is defined in recommendation
X.24 and their electrical characteristics in recommendation X.27.
X.21-bis is a CCITT recommendation that defines the analogue
interface to allow access to the digital circuit switched network using an
analogue circuit. X.21-bis provides procedures for sending and receiving
addressing information which enable a DTE to establish switched circuits with
other DTEs which have access to the digital network.
V.24 is also a CCITT recommendation, it provides procedures which
enable the DTE to operate over a leased analogue circuit connecting it to a
packet switching node or concentrator.
Now we will discuss in more detail the X.21 interface since it is the most
commonly used one.
X.21 Digital Interface.
In 1976 CCITT recommended a digital signaling interface called X.21. The
recommendation specifies how the DTE can setup and clear calls by exchanging
signals with the DCE.

The physical connector has 15 pins, but not all of them are used.The DTE
uses the T and C circuits to transmit data and control
information. The DCE uses the R and I circuits for data and
control. The S circuit contains a signal stream emitted by the DCE to
provide timing information so the DTE knows when each bit interval starts and
stops.The B circuit may also provide to group the bits into byte
frames. If this option is not provided the DCE and DTE must begin every control
sequence with at least two SYN characters to enable each other to deduce the
implied frame boundary.
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The Link Level




The link level (also called level 2, or frame level) ensures reliable
transfer of data between the DTE and the DCE, by transmitting the data as a
sequence of frames (a frame is an individual data unit which contains address,
control, information field etc.).
The functions performed by the link level include:

Transfer of data in an efficient and timely fashion.
Synchronization of the link to ensure that the receiver is in step with
the transmitter.
Detection of transmission errors and recovery from such errors
Identification and reporting of procedural errors to higher levels, for
recovery.
The link level uses data link control procedures which are compatible with
the High Level Data Link (HDLC) standardized by ISO, and with the Advanced Data
Communications Control Procedures (ADCCP) standardized by the U.S.American
National Standards Institute (ANSI).
There are several protocols which can be used in the link level:

Link Access Protocol,Balanced (LAPB) is derived from HDLC and is
the most commonly used. It enables to form a logical link connection besides
all the other characteristics of HDLC.
Link Access Protocol (LAP) is an earlier version of LAPB and is
seldom used today.
Link Access Procedure,D Channel (LAPD) is derived from LAPB and it
is used for Integrated Services Digital Networks (ISDN) i.e. it enables data
transmission between DTEs through D channel, especially between a DTE and an
ISDN node.
Logical Link Control (LLC) is an IEEE 802 Local Area Network (LAN)
protocol which enables X.25 packets to be transmitted through a LAN channel.

We now discuss LAPB in more detail since,as we mentioned before,it is the
most commonly used.
LAPB-Link Access Protocol, Balanced.
The LAPB protocol uses the following frame structure:


The Flag fields indicate the start and end of the frame.
The Address field contain the address of the DTE/DCE, it is most
important in multidrop lines, where it is used to identify one of the
terminals.
The Control field contains sequence numbers, commands and responses
for controlling the data flow between the DTE and the DCE.
The Checksum field indicates whether or not errors occur in the
transmission. It is a variation of the Cyclic Redundancy Code (CRC).
There are three kinds of frames:

Information: This kind of frame contains the actual information
being transfered. The control field in these frames contains the frame
sequence number.
Supervisory: There are various types of supervisory frames.

RECEIVE READY-Acknowledgment frame indicating the next frame expected.
REJECT-Negative acknowledgment frame used to indicate transmission error
detection.
RECEIVE NOT READY(RNR)-Just as RECEIVE READY but tells the sender to
stop sending due to temporary problems.
Unnumbered: This kind of frames is used only for control purposes.
LAPB also provides the following commands:

DISC (DISConnect)- Allows the machine to announce that it is going down.

SNRT (Set Normal Response Time)- Allows a machine that has just come
back on line to announce its presence.
FRMR (FRaMe Reject)- Used to indicate that a frame with correct checksum
but impossible semantics arrived.
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The Packet Level




The packet level (also called level 3 or network level) creates network
data units called packets which contain control information and user data.

The packet level provides procedures for handling the following services:


Virtual Circuit(VC) is a temporary association between two DTEs,
it is initiated by a DTE signaling a CALL REQUEST to the network. This
service ensures orderly(sequenced) delivery of packets in either direction
between two DTEs.Virtual Circuit is established whenever two DTEs want to
communicate.This service is the most frequently used by the X.25 protocol.
Permanent Virtual Circuit is a permanent association existing
between two DTEs, which does not require call setup(connect) or call
clearing (disconnect) action by the DTE.
Datagrams(DG) is a self contained user data unit, containing
sufficient information to be routed to the destination DTE(independently of
all other data units) without a need of a call to be established.The
data units are transmitted one at a time with no guarantee of final
delivery,and no guarantee of orderly delivery.Each datagram must contain
complete address and control information to enable it to be delivered to the
proper destination DTE.
Fast Select is a service which enables the control packet that
sets up the VC to carry data as well.
Other Services:the packet level also provides the call setup and
call clearing procedures required for the VC service. The packet level deals
with flow control to ensure that a user(DTE) does not overwhelm the other
user with packets, and to maintain timely and efficient delivery of
packets.The packet level also handles errors at packet level to abort or
restart VCs if necessary.
Call Setup
When DTE A wants to communicate with DTE B, it must set up a connection by
building a CALL REQUEST packet, and passing it to it's DCE.
DTE B gets the packet through the subnet and it's DCE. If DTE B wishes to
accept the call, it sends a CALL ACCEPTED packet back.
When DTE A receives the CALL ACCEPTED packet the Virtual Circuit is
established.
At this point the two DTEs may use a full-duplex connection to exchange
data packets. When either side wants to finish the call, it sends a CLEAR
REQUEST packet to the other side, which then sends a CLEAR CONFIRMATION packet
back as an acknowledgment.

