Label handling in packet networks

In a packet network for electronic transfer of data packets from one end station to another end station, there are, in the conventional way, a number of switch units. Each end station is connected to a switch unit, and the switch units perform the transfer of data packets along an established connection. The switch units are provided with switch ports on all of their connections. In these switch ports, a substitution is performed of labels of the data packets, both for data packets arriving to the switch unit and for data packets forwarded from the switch unit. The substitution prevents data packets belonging to different connections from having the same label and also offers distributed services in the data network. The data packet labels are constructed of a VPI field and a VCI field and, for data packets that arrive directly from or are sent directly to an end station, a GFC-field. In the substitution of the label of a data packet arriving to a switch unit, a table look-up is first performed in the associated switch port by way of the contents of the VPI field of the data packet. The result of the table look-up gives a direct information if the data packet belongs to a connection having a label comprising only a VPI field (VP connection), and then its contents are replaced by a record obtained in the table look-up. If the data packet has both a VPI field and a VCI field (VC connection), its VPI and VCI fields are replaced by new label portions obtained from a second table look-up.

BACKGROUND 
The present invention is related to networks and switches for the 
transmission or transfer of data packets or data cells and in particular a 
processing performed at the incoming and outgoing sides of a switch 
involving a translation of the labels, which data packets carry for 
identifying that they belong to a particular logical connection 
established through the network and the switch. 
This application claims priority from Swedish Patent Application No. 
9300793-8, filed Mar. 10, 1993, which is incorporated here by reference. 
In a data packet network it is required, in the transfer of data packets 
from sending units to receiving units through various nodes in the 
network, that each data packet can be uniquely assigned a connection, 
which has been established earlier in the setup of the logical connection 
through the network. It is achieved by providing the data packets with 
labels, i.e. a special field comprising identification information. In the 
transfer in the network through many nodes, where the data packet is 
switched from an incoming connection line to an outgoing connection line, 
it may occur that data packets have been provided with the same label what 
will cause difficulties or even errors in the transmission. 
Data packets belonging to different types of established connections may in 
addition have label fields of different formats, e.g. such that data 
packets belonging to some connections have a shorter label field while 
data packets belonging to other connections also have such a short label 
field but also in addition thereto a longer label field. In the second 
case the first short label field may then indicate a group of connections, 
of which the connection is a part. Such different parts may be present 
when for instance some established connections are assigned more permanent 
paths in the network than other connections. These different formats 
result in further problems in the processing of the labels of the 
individual data packets. 
The data packets considered here are in the common way transmitted through 
the network from one end station to another end station. The end stations 
are then provided with a single connection interface, both for sending and 
for receiving, coupled through a connection line to an associated 
connection node in the network. Special format restrictions may then be 
valid for the labels depending if a data packet is transmitted directly 
from such an end station or is received by an end station or if a data 
packet is in the process of being forwarded between two switching nodes in 
the system. 
The handling of in particular addresses in data packets is disclosed e.g. 
U.S. Pat. Nos. 4,894,822; No. 4,218,756; No. 3,979,733; and No. 4,494,230; 
and in European Patent Applications EP-A2 0 282 197 and EP-A2 0 206 403, 
and in Soviet Patent Application SU-A 478 445. 
In the European patent application EP-A2 0 406 842 a packet switch network 
is disclosed comprising label processing at incoming and outgoing links of 
a switch. Therein the numbers in data packets identifying the logical 
connection--VP or VC number--are substituted by new numbers found in lists 
stored in memories at the incoming and outgoing sides of the switch. When 
many logical connections are to be simultaneously active, the lists will 
be correspondingly long and memory space therefor must be provided in the 
construction of the switch. 
Label processing is also disclosed in European Patent Application EP-A2 0 
481 447, and in U.S. Pat. Nos. 4,947,388; No. 5,099,475; No. 4,995,032; 
and No. 5,166,926. 
SUMMARY 
It is an object of the invention to provide a network and a packet switch 
allowing an efficient use of the memory space available for storing label 
conversion lists at the incoming and outgoing sides of a switch. 
It is another object of the invention to provide a network and a switch 
allowing a correct handling of particular fields identifying the type of 
link, on which a data packet has arrived to a switch and on which it is to 
be transmitted from the switch. 
