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introduced. The VE interface has Ethernet characteristics and can be dynamically created. The
following is the protocol stack adopted by the VE interface:
• ATM PVC (the bottom layer)
• Ethernet (the link layer)
• Network layer and other upper layers (the same as those for common Ethernet interfaces)
ATM service types
ATM supports four service types: constant bit rate (CBR), unspecified bit rate (UBR), Real-Time
Variable Bit Rate (rt-VBR) and Non-Real-Time Variable Bit Rate (nrt-VBR). They are used for the
QoS purpose.
CBR
CBR provides ensured, constant bandwidth. The bandwidth assigned to the CBR service is decided
by the Peak Cell Rate (PCR). With CBR service, a source station can send ATM cells at PCR
constantly with assured QoS.
Usually, CBR is suitable for jitter-sensitive, real-time applications, such as audio and video.
rt-VBR
The rt-VBR service is provided for applications that have strict restrictions on delay and jitter, such as
audio and video.
An rt-VBR connection is described by the PCR, sustainable cell rate (SCR) and maximum burst size
(MBS). A station using the rt-VBR service is allowed to send burst traffic at PCR with the maximum
traffic size being MBS without packet loss and the average cell rate being SCR.
nrt-VBR
The nrt-VBR service supports non-real-time applications with burst traffic. An nrt-VBR connection is
described by PCR, SCR, and MBS. The nrt-VBR service is suitable for applications sensitive to cell
loss but not to delay.
UBR
The UBR service does not make any service quality commitment, guaranteeing neither cell loss ratio
(CLR) nor cell delay. When traffic congestion occurs, cells of the UBR service are always dropped
first. The UBR service is suitable for applications with low requirements for delay and bandwidth.
InARP
On an ATM PVC connection, you can use the Inverse Address Resolution Protocol (InARP) to obtain
the IP address of the remote end connected to the PVC. In this way, you do not need to manually
configure the IP address of the remote end. Figure 66
shows how InARP works.
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