Atm Service Types - HP MSR Router Series Wan Access Configuration Manual

them. Meanwhile, continuous bit streams received from physical media are restored to cells, which
are then passed to the ATM layer.
ATM layer—Resides over the physical layer, and implements cell-based communication with its
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peer layer by invoking the services provided by the physical layer. It is independent of physical
media, implementation of the physical layer, and types of services being carried. AAL passes
48-byte payloads, which are called segmentation and reassembly protocol data units (SAR-PDUs)
to the ATM layer. The ATM layer encapsulates the 48-byte payloads in 5-byte headers, and passes
53-byte cells to the physical layer. Other functions of the ATM layer include VPI/VCI transmission,
cell multiplexing/demultiplexing, and generic flow control.
ATM Adaptation Layer—Provides interfaces between high-level protocols and the ATM Layer. It
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forwards information between the ATM layer and upper-layer protocols. Four types of AAL are
available: AAL1, AAL2, AAL3/4, and AAL5, each of which supports specific services provided in
an ATM network. HP uses AAL5 for data communication services.
ATM upper-layer protocols—Responsible for WAN interconnection, Layer 3 interconnection, and
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multiprotocol over ATM (such as IP, IPoE, PPP, and PPPoE).

ATM service types

ATM supports the following service types:
Constant Bit Rate (CBR).
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Unspecified Bit Rate (UBR).
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Variable Bit Rate-Real Time (VBR-RT).
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Variable Bit Rate-Non Real Time (VBR-NRT).
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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.
Typically, CBR is suitable for jitter-sensitive, real-time applications, such as audio and video.
VBR-RT
The VBR-RT service is provided for applications that have strict restrictions on delay and jitter, such as
audio and video.
A VBR-RT connection is described by the PCR, sustainable cell rate (SCR), and maximum burst size (MBS).
With the VBR-RT service, a station can send burst traffic at PCR with the maximum traffic size being MBS
without cell loss and the average cell rate being SCR.
VBR-NRT
The VBR-NRT service supports non-real-time applications with burst traffic. A VBR-NRT connection is
described by PCR, SCR, and MBS. The VBR-NRT 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 dropped first. The UBR service
is suitable for applications with low requirements for delay and bandwidth.
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