The objects mentioned above are achieved by a network and a switch 
according to the invention, by means of which also the difficulties 
mentioned above are avoided. 
The network for the transfer of data packets thus in the common way 
comprises a number of end stations, from which the data packets are issued 
and which receive the data packets. The end stations comprise a connection 
or terminal point having a connection line extending to an associated 
switch unit. The switch units are nodes in the network and have a number 
of connection points of which some can extend to end stations and other to 
other switch units. Connection lines connect the different connection or 
coupling points of the switch units. Each connection point is bi-directed 
such that it comprises an output side and an input side. When information 
is to be forwarded from an end station to another one, in some way a 
connection is established in the network between these two end stations. 
This connection thus is a contemplated path through the network along 
which the various data packets in which the information to be transferred 
is carried, are guided through the network. 
All switch units are on the connection points thereof provided with ports 
and in these a substitution is performed of a label in each data packet 
passing through the connection line of the port. These labels are 
established in the establishment of the connection and indicate the 
connection to which a data packet belongs. The substitution of the label 
is performed to prevent that several connections have data packets with 
the same label and to simplify the handling of labels in the network, such 
that each switch unit not necessarily must be provided with information in 
regard of exactly all labels used in the network. The substitution of the 
label of a data packet in the connection point, where the data packet 
arrives to a switch unit, can be supplemented by a substitution also in 
the port associated with the connection point, from which the data packet 
is forwarded from the switch unit. 
As has already been mentioned, the substitution of the labels is performed 
to avoid that for instance data packets belonging to different 
connections, in their travel through a switch unit, will be provided with 
the same label. The label could in addition be used by each switch unit to 
send the data packet to the correct output connection point of the switch 
unit, but practically therefor a particular internal address is used being 
coded to control, in the simplest possible way, the various circuits from 
which the switching circuits or the switch core of the switch unit is 
constructed. The substitution of the label also on an output connection 
point is preferable, since hereby distributed services can be offered in a 
simple way in the network. Thus for instance a data packet which arrives 
to a switch unit, can be forwarded from the switch unit on several 
different connection lines and on these connection lines be provided with, 
when it is necessary, different labels in order that a confusion with 
labels of already existent connections will not occur. 
In the substitution of the label of a data packet which is performed in the 
ports of the switch unit, in some way a list is established containing 
records with information relating to the method of substitution of the 
label of a data packet and also the new internal labels. For data packets 
arriving to a port all the internal labels and the corresponding arriving 
labels may thus be stored in this list, where the internal labels thus are 
the labels which a data packet will carry in its travel through the switch 
unit. This list can then for instance only contain all internal labels in 
a suitable sequence such that an internal address in the list has a 
position corresponding to the arriving label thereof, that is generally 
such that the list is addressed by means of the contents of the label 
field of the data packet or generally such that a table record is accessed 
in some way by means of the label of the data packet. A corresponding list 
may of course be arranged for the substitution of the internal labels in a 
port, from which the data packets are forwarded, where thus all the 
internal labels and the corresponding forwarding labels are collected in a 
list. 
The labels of the data packets comprise two different parts, a first part 
and a second part, where only the first part is used for some connections, 
say for connections of a first kind, and both the first and the second 
part is used for other connections, say for connections of a second kind. 
A table look-up by means of the whole label field and for the substitution 
of the whole label could be performed if a sufficiently large and rapid 
memory was arranged for the list used in the table look-up. In order to 
reduce the requirement of memory a table look-up is performed, there is 
performed a first table look-up with the first part of the label of an 
arriving data packet as an entry or address in a first sublist. Hereby 
information can also be obtained in regard of the kind of connection to 
which the data packet belongs, i.e. if it belongs to a connection which 
has only a first label part, or if it belongs to the kind of connection 
which has both a first and a second label part. Further, in the first 
table look-up the new label part can be obtained which is to replace the 
first label part. In the case where the data packet belongs to a 
connection of the second kind, it could, however, also be considered that 
this substitution of the first part also is performed, but preferably the 
whole label comprising the first and second part is replaced by a new or a 
second table look-up in a second sublist. As an entry or address to this 
second list a pointer can be used based on both information obtained in 
the first table look-up and the second label part of the data packet. 
