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- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
PART 3
MODERN DATA
NETWORKS
- Networks and Telecommunications: Design and Operation, Second Edition.
Martin P. Clark
Copyright © 1991, 1997 John Wiley & Sons Ltd
ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
Packet Switching
Packet switching emerged in the 1970s as an efficient means of data conveyance. It overcame the
inability of circuit-switched (telephone) networks to provide efficiently for variable bandwidth
connections for bursty-type usage as required between computers, terminals and storagedevices.
In this chapter we discuss the basics of packet switching and ITU-T’s X.25 recommendation,
nowadays the worldwide technical standard interface to packet-switched networks. We then also
go on todiscuss the IBM company’s SNA (systems network architecture), a proprietary form of
packet switching, important because of its dominant role in IBM computer networks.
18.1 PACKET SWITCHING BASICS
Packet switching is so-called because the user’s overall message is broken up into a
number of smaller packets, each of which is sent separately. We illustrated the concept
in Figure 1.10 of Chapter 1. Each packet of data is labelled to identify its intended
destination, and protocol control information (PCZ) is added, as we saw in Chapter 9,
before it is sent. The receiving end re-assembles the packets in their proper order, with
the aid of sequencenumbers and the other PC1 fields. Each packet is carried across
the network in a store-and-forward fashion, taking the most efficient route available at
the time.
Packet switching is a form of statistical multiplexing, as we discovered in Chapter 9.
Figure 18.1 illustrates how a link within a packet switching network is used to carry the
jumbled-up packets of various different messages and the use of the information carried
in the packet header to sort arriving packets at the destination end into the separate
logical channels, virtual circuits ( VCs) or virtual calls (VCs).
Transmissioncapacity between pairs of nodesinapacket-switchednetwork is
generally not split up into rigidly separate physical channels, each of a fixed bandwidth.
Instead,theentireavailablebandwidth between two nodalpoints (switches) inthe
network is bundled together as a single high bitrate pipe, and all packets to be sent
between the two endpointsof the link share the same pipe (Figure 18.1). In this way, the
entire bandwidth (i.e. full bitspeed)can be used momentarily by any of the logical
channels sharing the connection. This means that individual packets are transported
more quickly and bursts of transmission can be accommodated.
341
- 342 PACKET SWITCHING
mm
n
U
packet may
switch
Figure 18.1 The statistical multiplexing principle of packet switching
A problem arises when more than one or all logical channels try to send packets at
once. This is accommodated by buffers at sending and receiving ends of the connection
as shown in Figure 18.2. These delay some of the simultaneous packets for an instant
until the line becomes free.
By use of buffers as shown in Figure 18.2, it is possible to run thetransmission link at
very close to100% utilization.This is achieved by sharingthecapacity between a
number of end devices (each with a logical channel). The statistical average of the total
bitrate of all the logical channels must be slightly lower than the line bitrate so that all
packetsmay be carried,butatany individualpoint in timethe buffers may be
accumulating packets or emptying their contents to the line.
Packet switching is able to carry logical channels of almost any average bitrate. Thus
a 128 kbit/s trunk between two packet switches might carry 6 logical channels of mixed
and varying bitrates 5.6 kbit/s, 11.4 kbit/s, 12.3 kbit/s,22.1 kbit/s, 28.7 kbit/s, 43.0 kbit/s
and still have capacity to spare. This compares with the two channels which a telephone
network would be able to carry using the same trunk capacity. (The excess capacity
of the telephone channels simply has to be wasted, and the other four channels cannot
be carried.)
packet
switch
buffer
Figure 18.2 The use of a buffer to accommodate simultaneous sending of packets by different
logical channels
- TRANSMISSION
DELAY IN PACKET-SWITCHED
NETWORKS 343
18.2 TRANSMISSION DELAY IN PACKET-SWITCHED NETWORKS
When using the trunksin a packet-switched networkat very close to full utilization, very
large buffers are required for each of the logical channels,to smooth out the bursts from
individual channels into a smooth output for carriage by the line. (This is rather like
having a very large water reservoir,collecting water during showersof rain, and varying
in water depth, butalways capableof outputting a constant volume of water for munic-
ipal use (Figure 18.3). The water reservoir is analagous to the data buffers, the showers
of rain to the bursts of data information, and the constant output to the information
carried by the line.)
