Figure 9-7 Basic LEC operational flow.
The READY_IND and READY_QUERY messages provide end-to-end, confirmation to both sides. When the called part receives the CONNECT_ACK message, it starts a timer for the supervision of
READY_IND. If the READY_IND is received before the timer expires, the called party stops the timer and goes into ready state to exchange data with the calling party. However, if the timer expires before
receiving the READY_IND, then the called party sends a READY_QUERY message to request
II. Classical IP and ARP over ATM
The ATM LAN Emulation which we discussed previously is designed to provide Layer 2 switching and bridging services to interconnect existing legacy LAN systems such as Ethernet and Token Ring-based
LANs to be interconnected through the high-speed ATM campus backbone network. The LANE emulates the MAC layer of the LAN protocol stack and is transparent to network Layer 3 level protocols such as
IP and IPX. In addition to Layer 2 emulation of Ethernet and Token Ring in the LAN environment Classical IP and ARP over the ATM concept has been described in RCF 1577 by the IETF organization in
order to extend the use of ATM in the IP-based LAN environment. This protocol is also referred to as Classical IP over ATM CLIP.
The key objectives of CLIP is to take advantage of higher wire speed offered by ATM technology while protecting investment in today’s IP based applications and to help eventual migration of legacy IP-based
legacy networks such as LANs to ATM-based networks.
The CLIP retains the IP characteristic at the Layer 3 network level for the existing applications while switching Layer 1 and 2 from Ethernet Token Ring to ATM. In other words, CLIP models an IP network
over an ATM network so that it is possible to run existing IP-based applications without modifications over an ATM network.
As shown in Figure 9-9, CLIP creates a Logical IP Subnetwork LIS over an ATM network. It is possible to create many such LIS over the same ATM network. A typical LIS is composed of many members hosts
and routers and a single ATMARP server. In a typical IP-based LAN network, the subnetwork boundary is defined by the physical location. On the other hand, the LIS provides a logical boundary for the IP
subnetwork; thus the location of its members is not a limiting factor.
Figure 9-8 Call setup procedure.
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