Further in some way information is also inserted in the data packet in 
regard of the kind of connection to Which the data packet belongs or 
generally if the data packet uses only the first label field or the two 
parts. This information is only retained internally in the switch unit and 
is removed on the output port, from which the data packet is forwarded 
from the switch unit. 
The table look-up can be performed in the same way in an output port, where 
the internal label is substituted by a forwarding label. 
Further data packets which arrive directly from an end station or are 
forwarded directly to an end station, can have a first label format, for 
instance in the shape of a short field, and the data packets being 
forwarded between switch units have a second label format, for instance in 
the shape of a longer field, such that the short field is a part of the 
longer field. In the establishment of a connection then the various label 
formats are established which are valid for data packets belonging to this 
connection, by the fact that indicator elements, for instance specific 
data bits in some register or memory, are transferred to various signal 
positions valid for the connection, i.e. if data bits are used, that they 
are set or reset. When a data packet then arrives to a switch unit and to 
a port thereof, it is decided in this arriving port by means of a the 
appropriate indicator element, if the packet from the switch unit is to be 
forwarded directly to an end station and, if it is the case, the label of 
the data packet is substituted by an internal label having the first 
format, and otherwise the label of the data packet is substituted by an 
internal label having the second format. In the case where the first label 
format is a short field being a part of a longer field, which constitutes 
the second label format, when necessary for some transfers, a longer field 
may be filled with data bits in the form of zeroes, such as in a transfer 
from shorter to longer fields for the label. 
The lists, in which the table look-up according to the above is performed, 
can also be organized such that the possible contents of a data packet 
arrived to or received by a port, this data packet arriving from the 
outside or from the switch unit itself, are stored in a sequential order. 
Then the new contents of the label field are obtained from the position or 
the sequential number of the contents of the data packet in this list. 
This organization of a list is chosen, when an ordinary list would require 
too much memory space.

DETAILED DESCRIPTION 
In FIG. 1 a network is schematically illustrated intended for the transfer 
of data packets, such as between end stations or terminals 1. The end 
stations 1 are thus both receivers and senders of the data packets, i.e. 
the origins or sources and the destinations of data packets considered 
here. The network in addition comprises a multitude of intermediate 
switching points or switches 3. Each end station 1 has basically only one 
connection point (not shown) which is bidirected and thus is arranged both 
for receiving and sending data packets. This connection point or unit is 
through appropriate lines coupled to a connection point 4 of a switching 
unit 3. The switch units 3 have several such connection points 4, which 
are also bidirected. The switching units 3 are, through some of their 
connection points, connected to other similar switching or exchange units 
3. 
When an end station 1 requests to start sending data packets in the network 
to another end station 1, a logical signal path is established through the 
network. Such a path can be established by the exchange of various 
messages or control information between the switch units 3. When a 
connection path is established, a message can be transmitted to the end 
station 1, which has requested the connection, that the connection now is 
established and is ready to be used for the transfer of data packets from 
the end station. 
For each switch unit 3 its connection points 4 are provided by switching 
ports 5, one switching port arranged for each connection point. Every 
switching port 5 is divided into an input, incoming or arriving side 5a 
and an output, outgoing or forwarding side 5b, such that each data packet 
outgoing from a switch unit 3 will pass through the output side 5b of a 
switching port 5 and a data packet arriving to a switch unit 3 will pass 
through the input side 5a of a switching port 5. The physical links or 
connections between a end station 1 and its associated switch unit 3, this 
type of links in the figures being indicated as UNI, can be constructed 
differently and have other requirements on the connection and on the 
structure of the data packets than links or connections between two switch 
units, which in the figures are indicated as being type NNI. 
In the transport through the network the data packets are provided with a 
field for identification of the packet and also containing other 
information relating to the handling of the contents of the data packet, 
and further the real or proper information field of the packet containing 
the user information which is to be transported from the source end 
station to the destination station. The identification field comprises a 
label field, the general configuration of which is illustrated at the top 
in FIGS. 2 and 3. The number or generally the information in this field 
identifies the established connection, to which the data packet belongs. 
