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_______________________________________________________________________________
An Introduction to Packet Switched Networks Part I
Written by Blade Runner on 08/20/88
A Telecom Computer Security Bulletin File
_______________________________________________________________________________
1. GENERALITIES
The growth of data transmission services present more problems due to
several types of protocols, either in asyncronous or syncronous ways, and
it needs higher speeds that can allow a faster service.
To guarantee a service with these characteristics it is helpful to work
on lines, making them less afflicted from line noise, and on data, with the
adding of redundancy codes for data correction procedures; we also have to
find a solution that can allow the use of all protocols, either asyncronous
or syncronous, on the same physical media in order to use the same line for
both means.
At that, we must allow the capability to optimize data transmission
systems, by arise the connections wires between two points, so the safety
of the system, in it's globality from computer to terminals, will be
augmented and made as best is possible.
The International Standard Organization (ISO) has studied a way that
works with all connection needs between computers and terminals, building
a model to refer that can be used as common mind to develop branch reagles.
The ISO's model is OSI Architecture (Open System Interconnection),
where "Open" means that system is open to other systems that have well
fixed standards.
In that architecture a data transmission system, meant as globality from
computer to transmission line, is sub-divided into seven levels, each one
is doing a colloquial protocol with an analogous level that look for
interconnection between the two, following a logical wire equal to this
which is shown in the following scheme:
A B
+----------+ +----------+
| 7 |< ---------------------- >| 7 |
|----------| |----------|
| 6 |< ---------------------- >| 6 |
|----------| |----------|
| 5 |< ---------------------- >| 5 |
|----------| |----------|
| 4 |< ----------------------->| 4 |
|----------| +----------+ |----------|
| 3 |< --- >| |< --- >| 3 |
|----------| |----------| |----------|
| 2 |< --- >| |< --- >| 2 |
|----------| |----------| |----------|
| 1 |< --- >| |< --- >| 1 |
+----------+ +----------+ +----------+
|
V
Physical device of interconnection
The seven levels are called:
Application Level - 7
Greeting Level - 6
Session Level - 5
Carrying Level - 4
Network Level - 3
Line Level - 2
Physical Level - 1
The higher levels are strictly wired to the computer world and intellig-
ent terminals (levels 5,6 and 7) while lower levels are wired to network
interconnection problems, about transmitting devices and error correction.
The physical device of interconnection between two OSI systems will
provide a three-level structure, making able the structurization of a net,
e.g., that of following figure:
users users
+-----+ | | | | | |
| OP1 | | | | | | |
+-----+ +------+ +------+
|____| city |<----------------------->| city |
____| A | | B |____
/ +------+ +------+ \
| | \ / | |
| | \ / | |
| | \ / | |
| | \ / | |
| | \ / | |
| | \ / | |
| | X | |
| | / \ | |
| | / \ | |
| | / \ | |
| | / \ | |
| | / \ | |
| | / \ | |
| +------+ +------+ |
| | city |<----------------------->| city | |
| | C | | D | |
| +------+ +------+ |
| \ / |
| \ / |
| \ / |
| +------+ |
\--------------------->| city |<-----------------/
| E |----\
+------+ +------+
| | | | OP2 |
| | | +------+
users
We can make the following assumptions about the above structure:
a) The net is composed by a certain number of devices (in table: A, B,
C, D, E), called nodes, connected between themselves at high speed so that
operator "1" can connect to operator "2" by several ways, following more
than 1 directive (e.g. "A, C, E" or "A, C, D, E" or "A, B, D, E") and
following traditional system he can use only one way, the "A,E" connection;
b) The user doesn't need more than one physical line in his "possession"
when connecting (e.g. OP1 with OP2), but only of links to network (in table
1 links to the "A and E" nodes) with short wiring, that are possible with a
modem at base band then at low prices.
The structure that is nearest to the OSI model is called "PSS" (Packet
Switching Network) and was analyzed from CCITT (International Consultative
Commitee for Telephonist and Telegraphy) with the X25 recommendation. This
recommendation defines the interface between DTE (Data Terminal Equipment)
and DCE (Data Circuit Terminating Equipment) for terminals that work with
the PSS net.
The DTE is strictly known as the source (or the receiver) of data pack-
ets to (or from) the network and physically can be a Host computer, a Front
End Processor or an Intelligent terminal.
The DCE, strictly talking, as common consent, is the device that maybe
converts these packet-signals received from DTE in a way that might be
transmitted on lines (e.g. the modem, the TDM, the line couplers), but in
the sense used by X25, it can be assumed the means of node access or swit-
ching node to which DTE is connected.