We can make sure that the packets accumulated in the buffer are despatched on a
jirst-in-jirst out (FIFO) basis to fairly share out the
queueing delays which result, but it is
critical to ensure that the queueing delay does not become unacceptably long. The
chance of a very long delay is much greater when close to 100% utilization of the line is
expected. (Imagine waiting in line for a bus, all of the seats of which had to be full
before it pulled away; either the bus doesn’t come very often, or there is a very long
queue to ensure that all the seats can be filled).
A certainamount of queueing delaycaused by buffering is not noticeable to
computer users (a $ second is a very long queueing delay in packet switching network
terms). Even if a typed character did not appear on the computer screen until a f second
afterhittingthekeyboard,the user is unlikely to notice. A variation inthedelay
(sometimes a $ second, and sometimes no delay) is also unimportant. (The fact that
some characters appear on the screen more quickly than the f second maximum delay
will not be noticed.) On the other hand, once the average delay becomes much longer,
then computer work may become frustrating, so that much longer queueing delays are
unacceptable.
There is an entirestatistical science used to estimatequeueingdelays. Themost
important formulais the Erlang call-waiting formula, which we willdiscuss in Chapter 30.
In simple terms, however, the unacceptability of long queueing delays means that the
- 344 PACKET SWITCHING
trunks in packet-switching networks may not be utilized at 100%. Typical acceptable
maximum utilization is around 50%. Despite this fact, they are still more efficient than
circuit-switched networks to carry data tra@c (i.e. information between computers).
18.3 ROUTINGIN PACKET-SWITCHED NETWORKS
Packets are routed across the individual paths within the network according to the
prevailing traffic conditions, the link error reliability, and the shortest path to the desti-
nation, according to one of two main techniques, so-called path-oriented routing and
dutagram routing. The routes chosen are controlled by the (layer 3) software of the
packet switch, together with routing information pre-set by the network operator.
Path-orientedrouting is nowadaysthemostcommontechnique.In path-oriented
routing, a fixed path is chosen for a given logical channel (i.e. virtual circuit, V C ) at the
time of call set up. The pathitself is chosen based on the current loadingof the network
and the available topology. In the case of the virtual call (i.e. switchedvirtualcon-
nection) service, the required destination of the path is indicated using an X.121 address
carried by a layer 3 packet, called the call set-up packet. This packet has the equivalent
function to the dialled digits of a telephone number in setting up a telephone call.
Should any link in the path become unavailable during the course of the call (say
because of a transmission failure), then an altenative path is sought, without breaking
the connection. The packets are stored for a short while, while the new path is found
and then sent over this path. (Figure 18.4).
The advantage of path-orientedrouting is that the packets pertaining to a given
logical connection all take the same path, all suffer about the same amount of queue-
ing delay in the buffers and arrive pretty much in the same order as they were sent
(allowing for any lost on the way). This makes the job of re-sequencing the packets at
the receiving end much easier, as well as the job of directing the packets through the
network. It alsoleads tomorepredictable delayperformancefortheend user or
computerapplication. The packetswitchingnetworkcomponents can therefore be
relatively simple and cheap.
The disadvantage of path-oriented routing is the inability of the network on a more
immediate basis to employ alternative routing to better utilize the network as a whole
(A31ppq-W
packet
C-path back-up
packet
fcl p (c31
c1 1
4
*
normal A-connection-path
switch
4q m p 1
," normal C-path (currently failed)
A'
Figure 18.4 Circumventing a transmission link failure using path-oriented routing
- ROUTING IN PACKET-SWITCHEDNETWORKS 345
m
L
U
U)
2
- 346 PACKET SWITCHING
during periodsof sudden surge in demand resulting from simultaneous packet bursts by
many logical channels sharing the same path. The second type of routing, datagram
routing, allows for more dynamic routing of individual packets (Figure 18.5), and thus
has the potential for better overall networkefficiency. The technique, however, requires
more sophisticated equipment, and powerful switch processors capable of determining
routes for individual packets.
Packet switching gives good end-to-end reliability, with well-designed switches and
networks it is possible to bypass network failures (even during the progress of a call).
Packet switching is also efficient in its use of network links andresources, sharing them
between a number of calls, thereby increasing their utilization.