The DTE determines the circuit number on outgoing calls and the DCE
determines the circuit number on incoming calls If both simultaneously choose
the same number then Call Collision occurs. X.25 specifies that in this
case, the outgoing call is put through and the incoming one is cancelled.
Packets Format
We will examine now the format of the packets in X.25 protocol.

The Control Packet
The format of the control packets is as follows:
The control packet as well as all X.25 packets begins with a 3-byte
header. Bytes 1,2 contain the Group and the Channel fields that together
form a 12 bit virtual circuit number. Number 0 is reserved for future use,
so a DTE may have up to 4095 virtual circuits at a time.
The CALL REQUEST packet:
The additional information of the CALL REQUEST packet is as
follows:

The Length Of Calling Address and Length Of Called Address fields tell
how long the calling and called addresses are, respectively. The next two
fields are the addresses, both addresses are encoded as decimal digits, 4
bits for digit.
The addressing system used in X.25 is defined in CCITT recommendation
X.121. This system is similar to the public switched telephone network, with
each host identified by a decimal number consisting of country code, a
network code, and an address within the specified network. The full address
may contain up to 14 digits, of which the first three indicate the country ,
and the next one indicates the network number(for countries with many public
networks multiple country codes exist). The division of the remaining 10
digits is not specified by X.121, permitting each network to allocate the 10
billion addresses itself.
The Facilities Length field tells how many bytes of facilities field
follow. The Facilities field itself is used to request special features for
this connection.
The specific features may vary from network to network. Possible features
are reverse charging(collect calls), simplex instead of full-duplex virtual
circuit, maximum packet length and a window size rather than using the
defaults of 128 bytes and 2 packets.
The last field is The User Data field which allows the DTE to send up to
16 bytes of data together with the CALL REQUEST packet.
Other Control Packets are:

The CALL ACCEPTED packet is sent by the callee DTE if it accepts the
call.
The CLEAR REQUEST is sent due to various reasons,the fourth byte of
the packet tells why the connection is being cleared. It is acknowledged
by a CLEAR REQUEST CONFIRMATION packet.
The INTERRUPT packet allows a short (32 bytes) signal to be sent out
of sequence. It is acknowledged by INTERRUPT CONFIRMATION packet.
The RECEIVE READY (RR) packet is used to send separate acknowledgments
where there is no reverse traffic. The ppp field (three first bits of the
type field) tells which packet is expected next.
The RECEIVE NOT READY (RNR) packet allows a DTE to tell the other side
to stop sending packets to it for a while.
The REJECT packet allows a DTE to request retransmission of a series
of packets. The ppp field gives the first sequence number desired.
The RESET and RESTART packets are used to recover from varying degrees
of trouble. Both are acknowledged by RESET CONFIRMATION and RESTART
CONFIRMATION respectively.
The DIAGNOSTIC packet is also provided, to allow the network to inform
the user of problems.
The Data Packet:
The format of a data packet is shown bellow:
The Q bit indicates qualified data, the intention is to allow protocols
in the higher layers to set this bit to 1, to separate their control packets
from their data packets. The control field is always 0 for data packets.

The Sequence and Piggyback fields are used for flow control, using a
sliding window. The sequence numbers are modulo 8 if Modulo is 01, and
modulo 128 if Modulo is 10 (00 and 11 are illegal). If modulo 128 sequence
numbers are used, the header is extended with an extra byte to accommodate
longer Sequence and Piggyback fields.
The D bit determines the meaning of the Piggyback field. D=0, means that
the local DCE has received the packet, but not that the remote DTE has
received it. D=1, means that the packet has been successfully delivered to
the remote DTE.
The More field allows a DTE to indicate that a group of packets belong
together.
The standard is that carriers are required to support a maximum packet
length of 128 data bytes. However it also allows carriers to provide
optional maximum lengths from 16 up to 4096 bytes (in powers of 2).
X.25 State Diagrams
The X.25 standard contains several state diagrams to describe event
sequences such as call setup and call clearing. The diagram bellow shows the
subphases of call setup:
Initially, the interface is in state P1.A CALL REQUEST or INCOMING CALL
changes the state to P2 or P3, respectively. From these states the data
transfer state, P4, can be reached, either directly, or via P5. Similar
diagrams are provided for Call Clearing, Resetting, and Restarting.
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R e f e r e n c e s





Computer Networks / Andrew S.Tanenbaum
Data and Computer Communication / William Stallings
Telecommunication Networks: Protocols, Modeling and Analysis / Mischa
Schwartz
Interconnections: Bridges and Routers / Radia Perelman
Computer Networks and Their Protocols / D.W. Davies, D.L.A. Barber, W.L.
Prince and C.M. Solomonides
Computer Networks Architecture and Protocols / Paul E.Green
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