System information

Chapter 1. Planning interoperability and migration 3
1.1.2 Cutover migration
The term cutover migration describes the case where all or almost all existing
equipment is taken out at once, or over a brief period of time, and replaced
with new equipment. See Figure 2. This method is sometimes referred to as
forklift migration. A cutover migration may be staged, taking a piece of the
network at a time.
Figure 2. Cutover migration; all boxes are replaced
Many Ethernet users have gone through migration from 10Base2 to
10Base-T, and again from 10Base-T to 100Base-T. However, interoperability
between the network devices is straightforward, leaving the customer to
migrate at his own pace.
1.1.3 Overlaying network
It may be desirable to operate two networks simultaneously - the new network
together with the existing, old network. See Figure 3. This is often the case if
the new network differs considerably from the existing network, or if
applications or existing equipment depend on the old network and cannot be
converted easily or economically to connect to the new network.
For example, devices may depend on specific application code. This was
often the case with older printers using a BSC or asynchronous protocol.
Altering the old applications may not be feasible, and the only option is to
keep the old network in parallel with the new until the time when the old
applications are phased out and the old printers may be replaced with new
ones that can connect to the new network.
Another instance where an overlay migration approach may prove desirable is
in the case of a circuit-switched or cell-switched network providing
connectivity between private telephone exchanges (PBXs). To extend such a
network with the capability to support current frame-based and Ethernet
technologies such as Voice over IP (VoIP), an overlay network may be
deployed.
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