18.4 ITU-T RECOMMENDATION X.25
Most packet-switched networks use the protocol standards set by ITU-T’s recommen-
dation X.25. Thissets out the mannerin which adata terminal equipment ( D T E )should
interact with a data circuit terminating equipment ( D C E ) , forming the interface to a
packet-switched network. The relationship is shown in Figure 18.6.
The X.25 recommendation defines theprotocolsbetween DTE (e.g. personal
computer computer
or terminal
controller (e.g. IBM 3174)) and DCE (i.e.
the
connection point to a wide area network, W A N )corresponding to OS1 layers 1, 2 and 3
(Figure 18.7) which we learned about in Chapter 9).
The physical connection may either be X.21 (digital leaseline) or X.21 bis (V.24/V.28
modem in conjunction with an analogue leaseline: Chapter 9). Alternatively, the X.31
recommendation (Chapter 10) specifies how the physical connection (DTE/DCE) may
be achieved via an ISDN (integrated digital services network). Finally, recommenda-
tion X.32 specifies the use of a dial-up connection for apacket mode connection via the
telephone or ISDN network to an X.25 packet exchange.
The X.25 recommendation itself defines theOS1 Iayer 2 and layer 3 protocols. These
are called the link access procedures (LAPB and L A P ) and thepacket level interface. The
link access procedureassures the correct carriage data across the
of link connecting DTE
DTE DCE DCE DTE
I I
I I l
*X25 --U I c X 2 5 - W
Packet switched network
Figure 18.6 The X.25 interface to packet switched networks
- ITU-T X.25 347
E
layer 3 protocol
(packet layer) * X.25 packet level interface -
layer 2 protocol X.25 LAPB (link access procedure)
(NETWORK)
(link layer)
layer 1 protocol X.21, X.Slbis, X.31 or X.32
(physical layer)
DTE
DCE
Figure 18.7 OS1 layered model representation of ITU-T recommendation X.25
to DCE and for multiplexing of logical channels; the packet level interface meanwhile
guarantees the end-to-end carriage of information across the network as a whole.
The LAPB (link access procedure balanced) protocol provides for the I S 0 balanced
class o procedure and also allows for use of multiple physical circuits making up a
f
single logical link. LAP is an olderand simpler procedure only suitable for single
physical circuits without balanced operation.The link access procedures use the
principles and terminology of high-level data link control ( H D L C ) as defined by ISO.
This procedure ensures the correct and error-free transmission of data information
across the link from DTE to DCE. Itdoes not, however, enable the DCE (in the form
of a dataswitchingexchange, D S E ) to determinewheretheinformationshould be
forwarded to withinthenetwork or ensure its correct and error-free arrival at the
distant side of the packet network. Thisis the job of the OS1 layer 3 protocol, the X.25
packet level interface.
During the set-upof a switched virtual circuit (SVC, also called a virtual call ( V C ) )it
is a level 3 call set up packet which delivers the DSE the data network address of the
remote DTE. Level 3 packets confirm the set-upof the connection to the initiating DTE
and then passend to end through the network, allowing user data tobe carried between
the DTEs.
A packet of data carried by the X.25 protocol may be anything between three and
about 4100octets (bytes: 8 bits). Up to 4096 alphanumeric characters of user
information may be carried in a single packet.
In slang usage, many people refer to ‘X.25 networks’. In general they mean packet
switching networks to which X.25 compliant DTEsmay be connected, forrecommenda-
tion X.25 describes only the U N I (user-network interface, see Chapter 7). The X.25
protocol allows DTEs madeby any manufacturer to communicate across the network.
You should not be tempted into believing that the protocol used between the various
packet data switching exchanges (DSEs) within the network is also X.25. Generally,
packet-switched data networks arebuilt from a number of individual exchanges, but all
of them provided by the same manufacturer. The protocol used for the carriage of
the data between the exchanges is normally an X.25-like, but enhanced ‘proprietary’
protocol. Examples include those used by Northern Telecom (Nortel), Telenet, BBS,
Tymnet and France’s Transpac.
Proprietary trunk protocols typically allowthecarriage of sophisticatednetwork
management and charging information back to the network control centre. In addition,
- 348 PACKET SWITCHING
they may allow for dynamic adjustment of the traffic paths taken through the network,
so giving better overall network performance during heavy traffic loading.