A data packet forwarded on link from and to an end station has a label 
field with the configuration as indicated at the top of FIG. 2. The label 
field comprises three subfields, a field of four bits, in the figure 
indicated as GFC, a field of eight bits, in the figure indicated as VPI, 
and a field of sixteen bits, in the figure indicated as VCI. When a packet 
comprising a label field having this configuration is forwarded from an 
end station 1 to the associated switch unit, in the switching port 5 of 
the switch unit, to which the data packet will first arrive in the travel 
through the network, the information in the VPI field and possibly also 
the information in the VCI field are replaced or translated. If the data 
packet from this most adjacent switch unit 3 is to be sent back 
immediately to an end station 1 without any transport through other switch 
units, the field GFC of the data packet is not affected. Otherwise, when 
the data packet is to be transmitted further in the network and travel 
through another or several switch units 3, the GFC field is removed and 
the data packet is given the configuration illustrated at the top of FIG. 
3. 
The actual labels are comprised or written in the fields called VPI and VCI 
respectively. Some connections, called VP connections, have only 
information in the VPI field, while other connections, called VC 
connections, have information both in the field VPI and the field VCI. 
When a data packet arrives to a switch unit 3, it may happen, that its 
label as specified in the fields VPI and/or VCI coincides with the label 
of another connection and that data packets of this other connection 
arrive to the same switch unit 3 on another input port. To avoid a 
confusion of such labels the label fields are translated to or substituted 
by an internal label in the switching port or interface unit 5, on which 
the data packet arrives to the switch unit 3. In the case where data 
packets arrive directly from an end station, their label field thus has 
the configuration as illustrated at the top in FIG. 2. The label field is 
accessed in the switching port 5, i.e. in the input side 5a thereof, in 
order to be processed. 
At the bottom in FIG. 2 the procedure is illustrated, in this case with 
data packets arriving directly from an end station, for changing the label 
field in the switching port 5. From the label field first the information 
carried in the VPI field is accessed. By means of this information a table 
look-up in a database or register is performed. From this table look-up 
information is first obtained, whether the data packet belongs to a VP 
connection or a VC connection. Further also information is obtained, 
whether the data packet is to be forwarded to an end station connection of 
the same kind, which here as above is called a UNI link, or if the packet 
is to be conveyed further in the network, the latter case as above also 
being called that the packet is to be forwarded next through an NNI link. 
For the case when a packet arrives on a UNI link, the packet belongs to a 
VP connection and is to be forwarded directly to another end station 
connected to the same switch unit, that is to a UNI link, only the 
contents of the VPI field in the original data packet are substituted by a 
new VPI number, which is obtained directly from the table look-up, without 
any change of the length of the VPI field. If the data packet belongs to a 
VP connection but instead is to be forwarded to an NNI link, the VPI field 
is enlarged to comprise also the GFC field, such that the information in 
the GFC field and the VPI field is replaced by a new longer VPI number 
obtained in the table look-up. In the latter case, the GFC field is thus 
removed and in both cases the information in the VCI field is not 
affected. 
If the data packet in the table look-up instead is decided to belong to a 
VC connection, in the table look-up new information is obtained which is 
used for an additional table look-up. At the same time other information 
is also obtained, in particular as above information whether the data 
packet is to be sent to a UNI link or to an NNI link. If the data packet 
is to be sent to a UNI link, the information in the VPI and VCI fields is 
substituted by the information obtained in this later table look-up, thus 
the VPI and VCI fields of the packet being merged to one field. In the 
corresponding way the GFC, VPI and VCI fields are merged to one field and 
then thus the contents of these GFC, VPI and VCI fields are replaced by a 
new unified VPI and VCI number, when the packet is to be forwarded to an 
NNI link. In the latter case, thus also the information in the GFC field 
is overwritten. 
Further, both for VP and VC connections an additional information unit is 
inserted in the data packet in its identification part but outside the 
label field, in the shape of an addressing mode bit AM, indicating if the 
data packet belongs to a VP connection or to a VC connection. 
A data packet arriving to a switch unit 3 on a connection line, which is 
not directly connected to an end station 1, has the configuration 
illustrated at the top in FIG. 3. Here there is a VPI field comprising 12 
bits and a VCI field comprising 16 bits. In the substitution or 
replacement of the information in the VPI and VCI label fields inside the 
identification field of the arriving data packet a procedure is performed 
analogous to the one described above, with the exception that here there 
is no GFC field and, that when the packet is to be sent to a UNI link, a 
zeroed or reset GFC field is inserted, compare the lower part of FIG. 3. 
