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ATM Protocols and Specifications 119
ATM Virtual Circuits
ATM virtual circuits operate in one of the following service categories:
·
Constant Bit Rate (CBR)--CBR supports real-time applications that need a fixed
amount of bandwidth. CBR supports applications that allow only a limited number of
variations in delay. Voice and constant-bit-rate video are examples of two applications that
require CBR.
·
Variable Bit Rate (VBR)
-- Real-time Variable Bit Rate (rt-VBR)--rt-VBR supports time-sensitive
applications that have constrained delay and delay variation requirements. Voice
and variable-bit video are examples of two applications that might require rt-VBR.
-- Nonreal-time Variable Bit Rate (nrt-VBR)--nrt-VBR supports applications that
have no constraints on delay and delay variations. Packet data transfers, terminal
sessions, and file transfers fit the category of traffic that could utilize nrt-VBR.
·
Unspecified Bit Rate (UBR)--The UBR service can be classified as a "best shot" service.
It does not require constraints on delay and delay variations. It provides no specific quality
of service. It provides no guarantees. Any traffic that does not require performance
guarantees can be categorized under UBR.
·
Available Bit Rate (ABR)--The ABR service distributes bandwidth among users. The
objective of ABR is to make unused bandwidth available to end users in a fair and timely
manner. ABR does not provide guarantees for delay variation. So real-time applications,
such as voice and video, would not work well under ABR. Nonsensitive data traffic is the
best fit for the ABR service.
ATM Reference Model and the OSI Model
The three layers of the ATM Reference Model are the ATM physical layer, the ATM layer, and the
ATM adaptation layers (AAL). These three layers correlate to the physical and data-link layers of
the OSI model. The ATM physical layer closely relates to the physical layer (Layer 1) in the OSI
model. The ATM physical layer is responsible for tracking ATM cell boundaries, converting bits
interface ATM3/0.1 multipoint
ip address 1.1.1.1 255.255.255.0
atm esi-address 777777777777.00
atm arp-server nsap 47.009181000000001007386901.555555555555.00
RtrA#s
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Protocol Address Age (min) Hardware Addr Type Interface
Internet 1.1.1.2 0 0 / 55 ATM ATM3/0.1
Internet 1.1.1.1 0 0 / 54 ATM ATM3/0.1
Example 4-2
IP over ATM using SVC (Continued)
interface ATM3/0
87200333.book Page 119 Wednesday, August 22, 2001 2:18 PM