Where separate packet-switched
sub-networks(provided by different manufac-
turers) need to be interconnected, the X.75 (NNI) protocol is used. We discuss this
later in the chapter.
18.5 THE TECHNICAL DETAILS OF X.25
X.25 was one of the first data protocols to be well defined and standardized. As such it
has formed the basis on which later data transport protocols have been developed.
Understanding the principles in detail will give the reader a very good understanding of
all other dataswitching protocols, all which use similar principles. Therethus follows
of
a very detailed description.
18.6 X.25
LINK ACCESSPROCEDURE(LAPAND LAPB)
The link access procedure can be performed either in the basic mode (B = 0, called LAP)
or in the more advanced balanced mode (B = 1, called LAPB). Nowadays the LAPB
mode is more common.
There are two formsof LAPB; the basic form is called LAPB modulo 8,the extended
form is called LAPB modulo 128. Only the modulo 8 form is universally available. The
difference between the two forms is only the maximum value of the sequence number
given to consecutive packets before resetting to value ‘0’. LAPB allows for dataframes
to be carried across a physical layer connection between a DTE and a DCE. The frame
is structured in the manner shown in Figure 18.8.
The j a g is a delimiter between frames. The address (perhaps confusingly named, as
we discovered in Chapter 9) is a means of indicating whether the frame a command or
is
a response frame, and whether controlis with the DTE or DCE. It coded as shown in
is
Table 18.1.
Flag Address Control Information Frame Check Flag
01111110 (8 bits) (8 bits) (N bits) Sequence (of next frame)
(16 bits)
Figure 18.8 X.25 LAPB modulo 8 frame
Table 18.1 X.25 LAPB address field coding
Single
link procedure Multiple link procedure
command from DCE to DTE address A - 1l000000 address C - 11110000
response of DTE to DCE address A - 1 l000000 address C - 11 110000
command from DTE to DCE address B - 10000000 address D - 11l00000
response of DCE to DTE address B - 10000000 address D - 11l00000
- ACCESS LINK
X.25 (LAP
PROCEDURE AND LAPB) 349
Table 18.2 X.25 LAPB modulo 8 control field command and response coding
~ ~ ~ ~ ~ ~~~~~~~~~~~~
Format Command Response bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 bit 7 bit 8
Information I (information) 0 P
transfer
Supervisory (receive
RR ready)
RNR (receive not
ready)
REJ (reject)
Unnumbered SABM (set
asynchronous
balanced mode)
DISC (disconnect) 1 1 0 0 P 0 1 0
DM (disconnect l l l l F l l O
mode)
UA (unnumbered l l O O F l l O
acknowledgement)
FRMR l l l O F O O l
(frame reject)
The controlfieldcontains either command or a response, and sequence numbers where
a
applicable as a reference when acknowledging receipt of a previous frame. There three
are
types of control field format,correspondingtothenumberedinformationtrans-
fer of I-frames (I format), numbered supervisory functions(S format) and unnumbered
control functions (U format).
The control field is coded as detailed in Table 18.2.
The frame check sequence ( F C S )field is a string of bits which help to determine at the
receiving end whether the data in the frame has in any been corrupted. Itis a 16 bit
way
field, created using the properties of a cyclic code, hence the term cyclic redundancy
check. The exact 16 bit sequence sent is the ones complement of the sum (in binary) of
the following two parts.
(1) The remainder of xk(xI5+ xI4 + xI3 + XI* xl1+ x10+ x9 + x8 + x7 + x6
+
+X5 + + + + + +
X4 x3 X2 XI X 1)
divided (in binary) by x16 + x L 2 x5 + 1
+
where k is the
number of bits in frame
the excluding flag and FCS.
(excluding flag and FCS) and xI6,
(2) The remainderof the productof the frame content
when divided by X'6 +
X12 + X5 + 1
There are six configurable parameters when setting up LAPB connections. These, their
meaning and typical value settings are given in Table 18.3.