Thus first a table look-up is performed by means of the contents of the VPI 
field. From the table look-up information is first obtained if the data 
packet belongs to a VP or a VC connection. In the former case also 
information is obtained if the packet is to be sent to a UNI or NNI 
connection. If the packet is decided to belong to a VP connection and is 
to be sent to a UNI link, in the beginning of the VPI field a reset GFC 
field comprising four zero bits is inserted, such that the VPI field is 
shortened, and further in this new shorter VPI field information is 
inserted which has been obtained in the table look-up. If the packet 
instead is to be next transferred through an NNI link, the information in 
the VPI field of the data packet is substituted by information obtained in 
the table look-up. In these two cases the information in the VCI field is 
unaffected. When it in the table look-up instead has been decided that the 
data packet belongs to a VC connection, an additional table look-up is 
performed based both on the information which has been obtained in the 
first table look-up and by means of the information present in the VCI 
field of the data packet. In this second table look-up, as above, 
information is obtained, whether the data packet is to be forwarded on a 
UNI or an NNI link. In the UNI case, in the beginning of the label, as 
above a reset GFC field is inserted, which thus will be a part of the 
original VPI field. Further a new label is obtained, which for 
transmission through a UNI link has a correspondingly shorter length and 
in the shape of a unified VPI and VCI number it is inserted in both the 
rest of the VPI field and the whole VCI field in the original data packet. 
In the transmission through an NNI link the new unified VPI and VCI number 
has a whole length and replaces the all of the original VPI and VCI 
fields. Further, the addressing mode bit AM is inserted in the data packet 
in the same way as described above. 
In FIGS. 4 and 5 it is schematically illustrated a possible organization of 
the tables, in which table look-ups are performed in the translations or 
substitutions, described above with reference to FIGS. 2 and 3, of the 
labels on an incoming link. 
As is illustrated in FIG. 4, for each VPI number, that is for each contents 
of the VPI field, of an incoming data packet there is a record of three 
fields. The contents of the field illustrated rightmost in the figure 
indicate if the connection is a VP connection or a VC connection. This 
field is a simple flag field comprising one bit and storing the value 1 or 
0. There is another similar flag field comprising one bit, which is shown 
to the left of the mentioned field. This middle field contains a flag, 
indicating if the connection will next continue to a link of type UNI or 
type NNI. Further there is in each record in the table a longer part 
comprising 12 bits, in which, for VP connections, the new internal label 
is located or stored. In the case where the connection will next continue 
to a UNI link, however, the four first bits in this field are not used. In 
the case where the connection is a VC connection, there is instead of a 
new internal label a base address located in the mentioned field. This 
base address is used in the second table look-up. 
In FIG. 5 the structure of the second table is schematically illustrated, 
which is used only for VC connections. The table contains a number of 
records, which are addressed by means of both the base address which has 
been taken from the table of FIG. 4, and the VCI number of the data 
packet, that is the contents of the VCI field of the packet, for instance 
by an addition of the base address and the VCI number of the data packet. 
In the same way as in the table of FIG. 4, there is in each record a 
flagfield of one bit length storing a flag indicating if the connection 
next continues to a UNI or NNI connection respectively. In a longer field 
the information is located which is to replace the contents of the VPI and 
VCI fields of the data packet. In a practical case with a number of 
simultaneously established connections which is not too great, this new 
field can for instance comprise 16 bits and contain the new internal 
label, which in this case then will replace the information only in the 
VCI field of the data packet, while in the information in the VPI field is 
reset. If desired, the record can of course contain a still longer field 
at most comprising 28 bits, if the number of connections simultaneously 
established is very large. 
An alternative structure of a table used for the label translation of an 
arriving data packet is illustrated in FIG. 6. Here for an incoming link 
of NNI type, all possible contents of the whole label field considered as 
one field, i.e. the joined VPI and VCI fields of an arriving data packet, 
are listed or stored in a sequential order. For an incoming link of UNI 
type the unified contents of the VPI and VCI fields of the incoming data 
packets, in the same way considered as one field, are inserted and stored 
in a sequential order (this case is not shown). The whole internal label 
of a data packet belonging to some connection is in this case obtained 
from the position of the label of the data packet in the list of contents, 
or from the address of the position in a memory, where exactly these 
contents are stored. For every possible contents there is like above a 
simple bit position having a flag indicating if the data packet is to be 
forwarded next to an output of the switch unit connected to a link of UNI 
or NNI type. 
