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ATM Cell Fields
The five-octet header contains the following fields:
GFC (Generic Flow Control)
VPI (Virtual Path Identifier)
VCI (Virtual Channel Identifier)
PT (Payload Type)
CLP (Cell Loss Priority)
HEC (Header Error Control)
The 4-bit GFC is used to control traffic between the user and ATM network, thus it is only seen in the UNI, not the NNI. It provides local functions only.
The VPI identifies a virtual path link between two nodes. The VPI allows a VP switch to route cells based on information held within the node’s switching table. The VCI identifies a VCL between two adjacent nodes. The VCI (along with the VPI) allows a VC switch to route cells based on information held within the node’s switching table.
The 3-bit PT indicates the nature of the payload information. It distinguishes between user data and management data.
Cells with their CLP set to 1 are deemed eligible for discard by the network in the event that congestion is encountered within the network. Thus it provides a method for prioritizing cells.
HEC is used to carry out a CRC of the header. The HEC does not report on the payload of the cell, only the header. If an error is found, the cell is discarded. Though this field is provided by the ATM layer, it is populated and used by the physical layer.
ATM Forum
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ITU-T and ATM Forum Correlation
An ATC (ATM Transfer Capability) is intended to represent a class of ATM connections that have homogeneous characteristics in terms of traffic pattern, QoS requirements and possible use of control mechanisms, making it suitable for a given type of resource allocation.
ATC is an ITU-T definition; the ATMF (ATM Forum) equivalent definition is termed an ATM Service Category. A comparison of the two definitions are shown.
Referring principally to the ITU-T definitions, the transfer capabilities are defined as follows and summarized below:
DBR (Deterministic Bit Rate)
SBR (Statistical Bit Rate)
ABR (Available Bit Rate)
ABT (ATM Block Transfer)
The DBR is based on a constant (maximum) bandwidth allocation. It is termed CBR (Constant Bit Rate) by ATMF.
The SBR is based on an average bandwidth allocation. This is termed VBR (Variable Bit Rate) by ATMF.
The ABR is based on dynamic bandwidth allocation, where the allocated resources vary with time depending on network availability.
The ABT is based on block or burst allocation, and is specific to ITU-T.
In addition, the ATMF has split the VBR category into rt-VBR (real-time Variable Bit Rate) and nrt-VBR (non-real-time Variable Bit Rate) and has introduced the ATM Service Category known as UBR (Unspecified Bit Rate), which is considered only by ATMF. Here, no explicit resource allocation is performed; neither bandwidth nor QoS objectives are specified.
Options for Layer 2 Virtual Circuits
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$70&HOOV ATM Transfer Capability Connection Traffic Descriptor Source Traffic Descriptor Peak Cell Rate (PCR) Sustainable Cell Rate (SCR) Minimum Cell Rate (MCR)
QoS Class
QoS Parameters (i.e. CLR, CTD, CDV)
TY2702/v3.1
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Traffic Contract
A traffic contract specifies the negotiated characteristics of the ATM layer connection at the UNI.
The network must only accept a new connection if the traffic contract can be honoured and must ensure that the user adheres to this contract during the connection by policing the connection traffic flow.
The traffic contract identifies an ATC (ATM Transfer Capability). The ATC is realized by a source traffic descriptor (e.g. peak cell rate, mean cell rate), associated CDV (Cell Delay Variation) tolerances and requested QoS class. All are declared by the user at connection set-up by means of signalling (SVC) or subscription (PVC).
Once the connection has been accepted, the value of the CAC (Connection Admission Control) and UPC/NPC (Usage Parameter and Network Parameter Control) parameters are set by the network at the UNI and the NNI respectively. CAC represents a set of actions taken by the network at call set-up in order to accept or reject an ATM connection. UPC/NPC represent a set of actions taken by the network to police ATM connection usage, e.g traffic volume and cell routing. In other words UPC/NPC enforce compliance of ATM connection usage with the traffic contract.
Typically provided by SDH
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OAM Hierarchical Levels
OAM functions are performed on five OAM hierarchical levels associated with the ATM and physical layers of the ATM reference model. These functions, identified by the ITU-T, result in five bidirectional information flows, referred to as OAM flows. Not all of these flows need to be present, as OAM functions of a missing level are performed at the next higher layer. The levels are as follows:
Virtual Channel Level (F5)
This extends between network elements performing VCI termination functions, such as VC switches and CEQ (Customer Equipment). This level encompasses one or more VCLs (Virtual Channel Links) and may span a complete VCC (Virtual Channel Connection). Note that a virtual channel link is transported over a VPC and that a VCC is a concatenation of VCLs. Payload Type indicates the presence of OAM cells at this level.
Virtual Path Level (F4)
This extends between network elements performing VPI termination functions, such as VP cross-connects and CEQ. This level encompasses one or more VPLs (Virtual Path Links) and may span a complete VPC (Virtual Path Connection). A reserved VCI value is used to indicate presence of OAM cells.
Transmission Path Level (F3)
This extends between network elements assembling/disassembling the payload of a transmission system and associating it with its OAM functions. Cell delineation and HEC functions are required at each endpoint. The transmission path is connected through one or more digital sections.
Digital Section Level (F2)
This extends between section endpoints and comprises a maintenance entity.
Regenerator Section Level (F1)
A regenerator section is a portion of a digital section and as such is a maintenance sub-entity (such as a repeater).
It should be borne in mind that the Physical Layer within the ATM model may be realized by technologies such as SDH or a cell-based technology. SDH will exchange its OAM F1 to F3 information within its own synchronous overhead channels, while a cell-based technology will need to identify cells specifically marked as OAM.
Options for Layer 2 Virtual Circuits
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VC-4 Path Overhead
H4 byte used as a pointer to indicate the first occurrence of an ATM cell header in the same row as the H4 byte.
ATM Cell
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Mapping ATM Cells into SDH (Cell Delineation)
The mapping of ATM cells is performed by aligning the byte structure of every cell with the byte structure of the VC used.
According to the ITU-T, there are several ways in which ATM cells can be mapped into an SDH frame, including VC-4 use.
Considering the VC-4 option, the diagram shows how ATM cells containing 53 octets could be mapped into the Virtual Container payload.
The VC-4 is a standard 261-column payload, with the first column reserved for the VC-4 POH (Path Overhead); the remaining 260 columns are designated as the payload. The ATM cells are transported as a stream in the payload area, where each cell has the characteristic 53 octets, i.e. 5-octet header and 48-octet data.
The ATM cell header can be thought of as the cell’s FAW (Frame Alignment Word), which facilitates cell delineation.
The H4 byte in the VC-4 POH is identified in G.707 as being ‘a generalized position indicator for payloads, which is payload dependent’. Thus the H4 byte is used as the pointer, indicating the first occurrence of an ATM cell header, which is found in the same row as the H4 byte.
The 48 bytes of the ATM cell for data are scrambled prior to transmission to prevent the ATM signal from replicating the SDH frame sequence. The 5-byte header is not scrambled, however, as this would affect ATM cell delineation.
The H4 byte indicates the start of an ATM cell to the scrambler/descrambler.