- 350 PACKET SWITCHING
Table 18.3 Optional parameter settings for X.25 LAPB
Meaning Parameter
Frame window size This is the maximum permitted number of 7
outstanding frames which may be sent without
receiving an acknowledgement
N1 parameter This is the maximum number of bits in an I-frame 2096
N2 parameter This is the maximum number of attempts which 10 attempts
may be used to complete a transmission
Timer T1 The timer T1 is the timeout period after which a 3 seconds
retransmission may be initiated
Parameter T2 This is the maximum time allowed before a n 0.2 seconds
acknowledging frame must be initiated
Timer T3 Should the channel remain idle longer than this time, 0 (not used)
then the link shall be assumed to be non-active
18.7 X.25 PACKET LEVEL INTERFACE (LAYER 3 PROTOCOL)
The information carried within a LAPB I-frame will be a packet of user data informa-
tion structured in t h e format as defined by the X.25 packet level interface.
The general format of a packet is as s h o w n i n Figure 18.9.
Octet 8 7 6 5 4 3 2 1
1 General format identifier Logical channel groupnumber
I
2 Logical channel number
clearing cause, resetting cause interrupt data
or
31
4 diagnostic code I
5 Packet type identifier
I
41 Address
digit 1 I
I
Address
digit 2 I
I I
X length
Facility (1 octet)
Figure 18.9 X.25 packet format (layer 3)
- X.25 PACKET LEVEL INTERFACE (LAYER 3 PROTOCOL) 351
Table 18.4 X.25 packet-coding of the general format identifier
General format identifier
bit 8 bit 7 bit 6 bit 5
Call set-up sequence modulo
packets number 8 X X 0 1
sequence number modulo I28 X X 1 0
Clearing number
packets modulo
sequence 8 X 0 0 1
sequence number modulo 128 X 0 1 0
Flow control, interrupt, sequence
number
modulo 8 0 0 0 1
reset, restart, registration sequence
number
modulo
128 0 0 1 0
and diagnostic packets
Data packets sequence number modulo 8 X X 0 1
sequence number modulo 128 X X 1 0
general format identifier extension 0 0 1 1
reserved for other applications * * 0 0
The minimum number of octets a packet is three, the general format identifier, the
in
logical channel identifier and the packet typeidentifier. The other packettypes are
added as required. As is normal, the least significant bits of each octet (i.e. bit 1) is
transmitted first.
The general format identijier field provides information about the nature the rest of
of
the packet header (the first three octets of the packet). It is coded according to the
values set out in Table 18.4.
The logical channel number is a reference number allowing the DTE and DCE to
distinguish to which logical connection (or statistically multiplexed virtual channel) the
packet belongs. Thus, in theory, up to 4096 logical channels may be supported by a
single DTE /DCE connection simultaneously.
The packet type identijier is coded according to Table 18.5. As can be seen from
the table, certain packet types are needed for virtual calls (VCs, also called switched
virtual channels, or SVCs), and a lesser number of packet types are required to sup-
port permanent virtualchannels (or PVCs). The difference between an SVC and a
PVC is essentially the difference between an ordinarytelephone line and apoint-
to-point leaseline. With an ordinary telephone line and an SVC each connection is set
up on demand by dialling the number of the desired destination. With a telephone
leaseline or a PVC the connection is permanently connected between the same two
endpoints.
