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X.25 Layered Model 231

In an SVC deployment, the provider incurs significantly less overhead. The circuits are not
permanently nailed up, so bandwidth is more efficiently utilized. This fact also makes it
possible to dynamically route around network failures with no manual intervention. In addition,
downtime is decreased in the event of a failure. SVC implementations are similar to ISDN
calls--there is a call setup, data transfer, and a call release.

When a VC is established, the DTE sends a packet to the other end of the connection using the
proper VC. The DCE examines the VC identifier to make a routing determination to get the
packet through the X.25 network. The Layer 3 X.25 protocol enables the DCE to MUX data
from all the DTEs on the destination side of the network.

X.25 supports multiple variations for VC configuration. A single X.25 interface can support up
to 4095 individual VCs. The options available are as follows:

Single Protocol Single Circuit--A single protocol is sent across the X.25 link utilizing
a single VC.

Single Protocol Multiple Circuit--A single protocol is multiplexed over multiple VCs.
SVCs can be combined to improve throughput for a given protocol.

Multiple Protocol Single Circuit--Multiple protocols are multiplexed across a single
circuit. Up to nine protocols can be sent across a single VC.

Multiple Protocol Multiple Circuit--Multiple protocols are sent across multiple VCs.
This deployment can utilize a single VC per protocol, or multiplexed protocols across
multiple VCs.

X.121 Addressing

The X.25 header at Layer 3 comprises a general format identifier (GFI), a logical channel
identifier (LCI), and a packet type identifier (PTI). GFI is a four-bit field. The LCI is 12 bits and
is locally significant at the DTE/DCE interface. The PTI field identifies one of X.25's 17 packet
types. Those packet types are beyond our scope at this time.

Addressing fields in call setup packets contain the source and destination DTE addresses. These
addresses are used to establish the VCs that make X.25 communication possible.

ITU-T recommendation X.121 specifies the source and destination address formats. X.121
addresses (also known as international data numbers, or IDNs) vary in length and can be up to
14 decimal digits long. The forth byte in the call setup packet specifies the source DTE and
destination DTE address lengths.

The first four digits of an IDN are called the data network identification code (DNIC). The
DNIC is divided into two parts, the first three digits specifying the country and the last digit
specifying the PSN itself. The remaining digits are called the national terminal number (NTN)
and are used to identify the specific DTE on the PSN. Figure 8-4 depicts the addressing format.