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3. Some subnets use virtual circuits, some datagrams. This situation has no easy solution. A practical way of addressing it is to use either a network level virtual
circuit protocol or a network level datagram protocol for internetworking and require all participating subnets and IWUs that do not support that mode to
implement both modes.
Given that the subnets may use different network architectures and different protocols, many incompatibilities need to be overcome by the IWUs and gateways,
including the following:
•
Different types of service and network user interfaces
•
Different message formats
•
Different addressing schemes
•
Different packet switching modes
•
Different routing methods
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Different error handling methods
•
Different security measures
4.5. Network Layer Standards
Network services discussed in Section 4.1 are defined by the ISO 8348 and CCITT X.213 standards. ISO 8880-2 and ISO 8880-3, respectively, provide
standards for the provision and support of network services for the virtual circuit and datagram models. Internetworking and the internal breakdown of the network layer
into sublayers is covered by the ISO 8648 standard. CCITT X.121 provides a numbering plan for data network addressing on an international basis.
There are many other standards pertaining to the network layer. Below we will look at three influential networking and internetworking standards: X.25, X.75, and
ISO 8473.
4.5.1. CCITT X.25
X.25 is probably the most widely-known network layer standard. It has been in existence since 1974 and has been revised a number of times. X.25 enjoys
widespread use in the industry for connecting DTEs to packet networks, and has found its way to many network products.
X.25 encompasses the bottom three layers of the OSI model. For its bottom two layers, however, it simply refers to other standards. It recommends X.21 and
X.21 bis see Chapter 2 for its physical layer, and LAP-B see Chapter 3 for its
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data link layer. Many vendors have also used other standards instead of these in their products.
Strictly speaking, X.25 provides no routing or switching functions. It simply provides an interface between DTEs and network DCEs. As far as X.25 is
concerned, the network is an abstract entity, capable of providing end-to-end connections. Figure 4.50 illustrates the role of X.25 in a typical end-to-end
connection. The two logical channels between DTEs and DCEs at either end are joined through the packet network to complete an external virtual circuit. X.25
makes no assumptions about the internal mode of the packet network. This may be virtual circuit or datagram.
Figure 4.50 X.25 scope and layers.
Packet Network X.25
X.21 LAPB
L6 L4
L7 L5
X.25 X.21
LAPB X.25
X.21 LAPB
DCE DCE
X.25 X.25
X.25 X.21
LAPB L6
L4 L7
L5
Logical Channel Logical Channel
External Virtual Circuit
X.25 provides three types of external virtual circuits which, although similar in many respects, serve different applications:
•
Virtual Call VC. Virtual calls consist of three sequential phases: i call setup
which involves the caller and the callee exchanging call request and call accept packets, ii data transfer which involves the two ends exchanging data packets,
and iii call clear which involves the exchanging of call clear packets.
•
Permanent Virtual Circuit PVC. The virtual circuit is permanently assigned
by the network, hence the DTEs are guaranteed that a connection will always be available. There is no need for call setup or call clearing, and the data transfer is
as in VC.
•
Fast Select Call . Fast select calls are similar to virtual calls, except that the call
setup and the call clear packets may contain up to 128 octets of user data. A variation of this type of circuit provides for the immediate clearing of the call after
the first phase. Fast select calls in effect emulate a datagram facility, which is useful for situations where the communication between the two DTEs is limited to
a couple of short messages.
X.25 provides for sequencing of packets using the send and receive sequence numbers. Flow control is based on the sliding window protocol. Error control uses
the Go-Back-N method. All these are used in a similar fashion to their use in HDLC.
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There are three general categories of X.25 packet formats: data packets, control
packets, and interrupt packets see Figure 4.51. Most packet types fall
into the control packet category. Interrupt packets are used for implementing the network layer’s expedited data service. These are assigned a higher priority, can
carry up to 32 octets of user data, and bypass the normal flow control procedures. To avoid flooding the network, only one unconfirmed interrupt packet is allowed at a
time.
Figure 4.51 X.25 packet format categories. Data Packet:
Packet Format Logical ChannelGroup Numbers
SendReceive Sequence Numbers User Data
Control Packet: Packet Format
Logical ChannelGroup Numbers Packet Type
Packet Type-dependent Control Info e.g., source and destination DTE
addresses, facilities, data
Interrupt Packet: Packet Format
Logical ChannelGroup Numbers Packet Type: ‘interrupt packet’
Interrupt-related Data
X.25 packets support all the network services described in section 4.1, as well as three other services: diagnostic, registration, and restart. Diagnostic packets are
used to communicate additional error-related information between X.25 stations. Registration packets enable X.25 subscribers to optimize the network services
assigned to them by conveying appropriate requests to the network. Restart packets reset all the logical channels and all the virtual circuits; they are useful for dealing with
serious network failures.
Other protocols often associated with X.25 are: X.3, X.28, and X.29 collectively referred to as triple-X. These define the functions and the interfaces for
a Packet AssemblerDisassembler PAD. The role of the PAD is to allow conventional character-based terminals to be connected to X.25. It assembles the
characters from a terminal into a packet ready for transmission and disassembles an incoming packet into a character sequence for terminal consumption. X.3 defines the
PAD parameters. X.28 defines the interface between the PAD and the terminal.
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X.29 defines the interface between the PAD and the remote host. Figure 4.52 illustrates.
It is worth noting that the triple-X have no clear place in the OSI model. Also, their role will diminish over time as X.25 terminals become more widespread.
Figure 4.52 The triple-X protocols.
X.25 DTE
Dumb Terminal X.28
PAD X3
X.29 Packet Network
DCE DCE
X.25
4.5.2. CCITT X.75