In clear request and clear indication packets, the clearing cause is included at octet
4. In reset request packets the reset cause appears in octet 4, while in interrupt data
packets. Interrupt data, when sent, also appears in octet 4. This is data which is not
subject to normal flow control,in effect allowingthe DTEto overrideprevious
commandstothedistantDTE (like a ‘break’ or ‘escape’ key). Callrequest, call
accepted and call connected packets contain from octet 4 onwards the address block
field, andoptional facilitylength and facility $eld andthe calleduser data. The
- 352 PACKET SWITCHING
Table 18.5 X.25packetidentifiercoding
Packet type
From to From to
DCEDTE DTE DCE VC PVC bit bitbitbit
bitbit
bitbit
8 1 6 5 4 3 2 1
~
Call set-up and clearing
questcall callincoming J 0 0 0 0 1 0 1 1
accepted call
connected
call J 0 0 0 0 1 1 1 1
request
indication
clear
clear J 0 0 0 1 0 0 1 1
DCE clear confirmation clear
DTE confirmation J 0 0 0 1 0 1 1 1
Data and interrupt
DCE data DTE data J J x x x x x x x o
DCE DTE
interrupt
interrupt J J 0 0 1 0 0 0 1 1
DCE interrupt confirmation DTE interrupt confirmation J J O O 1 0 0 1 1 1
Flow control and reset
DCE RR (modulo 8) DTE RR (modulo 8) J J x x x 0 0 0 0 1
DCE RR (modulo DTE
128) RR (modulo128) J J 0 0 0 0 0 0 0 1
DCE RNR (modulo 8) DTE RNR (modulo 8) J J x x x 0 0 1 0 1
DCE RNR (modulo 128) DTE RNR (modulo 128) J J 0 0 0 0 0 1 0 1
DTE REJ (modulo 8) J J x x x 0 1 0 0 1
DTE REJ (modulo 128) J J 0 0 0 0 1 0 0 1
request reset
indication
reset J J O O O 1 1 0 1 1
DCE reset
confirmation reset
DTE confirmation J J O O O 1 1 1 1 1
Restart
indication
request
restart
restart J J 1 1 1 1 1 0 1 1
DCE restart
confirmation restart
DTE confirmation J J 1 1 1 1 1 1 1 1
Diagnostic J J 1 1 1 1 0 0 0 1
Registration
registration request J J l 1 1 1 0 0 1 1
registration confirmation J J 1 1 1 1 0 1 1 1
diagnostic code isincluded indiagnostictypepackets andalsooptionallyin clear
request and reset request packets.
The address block field is coded as shown in Figure 18.10. The calling DTE and called
DTE address length fields are coded in binary. The address digits themselves, however,
are in digit
codedfour blocks, coded
binary decimal,according to
ITU-T
recommendation X.121. Ifnecessary,bits 1-4 of the last octet are filled with Os to
maintain octet alignment.
Thefacility length field merely indicates as a binary number the length of the facility
field which follows. The facilities field allows the calling DTE to request a number of
optional services. These (where made available) are as listed in Table 18.6.
- (LAYER PACKET
INTERFACE
LEVEL
X.25 3 PROTOCOL) 353
8 7 6 5 4 3 2 1
called DTE address length calling DTE address length
called DTE address
l I
I
I I
I calling DTE address
I
I
I 0 0 0 0
I
I
I
Figure 18.10 Format of the X.25 packet address block
Table 18.6 X.25networkfacilities
Description X.25 Optional
facilities
on-line facility registration if subscribed to, allows user to request facility
registration
extended packet sequence numbering packets numbered modulo 128 rather than normal
modulo 8
D bit modification support of D-bit procedure
packet retransmission allows DTE to request DCE to restransmit packets
incoming calls barred prevents incoming calls being presented to DTE
outgoing calls barred prevents DCE from accepting outgoing calls
one-way logical channel outgoing restricts logical channel use to originating outgoing
virtual calls only
one-way logical channel incoming restricts logical channel use to incoming virtual calls
only
non-standard default packet sizes provides for the selection of non-default packet sizes
non-standard default window sizes provides for the selectionof non-default window sizes
default throughput classes assignment provides for the selection of default throughput
classes
flow control parameter negotiation allows the DTE to alter window and packet sizes for
each virtual call
throughput class negotiation allows throughput class (i.e. bitspeed) negotiation for
each call
closed user group restricts incoming calls to the DTE to be from other
specific DTEs which are members of a closed user
group
bilateral closed user group a closed user group of only two DTEs
fast select allows the call request packet to contain up to 128
octets of user information, thereby speeding the
sending of short user messages
fast select acceptance authorizes DCE to forward fast select packets to the
DTE
- 354 PACKET
Table 18.6 (continued)
X.25 Optional facilities Description
reverse charging allows calls to be requested for charging to recipients
rather than callers
reverse charging acceptance DTE authorization to the DCE that it is willing to
accept reverse charge calls
local charging prevention prevents the DCE from allowing virtual callsto be set
up for which the local DTE will be charged
network user identification (NUI) an authorisation procedure allowing for the dial-into
an X.25 network port across a public telephone
network
charging information DTE may request charging information
RPOA related facilities allows the calling DTE to specify transit networks
through which the call should be routed
hunt group allows several separate DTE/DCE network
connections to share the same network address.
