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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
X.25 is a subnet standard; it operates within a single network. A complementary standard, X.75, has been developed to support the internetworking of X.25 subnets,
although it can also be used to interconnect subnets based on other protocols.
Like X.25, X.75 consists of three layers: physical, data link, and network. For its physical and data link layers, it may use standards similar to those used by X.25
e.g., X.21 and LAP-B. At the network level, X.75 accounts for the upper two sublayers, sitting on top of X.25 which accounts for the lower sublayer see Figure
4.49.
Operationally, X.75 provides a virtual circuit end-to-end connection between two DTEs on separate subnets, by interconnecting the latter and any intermediate
subnets see Figure 4.53. The interconnections are facilitated by Signaling Terminals
STEs which act as partial IWUs or routers, each implementing the X.75 protocol stack. The two DTEs are connected by a virtual circuit which is
comprised of a number of ‘smaller’ virtual circuits within and in between the subnets.
Figure 4.53 Internetworking with X.75.
Packet DCE
STE
X.25 Network
X.21 LAPB
X.25 X.75
Packet DCE
STE Network
X.25 Packet
STE Network
STE X.21
LAPB X.25
X.75 X.21
LAPB X.25
X.75 X.21
LAPB X.25
X.75
X.75 X.75
DTE DTE
X.75 packets implement the international packet-switched services for internetworking defined by the X.75 standard. In many respects, they are similar to
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X.25 packets, except that they are used for virtual circuits between STEs as opposed to virtual circuits between DTEs and DCEs.
4.5.3. IP
The Internet Protocol IP is a connectionless datagram protocol developed by the US Department of Defense Advanced Research Projects Agency DARPA. It
currently enjoys widespread use around the world. An IP network consists of a set of subnets and a set of intermediate stations which act as gateways. The subnets may
have totally different characteristics. The gateways perform routing functions as well as handling protocol conversion between the subnets.
IP datagrams are referred to as Internet Protocol Data Units, or IPDUs see
Figure 4.54. This datagram is transported as user data within the packets of the subnets. When a packet is about to cross a subnet boundary, the intervening
gateway extracts the datagram and encapsulates it into a new packet for use of the next subnet. Gateways and subnet stations maintain a static or dynamic routing
table which depicts the next gateway the IPDU should be forwarded to.
The interface for IP service users is quite simple, and consists of two primitives: send and deliver. An IP service user transmits data by issuing a send command
which contains various IPDU fields as parameters. When the IPDU is delivered to its final destination, the receiving station is issued with a deliver command which
contains the original data.
IP provides status reporting, diagnostics, and flow control capabilities through its Internet Control Message PDUs ICMPDUs. These are special PDUs used for
exchanging status and diagnostics information between the stations.
Figure 4.54 IPDU structure. Field
Description
Version Version of the IP.
Header Length Length of the header fields in octets.
Services Denotes the type of IP services required.
Total Length Total length of the IPDU.
Data Unit ID Denotes the first segment of a segmented PDU.
Flags For segmentation and error reporting.
Segment Offset Denotes the relative position of IPDU within PDU.
Lifetime IPDU lifetime.
Checksum Checksum for the IPDU header.
Addresses Source and destination NSAP addresses.
Options For source routing, route recording, QOS, etc.
Data Actual user data.
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4.5.4. ISO 8473