The X25 recomendation is therefore the local interface between a DTE
and a DCE (see next table).
+----------------+ +------------------+ +-----------------+
| Computer |<-->| network |<-->| Intelligent |
| | | | | terminal |
| | | |
| | | |
| | | |
| X.25 | X.75 | X.25 |
The above table is composed by several blocks, which we can observe as:
| Host | FEP |
|<----------->|<---------->| .
| | | .
+-------------+------------+ . +---------------------------------+
| | | . | modem --------|
| characters | X.25 |====.======|----> -----z----<---| |
| generator | generator | . | | node |
| | | . | |_______|
|-------------+------------+ . +---------------------------------|
| . |
| DTE . |
|<--------------->.<--------------------------------->|
| |
|<--------------------------------------------------->|
| |
We can see in "table 2" wires in the network domain are marked as X.75.
The CCITT has issued this X.75 recomendation (control procedures for
transit calls or terminals and data transfer systems on international
calls between PSS networks) that usually is used only for international
calls but can be used in country calls for interconnection between nodes.
The X.25 recomendation is not valid to simple terminals (start-stop)
and ACPs (Packet Adapter-Concentrator or PAD) that can be connected to
public data networks. Standards for these devices are regulated by X.3
recommendation (Protocol converters/adapters), X.28 (DTE/DCE Interface for
start-stop terminals who logon to ACP on a structured network in national
places) and X.29 (exchange procedure for information control and data
between ACP and X.25 terminal or other ACP).
2. X.25 STRUCTURE
In the "X.25 Interface", three levels are defined. Each level procedure
uses functions offered by the other level as soon under, but they dont care
how the lowest level is to be implemented. This is so that a particular
can be implemented in any of several levels, so long as it will obtain the
final results.
In addition, the X.25 recommendation will specify protocols and rules
that will decide the information exchange between simiar levels in DTE and
DCE, which can be understood from the next table:
| DTE |
|<------------------------>|
| |
--------+-------+-------+-------+ | +-------+-------+-------+
. | level | level | level | | | level | level | level |
. | 3 | 2 | 1 |--|->==<---| 1 | 2 | 3 |
--------+-------+-------+-------+ | +-------+-------+-------+
| | | | X.21 | | |
| | | | bis | | |
| | | |<---->| | |
| | |<--------------------------------->| |
| | HLDC Connection |
| |<------------------------------------------------->|
| Packet level X.25
|<------------------------------------------------------------...
Talking protocol
Every level will accumulate information from the lower level and it will
add a header with eventual redundancy codes before to make the information
transmitted through present the interface from the lowest level using a
step by step structure as shown in the next table (block 1 is inserted into
block 1 and so on).
LEVEL 1
Physical Interface: this defines the electric and physical
characteristics of the interface going on the used line or switched line
into network. Voltage tensions, connectors used, and transmission methods
are defined in level 1. The most important characteristic of this level is
that it supplies a point-to-point transmission, full-duplex, for digital
transmission.
LEVEL 2
Access to connection procedure: (LAP = Link Access Procedure or LAPB =
Line Access Procedure Balanced). This level will specify a control
procedure on data to correct mistakes due to physical level. It includes
control methods of Network Congestions during the DTE and DCE exchange.
This uses the media known as HDLC protocol (High Level Data Link Control
defined from ISO as the header building as an activation procedure of the
connection.
+---------------+
| message with |
| destination |
+---------------+
|
| packet level
|
V
+-------+----------------+
| packet| |
| header| |
| start | |
+-------+----------------+
|
| connection level
|
V
+--------+------------------------+---------+
| HLDC | . CRC |
| header | information . |
| start | . |
+--------+------------------------+---------+
|
|
|
V
+--------+-------------------------------------------+------+........
| | | | next
| flag | | flag | header
| | | |
+--------+-------------------------------------------+------+........
|
|
|
V
+-------------------------------------------------------------------+
| |
| bit string |
| |
+-------------------------------------------------------------------+
LEVEL 3
Packet Level: this level is the higest and specifies the way that
information are packet structured and the procedure in which to proceed
with connections. It has the function to Concentrator because it can mult-
iplex a number of logical channels into a unique physical channel, mixing
packets coming from differents channels. Each logical channel has an
independent control regarding packets and has a CRC for each channel.
Virtual circuits
The Third level has virtual channels, that are bi-directional associat-
ions between two DTE; via these associations packets are exchanged.