Incoming calls are routed to any free logical channel
within at any of the DTEs
call redirection and call deflection allows deflectionon busy or redirection on no answer
called line address modified notification notifies the calling DTE that the called address has
been modified (diverting to a new destination)
transit delay selection and indication allows the calling DTE to specify a desired transit
delay
TOA/NPi address subscription DTE/DCE to use TOA/NPi address format
18.8 TYPICAL PARAMETER DEFAULT SETTINGS USED IN
X.25 NETWORKS
A typical public X.25 network might be set with thefollowing default settings for the
up
packet level interface
window size 2
packet size 128
window negotiation allowed
packet negotiation allowed
throughput negotiation allowed
lowest logical channel number 1
highest logical channel number 16
customer DTE to select logical channel number, from highest number descending
- DISASSEMBLERS
PACKET 355
18.9 PACKET ASSEMBLER/DISASSEMBLERS
(PADS)
The X.25 protocol is suitable for the connection of synchronous communication devices
(i.e. computers) to a packet switching network. It is widely used, and many devices are
available worldwide (both DTEs and packet DSEs (data switching exchanges) which
supportit. However,directly X.25 protocol is unsuitableforconnecting slow and
relatively unintelligent asynchronous computer terminals to an X.25 packet network.
Instead a procedure is defined by ITU-T recommendation X.28.
Necessary interworking is achieved by means of a piece of equipment provided in
place of the DCE at the packet-switched exchange. This equipment is called a packet
assembler/disassembler or P A D for short. As its name suggests, it assembles packets
from the asynchronous terminal data for onward transmission, and it disassembles
packets received fromthefarendDTE, conveyingthem ontotheasynchronous
terminal as individual characters.
Three ITU-T recommendations define the operation of PADS.
0 X.3 defines the functions of the PAD (i.e. the conversion of asynchronous characters
into synchronous packets).
0 X.28 defines the control interface betweenPAD and asynchronous terminal. The X.28
parameters (Figure 18.18) define how thePAD is to react to the characters sent to it,
how often to despatch groups of characters as apacket, and
which characters typed by
the end terminal are be interpreted directlyby the PAD itself as control characters.
to
0 X.29 defines the interface between PAD and remote X25 DTE (usually some form
of host computer).
Figure 18.1 1 illustrates the inter-relationship of the three recommendations and Table
18.7 lists the PAD control parametersof recommendation X.28. In the final column of
packet-switched network
functions definedby X.3
L
PAD DCE
(packet-
V.24 I
I
I
interface
!
I
4 8
-I X.28 - - +
-H X.29: -
protocol
Figure 18.11 Thepacketassembler/disassembler (PAD)
- 356 PACKET
Table 18.7 PAD control parameters defined by ITU-T recommendation X.28
Purpose
Parameter Simple values Permitted standard
reference profile
parameter 1 PAD recallusing character 0 = no escape 1
1 = escape
32-126 = ASCII code of ‘escape’ character
parameter (i.e.
Echo character
2 is 0 = no echo 1
returned to bedisplayed on 1 =echo
screen)
parameter 3 Selectionof data fowarding 0 =forwarded only on ‘poll’fromnetwork 126
characters (e.g. ‘carriage 2 = forward on ‘carriage return’
return’ key) 8 =forward on (CR), (ESC), (DEL), (ENQ),
WK)
18=forward on (CR), (EDT), (ETX)
126 = forward on all control characters
parameter 4 Selection of idletimerdelay 0 = no delay
1-255 (delay, multiple of 10ms)
parameter 5 Ancillarydevice control 0 = X-ONand X-OFF not use
in
1 =X-ON and X-OFF in use
2 = X-ON and X-OFF in use during data
transfer
parameterControl
6 of PAD service 0 = n o signals
sent to DTE 1
and
command
signals 1 =signals
sent to DTE
other values define which signals may be used
parameter 7 Selection of reaction of 0 = n o reaction to ‘break’ 2
PAD to the‘break’key 1 = assembledcharactersand‘interrupt’sent
to host
2 = reset on ‘break’
other values define other actions
parameter 8 Discard output 0 = deliver data 0
l = discard data
parameterPadding
9 after carriage 0-255 (number characters
of to be
inserted 0
return’)
‘carriage after return