In FIG. 7 a block diagram is shown illustrating the label processing in the 
input side of an interface unit, where a table structure according to 
FIGS. 4 and 5 is used, the interface unit being connected to a switch unit 
3. The data packets first arrive to a buffer 601 and here, from a received 
packet, its label field is accessed or extracted in a block 603 and it is 
stored in a register or memory field 604. Then only the VPI field is taken 
out of the whole label field stored in 604 in a block 605, where the 
length of the VPI field is determined by means of a flag, which is stored 
in a memory cell 607 and has been set earlier in the configuration or 
setup of the switching port, to provide an indication if the input link is 
of UNI or NNI type. By means of the contents of the VPI field a table 
look-up is performed in a block 609 in a write and read memory 611 and a 
table record 610 is obtained temporarily stored in a memory field 610. By 
means of the result of the table look-up it is decided in a block 612, if 
the data packet belongs to a VP connection or a VP connection. In the 
former case the table look-up directly gives a new VPI field 614, which is 
formed in a block 613, and also a flag "UNI/NNI out" indicating, if the 
output link, to which the data packet will be forwarded next, is of UNI or 
NNI type. Both the new VPI field and the obtained flag of output link are 
fed to logic circuits 617 for the insertion of a new VPI field, the length 
thereof being determined by the flag "UNI/NNI out", and for a possible 
reset of a now formed GFC field, determined by the two flags "UNI/NNI in" 
and "UNI/NNI ut" in combination. Therefor the logic circuits 617 also need 
information whether the input link is of UNI or NNI type, which is 
obtained from the flag "UNI/NNI in" stored in the memory field 607. The 
logic circuits 617 for the insertion then place the new label information 
and the addressing mode bit AM in the data packet when it for instance 
travels through a buffer 619. The buffers 601 and 619 may be the same 
physical buffer. 
When in the block 612 for the decision based on the result of the table 
look-up in the block 609, it is instead decided, that the arrived data 
packet belongs to a VC connection, a new address is formed in a block 621 
by means of the contents of the VCI field of the data packet and also by 
means of the table record stored in the cell 610 containing the result of 
the table look-up already performed in the block 609. An alternative 
hereto is to directly use, instead of the result of this former table 
look-up, the contents of the VPI field used in the block 605. Then another 
table look-up is performed in a block 623 in the write and read memory 
611. Here as above a table record stored in a memory field 624 is obtained 
containing among other things a flag "UNI/NNI out" indicating, if the 
link, to which the data packet will be next transferred, is of UNI or NNI 
type respectively. Further, in a block 627 a new unified VPI and VCI field 
is obtained and temporarily stored in a memory field 628. The unified 
field stored in the field 628 obtained in this later table look-up is as 
above transferred to a logic block 620, to insert in the data packet in 
the buffer 609 the new label in the VCI field, in the corresponding case a 
reset of the part of the VPI field which will form a new GFC field, and 
possibly a further reset of the rest of the unified VCI and VPI field and 
insertion of the addressing mode bit AM which in this case indicates that 
the data packet belongs to a VC connection. Therefore, like the block 617, 
also this block 629 needs information from the memory cell 607, whether 
the link on which the data packet arrived is of UNI or NNI type 
respectively. 
From the second buffer 619 the data packet is then transferred to the 
associated switch unit 3. 
Inside the switching units 3 the data packets are transferred from an input 
connection point given in the establishment of the logical connection, to 
an output connection point by means of specific addresses, which are 
inserted in a particular, added internal header field, at the arrival of 
the data packet to a switch unit. These internal addresses are given in 
such a form that they easily can be decoded, for instance decoded directly 
by hardware, in the circuits inside the switch unit, without requiring any 
table look-up therefor. Data packets belonging to different connections 
can thus have the same internal address but they will always have 
different internal labels. At the arrival to an output connection point 
the internal header fields are removed and thus the internal address and 
then also a substitution or replacement is performed of the information in 
the label of the data packet. It will be described more closely with 
reference to FIGS. 8 and 9. 