It is like, via the several nodes in the network, a dedicated link betw-
ween the two DTE. These virtual circuits maybe temporary, and in this case
they are called "switched Virtual Circuits" (SVC) or fixed, called
"Permanent Switched Circuits" (PVC).
3. LEVEL 1 - Physical interfacing
This level is specified from physical characteristics of CCITT X.21
recommendation (physical interface between DTE and DCE; for asychronous
operations on data) and X.21bis (data network usage for designed terminals
to interface with syncronous modems of series V) used in a provisory way
to afford to use modems actually on market. Upon mentioned recomandations
are not depending on the transmission device as they provide that DCE
(modem or line coupler) will be the part that takes care of the line
technology.
The X.21 CCITT recomandation declares:
- Physical characteristics about the interface, the type of connectors
and the wire assignment (X.24, 8 ways, 15 pin);
- electrical characteristics of signals (X.26 and X.27 CCITT
characteristics as EIA RS 423 and RS 422 respectively);
- the serial asyncronous transmission;
- wires that must be point-to-point, working in full-duplex (from that
we can understand we cannot work in a multi-point structure);
- the necessary procedures to afford a switched connection;
- the necessary procedures to afford a dedicated connection.
The level 1 will consider only first 4 points suggested from X.21,
all others are of level 3.
Table 6 shows the circuitry, for functions exchange, provided from the
X.24 recommendation of CCITT.
This interface is absolutely transparent to data transfers thanks to
special C and I lines that are used to determine if the data on T and R
lines are controls signals or data signals.
The X.21 recommendation is supplied for interfacing devices at digital
level, so it is difficult to use for moment, the temporarly is used the
X.21bis recommendation that is compatible with actual series V modems.
Electrical characteristics about iterfacing circuitery for speeds less
than 20 kbit/s are conform to V.28 recomandation of CCITT that use a 25
pins connector with pins as standard from ISO with scheme # 2110 or with
X.26 recomandation that provide a 37 pins connector with ISO standard
scheme at # 4902. It is up to the local administrators to choose the con-
nector types and the interfacing type to offer as part of their service.
user Network
|<--------------------------------------->|<--------------->
+-------------+ +--------------+ | +---------------+
| |<-(T) xmit data->| | | | |
| |<-(C) control--->| |-|->| |
| DTE |<-(R) rec'd data>| DCE | | | node |
| |<-(I) info------>| | | | |
| |<-(S) time base->| |<|--| |
| | | | | | |
+-------------+ | +--------------+ | +---------------+
|
---------------------
Interfacing point
To obtain speeds in exceess of 20 kbit/s the electrical characteristics
are following what is provided by the V.35 recommendation that uses 34 pin
connectors as ISO standard draw #2593. The table's indicating interfacing
circuits considered by X.21bis recommendation.
Interface Circuit Description
102 Signal ground
103 Send data
104 Received data
105 Transmission request
106 Ready to transmit
107 DCE Ready
108/2 DTE Ready
109 Carrier detector
114 Time base for transmission
115 Time base for receiving
140 Loop remote probe
141 Local loop probe
142 Running test
4. LEVEL 2 - Link procedures
This level is a "point-to-point" link, and is normally known as the
"frame level" or "header level".
It follows terminologies and is under options specified from ISO HLDC
protocol.
4.1 Level 2 functions
Level 2 transforms to a physical circuit than can be affected by errors
in a logical connection between DTE and the network, a link that can be
understood as released from an error happening: this defines a correction
level based on automatic request about echoing as data is not considered
as transmitted since an error is received or a receive confirm has
been received. Only fully completed data are accepted from receiver.
In addition, this level will provide the ways for the recognizing of a
start and end header, the error recognizes about a bit (via a CRC
computation) and the loss of header (by count headers).
Basic directives of the system will provide:
- the "bit oriented" and no "char oriented" structure: this means that
information may be contained also in only one bit, and we are released from
a certain bit multiple as in the "character oriented way" in which the
information (character is linked to a table (e.g. ASCII 7 bit).
- the existence of CRC ad each end of header and sequential numbering
of headers.
- the correction of error by the re-transmition of data.
- the primary and secondary station definition without any particular
priority of the start of transmission.
- complete full duplex.
The wire specific at level 2 are the point-longs :
1) the structure of header: meant as format of header, then as length,
as CRC computation point, as sincronicity character;
2) procedure elements: allowed commands, answers and actions that must
be taken following the cases: these operations follow the HDLC;
3) class of long procedure: the HDLC will provide a certain number of
cases about the classes and procedures of link following the configuration
type and operating way; the X.25 recomandation uses two classes of link
procedures (see table 7):
- simmetric, usually called LAP (link access procedure),
- balanced, usually called LAPB (link access procedure balanced).