parameter 10 Line 0-255 length
folding (definesaof
characters)
line
in 0
parameter 11Binaryspeed 12 = 2 400 bit/s -
13 = 4 800 bit/s
14= 9 600 bit/s
16= 19200bit/s
19= 14400bit/s
20 = 28 800 bit/s
21 = 38 400 bit/s
other values give other speeds
- DISASSEMBLERS
PACKET 357
Table 18.7 (continued)
Parameter Purpose Simple values Permitted standard
reference profile
parameter 12Flow control of the PAD 0 = n o use of X-ON and X-OFF for flow 1
control by the
DTE
1 = use of X-ON and X-OFF for flow control
parameter 13
‘Linefeed’insertion after
0 =no linefeed
insertion 0
‘carriage
return’ 1 = linefeed after return’
‘carriage
other values define other actions
parameter
14
Linefeed 0-255 characters
padding (number
of inserted after
linefeed)
parameter 15
Editing 0 = editing
command
only
in mode
1 = editing in command and data transfer
modes
2 =extended editing
parameter16Characterdelete 0-127 (ASCII code of character used as 127
character delete)
parameter 17 Linedelete 0-127 (ASCII code of character used as line 24
delete)
parameter 18 Linedisplay 0-127 (ASCII code of character used to 18
request line display)
parameter 19 Editing PAD service 0 = no editing 1
signals 1 =editing for print terminals
2 =editing for display terminals
8, 32-126 this character replaces deleted
characters
parameter 20Echomask 0 = no echo mask 0
1 = no echo of ‘carriage return’
2 = no echo of ‘linefeed’
other values define other control characters
not echoed
parameter 21 Paritytreatment 0 =parity not generated or checked 0
1 =parity checked of input from DTE
2 =parity generated for output to DTE
3 =parity generated and checked
(combination of 1 and 2)
parameter 22Pagewait 0-255 (number of line feeds which may be 0
generated before an ‘abort’)
parameter 23 Size of input field 0-255 (number of characters) 0
parameter 24 End of frame signal 0 = not used 0
1-127 ASCII code of character to signify end
of frame
- 358 PACKET SWITCHING
Table 18.7 (continued)
Parameter Purpose Simple
Permitted values standard
reference profile
parameter 25 Additional data forwarding 0 =no extra data forwarding 0
character 1-127 = ASCII
code of extra
forwarding
character
parameter 26 Display
‘interrupt’
character 0 = no abort character 0
1-127 = ASCII code of ‘interrupt’ character
parameter 27 Display interrupt 0 = no confirmation 0
confirmation (‘prompt’ 1-127 = ASCII code of ‘interrupt’
character) confirmation character
parameter 28 Diacritic character coding 0 =not used 0
scheme
parameter 29 Extended echo mask 0 = not used 0
Table 18.7, the parameter settings are given for the simple standard profile. This is a
standard X.28 parameter setting set for emulating a low speed ‘transparent’ leaseline
between an asynchronous computer terminal and its terminal controller.
In caseyou are left wondering,havingstudiedTable 18.7, what exactly ‘X-ON’
and‘X-OFF’are, these arethealternativestates of a given control leadwithina
standard DTE-to-DCE interface(such as V.24 as defined in Chapter 9). When the
lead is set ‘X-ON’ then the DTE may send transmit data. When instead the lead is
set ‘X-OFF’ then no data may be transmitted. This ensures that the DCE is ready
to receive data before the DTE may send. Before call set up the lead is set X-OFF,
then X-ON aftercallset-up.Duringthecall,theleadmaybe reset to X-OFF to
slow up the receipt of data (i.e. for flow control) if the DCE or network becomes
overloaded.
A call can be set up from an asynchronous DTE (so-called startlstop terminal)simply
by typing in the X.121 data network address (Chapter 29). This procedure for call set-
up from a start-stop terminal is also defined by X.28.
18.10 ITU-T
RECOMMENDATION X.75
To connect different packet switched networks together, a network-network interface
(NNZ) or gateway protocol is defined by ITU-T in its recommendation X.75. X.75 can
be thought of as a super set of the X.25 protocol (Figure 18.12). It wasoriginally
developed for international interconnection of packet networks).
Like X.25, recommendation X.75 defines all the protocols forlayers 1-3. At layer 1, a
64 kbitls bearer conforming to Recommendation G.703 is stipulated. At layer 2, the
- ITU-T X.75 359
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