In FIG. 8, at the top thereof, thus the addressing mode bit AM and the 
label part are shown in the two different formats for a data packet which 
is to be transferred to an output connection point, which, through its 
associated switching port, is connected directly to an end station, i.e. a 
UNI link. As is further apparent from FIG. 8, it is decided by means of 
contents of the addressing mode bit, if the data packet belongs to a VP 
connection or a VC connection. The GFC field will in this case not be 
involved in the translation of the label part. It can contain information 
from earlier stages, if the data packet directly has arrived from a link 
of UNI type, or the field may contain only zeroes, if the packet has 
arrived to the switch unit on a link of NNI type. When the data packet, as 
is indicated by the AM bit, belongs to a VP connection, a table look-up is 
performed by means of the contents of the VPI field, and if the data 
packet belongs to a VC connection, all of the contents in the two fields 
VPI and VCI are used, where in many cases the VPI field will actually only 
contain zeroes and possibly the VCI field also in its most significant 
positions will have only zeroes. It is decided by the number of 
simultaneous connections which is to be handled by a switch unit and its 
switching ports. For a VP connection a new VPI number is obtained and it 
is inserted in the VPI field, the VCI field in this case being left 
unaffected. For a VC connection, from the table look-up a new VCI number 
is obtained, which is inserted in the VPI field of the data packet, and 
also a new VCI number which is inserted in the VCI field of the data 
packet. The addressing mode bit AM can be removed together with other 
information, which possibly has been added to the data packet in its 
travel through the switch unit, or be used for other purposes or simply be 
left. 
FIG. 9 is drawn in the same way as FIG. 8 and illustrates the case of 
substitution of the label fields for a data packet which is to be sent to 
a link of NNI type. In the data packet there is then no GFC field and the 
VPI field or the unified VPI and VCI field is then correspondingly longer. 
In the same way as in FIG. 8, in addition a table look-up is performed for 
the two cases of a VP connection and a VC connection. The sole difference 
is that the VPI field is longer. 
In FIG. 10 a block diagram is drawn illustrating the procedure of the 
substitution of labels is performed in a switching port of the output side 
of a switch unit. The data packets arrive from a switch unit 3 to a buffer 
901, from which in a block 903, the label field of the data packet and the 
AM bit thereof are accessed and stored in a register or memory field 905. 
By means of the contents of the AM bit it is decided in a block 907, if 
the data packet belongs to a VP connection or a VC connection. In the 
former case, in a block 909 a suitable address is formed by means of the 
address in the VPI field in regard of its length which is obtained by 
means of a flag "UNI/NNI out", which is obtained from a memory cell 911, 
containing information if the output connection point is of UNI or NNI 
type. The table look-up for this case with a VP connection is performed in 
a block 913 by means of the address formed in the block 909. The table 
look-up is performed in a write and read memory 915. If it instead is 
decided in the block 907, that the data packet belongs to a VC connection, 
in a block 917 an address is formed by means of the contents of the VCI 
field of the data packet, in the general case the contents of both the VPI 
and VCI fields, but in a preferred practical case the VPI field in the 
label of the data packet is reset in its transfer through the switch unit 
3. A table look-up is then performed in a block 919 by means of the 
address formed in the block 917. From the write and read memory 915 
information is obtained, which in a block 923 is used to form the contents 
of new VPI and VCI fields temporarily stored in a memory field 921. By 
means of the flag stored in the register 911 indicating, if the output 
link is of UNI or NNI type and hereby the length of the VPI field, the new 
label is inserted in the data packet in a block 925 comprising appropriate 
logic circuits. The data packet can be considered as being stored in a 
buffer 927, which actually can be identical to the earlier mentioned 
buffer 901, to which the cell stream arrives directly from the switch. 
From the buffer 907 the data packet passes in the outgoing data packet 
stream through a connection line to an end station. 
In a block 929 new contents, temporarily stored in a memory field 931, are 
formed in the case of a VP connection, by means of the result of the table 
look-up in the block 913. These contents in the memory 931 are to be 
inserted in the VPI field, and in the same way as for a VC connection 
there are logic circuits in a block 933 to insert these contents in the 
data packet which is supposed to be stored or hold in the buffer 927.