+----------------+ +-------------------+
| primary source | | receiver |
| A |-->OO OO-->| A |
+----------------+< >OO----------------OO +-------------------+
X \ /
/ \ X
+----------------+ OO----------------OO< >+-------------------+
| receiver | OO OO<_ | primary source |
| B |<_/ \| B |
+----------------+ +-------------------+
Simmetric LAP configuration
DTE DCE
+----------------------------+ +-----------------------------+
| source | | | | receiver |
| | primary or | | primary or | |
| | secondary |====>| secondary | |
|-------------| combinator | | combinator|---------------|
| receiver | | | | source |
| | |<====| | |
+----------------------------+ +-----------------------------+
Balanced LAPB configuration
In the first case the running can be compared with half-duplex running,
in the mean that initialization is done before in a way and then in the
other, before an error the channel can be re-initialized without other
aid. This can cause, in some operating conditions, malfunctioning phenomena
(see table 8).
The LAPB procedure will have none of these malfunctions because only
with a command will these do the re-initialization to both sides.
The B station will re-initialize, but primary station A can not perceive
therefore it has not requested no one correct recognizing of secondary
station. In this way we obtain a reset of counters only in one way.
Instant 1,2 Normal Running
/---------- ---------------\
| |
| +-----------+ Information +------------+ |
| | Primary A |---------------------------------->| Secondary | |
|--| |<----------------------------------| A |--|
| +-----------+ Correct receiving +------------+ |
| |
| |
| |
| |
| +-----------+ Information +------------+ |
| | Secondary |---------------------------------->| Primary B | |
\--| B |<----------------------------------| |--/
+-----------+ Correct receiving +------------+
Instant 3,4 Abnormal Running
/---------- -----------------\
| |
| +-----------+ Don't transmit +------------+ |
| | Primary A |---------------------------------->| Secondary | |
|--| |<----------------------------------| A |--|
| +-----------+ +------------+ |
| |
| |
| |
| |
| +-----------+ Reset (due to line error) +------------+ |
| | Secondary |---------------------------------->| Primary B | |
\--| B |<----------------------------------| |--/
+-----------+ Confirm +------------+
4.2 Header Structure
In the next table his supplies the Header structure.
8 bit 8 bit 8 bit variable => 0 16 bit 8 bit
+----------+----------+----------+-------......---+-----------+----------+
| Flag | Address | Control | Information | CRC Code | Flag |
| 01111110 | | | (data or ctrl) | | 01111110 |
+----------+----------+----------+-------......---+-----------+----------+
| Header start | |
|<------------------->| |
| |
| Stored bits |
|<------------------------------------------------>|
| |
The above table shows that the information field provides a variable
length but that length can't be a 8 multiple.
The Flag sequence (01111110) defines header boundaries and it can be
used to close a header and open another.
The same is also used as a syncro character and can be put a on line
when no one information header is yet present.
A header is not recognized it it does not have at its start and at its
end flag sequence, and if within there are not at least 32 bits present (8
for address, 8 for control and 16 as CRC).
The address was originally used from HLDC as an addressing function in
case of "multiple-point" wiring. The X.25 recomandation will provide that
the address function is used only to be able to distinguish commands and
replies in both ways. Its function is rendondancy, because there exists
some control bits to that specific function, but it can be used for addit-
ional researching of errors. Therefore it can distinguish data flow in
both ways and it can then recognize immediately some line loops.
Two are recognized addresses
A = 00000011
will determine commands header from DCE to DTE and answer headers from
DTE to DCE.
B = 00000001
will determine commands header from DTE to DCE and answer headers from
DCE to DTE.
The CONTROL field will identify headers and contain the count of them.
Three header types can be sended in line:
1) Information headers (I): are there who contains usefull data;
2) Supervision headers (S): are there only for control, used e.g. to
confirm a right receive, or for temporary hold of transmission;
3) Numbered headers (N): used e.g. as initialization of connection or
as closer ot connection: they have not CRC sequences, because they
transfer a know information, and did not provide neither an header count.
The format about control field will identify these three headers as
shown in next table:
Thus concludes the Part 1 of the TCSB Introduction to Packet Switched
Networks. Now go grab a hold of Part 2 and learn something.
_______________________________________________________________________________
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