What do you know about the ATM hierarchy model?

ATM Physical Layer

It is divided into physical media sub-layer and transmission aggregation sub-layer.

Physical Media sub-layer:

1) synchronous transmission and receipt by sending and receiving continuous bit streams with relevant scheduled information.

2) specify physical media for all physical media, including cable and connector types.
ATM interface:

The ATM physical layer is at the bottom of the ATM Protocol Reference Model. It involves specific transmission media, but its function does not depend on the transmission mechanism and speed used by the transmission media. The ATM physical layer transmits valid cells and corresponding scheduled signals between the upper layer and the transmission media.

1) single-channel ATM interface module that supports SONET and SDH to carry ATM services.

2) support for interface modules that carry ATM services on lines E1 and T1, and implement the IMAInverse Multiplexing for ATM feature. What is IMA?

1) The purpose of IMA is to reuse an ATM Cell to multiple physical links, and then restore the original cell stream from these physical links at the remote end. The multiplexing of ATM Cell streams is based on a single cell on multiple physical links. It can meet the high rate, maintain the order of cells, and facilitate ATM management.

2) IMA group: consists of one or more links to provide a higher bandwidth Logical Link. The bandwidth of this logical link is approximately the sum of the bandwidth of all member links.

3) in the sending direction, IMA receives ATM cells from the ATM layer and places the cells in a circular order on each member link of the IMA group. After arriving at the receiving end, these cells are combined into original cell streams and passed to the ATM layer. The process of direction multiplexing is transparent to the ATM layer.

4) The IMA interface periodically sends some special cells. The information contained in these cells is used by the receiver of the IMA virtual link to reconstruct the ATM Cell stream. Before recreating an ATM Cell stream, the receiver must adjust the link differential latency and eliminate the cell delay jitter introduced by the control cell. These Cells are called the IMA Control Protocol cell ICPIMA Control Protocol Cells) and are used to define IMA frames.

5) when sending, the sender must align the IMA frames on all links. In this way, the receiver can test the differential latency between links based on the arrival time of the IMA frames on different links and adjust the latency accordingly. The Cells are continuously sent at the sending end. If there are no ATM-layer Cells between the ICPs of an ima frame, the IMA sender adds the filled Cells to Filler Cells) to keep the physical layer continuous cell streams, which are discarded at the receiving end. Transmit aggregation Sub-layer TCTransmission Convergence Sub-layer ):

1) Generate and authenticate the header Error Control HECHeader Error Control) sequence.

2) Transfer frame adaptation

3) Generate and restore transmission frames

4) Check the cell header at the top of the message management module of the physical layer and perform 1-Bit Error Correction through the 8-bit HEC. If there are multiple errors, discard them.

5) Division of cells. It usually starts to process and exchange cells after observing 7 valid 53-byte cells.

6) The TC sub-layer encapsulates ATM cells into the frame structure that can be received by a specific physical layer application.

ATM-ATM layer

Basic functions of the ATM layer:

1) the ATM layer is located on the physical layer and is responsible for switching and multiplexing of cells through the ATM network.

2) The net load of 48 bytes is entered into the ATM layer. The net load of these 48 bytes is called the Data Unit SAR-PDUSegmentation And Reassembly Data Unit of the segmentation And restructuring protocols)

3) the ATM layer adds a 5-byte cell header and verifies the new source, and then transmits the 53-byte cell to the physical layer for transmission.

4) It also transmits the virtual circuit number VPI/VCI to multiplexing/reuse cells, and generally controls the Flow of GFCGeneric)

Three virtual links in ATM: each virtual link is created to obtain a forwarding table. When an ATM switch receives a cell through the VPI/VCI link, it first checks whether the VPI/VCI configured locally matches. If yes, it determines the output port of the Connection Based on the forwarding table, determine the VPI/VCI value corresponding to the output link, and then use the VPI/VCI to forward the cells on this virtual link.

Permanent virtual circuit PVC: PVC is statically configured by the Administrator and cannot be released once connected. Suitable for always using connections with advanced requirements.

Switch virtual circuit SVCswitched virtual circuit): established by means of signaling, you can establish connections and release through command. When receiving a creation request from another node, each node needs to send a connection response to the node if it meets the configuration requirements. After the connection is established, the node sends a connection creation request to the next target.

Soft VCSoft virtual Circuit ):

This type means that the ATM network is based on SVC, and the peripheral devices access the ATM network through PVC. The creation process is like SVC, but the PVC must be manually configured between the peripheral device and the entrance exit of the ATM switch. The advantage of this method is that PVC is connected to the user to facilitate user management, and SVC can ensure the reasonable use of the link.

VPI/VCI:

1) In an ATM, a pair of VPI/VCI combinations are used to identify a logical connection. The value of VCPI/VCI is meaningful only on the local interface.

2) VPI is used to identify the virtual channel number, and VCI is used to identify the virtual circuit number in the virtual channel. The combination of the two constitutes the connection identifier. It is possible that a VPI contains one or more VCI.

3) in ATM communication, the cell is entering the switch with the VPI/vci id, and then the Switch replaces the new VPI/VCI at the exit according to the route table to complete VP switching or VC switching. There is a corresponding form in the ATM switch, which is similar to the MAC address table. The corresponding VPI/VCI form of the ATM is contained in the cell end-VPI/VCI value-output cell port-need to replace VPI? VCI value ).

ATM Cell Header Format: ATM has two types of Cell Header formats: User network interface UNIuser to network interface) and network interface NNInetwork to network interface. The UNI cell header is used for communication between an ATM terminal and an ATM switching node in an ATM private network. The NNI header is used for communication between ATM switching nodes.

ATM Adaptation Layer

AAL layer: AAL is the interface between the high-level protocol and the ATM layer. It is responsible for transferring information between the ATM layer and the high-level protocol. Five types correspond to five types of businesses, among which AAL5 supports data communication services. The AAL layer is located above the ATM layer and corresponds to the data link layer of the OSI reference model.

1) Aggregation sub-layer CSconvergence sub-layer)

It includes the SSCSservice special convergence sub-layer), which is responsible for associating with various business features. The common part of the aggregation sub-layer CPCScommon part convergence sub-layer ), by adding variable-length characters before and after a frame to form a frame, the error is detected.

2) Grouping and restructuring sub-layers:

Grouping And restructuring sub-layer SARSegmentation And reassembly) is used to divide aggregate frames into 48 bytes of net load when the peripheral device sends data externally. When the peripheral device receives data, reassemble the 48-byte net load into a frame.

AAL type:

AAL1: used for CBRconstant bit rate) type, send data at a fixed interval.

AAL: It can transmit compressed speech and practice common channel signaling in ISDN.

A_3/4: the first method is to implement the cell delay technology, which specifies connection-oriented and connectionless data transmission.

AAL5: ability to process connection-oriented and connectionless data.

Top layer of ATM:

The ATM high-level layer accepts user data, arranges it into data packets, and then submits it to the ATM Adaptation Layer for processing.