ATM hierarchy model How much do you know?

Physical layer of ATM


It is divided into physical media sublayer and transfer aggregation sublayer.


Physical Media child layer:


(1) The synchronization of sending and receiving is realized by sending and receiving a continuous bit stream with relevant timing information.


(2) Specify the physical media for all physical media, including the types of cables and connectors.


ATM interface:


The ATM physical layer is at the bottom of the ATM protocol reference Model. The specific transport media is involved, but its function does not depend on the transmission mechanism and rate used by the transport media. The ATM physical layer mainly transmits the effective cell and the corresponding timing signal between the high level and the transmission medium.


(1) A single channel ATM interface module which supports SONET and SDH to carry ATM services.


(2) Support the interface module of E1 and T1 line to carry ATM service, realize the characteristic of IMA (inverse multiplexing for ATM). What is IMA.


(1) The purpose of IMA exists is to reuse an ATM signaling direction to multiple physical links, and then recover the original source stream from these physical links. The multiplexing of ATM signal streams is based on a single element on multiple physical links. It can satisfy the higher rate, keep the order of the letter element, and benefit the ATM management.


(2) IMA Group: A logical link consisting of one or more links to provide higher bandwidth. The bandwidth of this logical link is approximately equal to the sum of the bandwidth of all member links.


(3) in the direction of transmission, the IMA receives ATM elements from the ATM layer and places the order of the letters into the member chains of the IMA group in a circular manner, and when the receiver is reached, the elements are then grouped back into the original stream and passed to the ATM layer. The process of directional multiplexing is transparent to the ATM layer.


(4) The IMA interface periodically sends special elements that contain information that is used by the receiver end of the IMA virtual link to reconstruct the ATM stream. Before the receiver can reconstruct the ATM signal stream, it is necessary to adjust the link difference delay and eliminate the signal delay jitter introduced by the control signal. This type of letter, called the IMA Control Protocol, is a Protocol Cells, which defines the IMA frame.


(5) The sending end must align the IMA frame on all links when it is sent. In this way, the receiving end can test the difference delay between the links according to the arrival time of the IMA frames on the different links, and adjust them accordingly. The letter is sent continuously at the sender end, if a letter with no ATM layer can be sent between the ICP elements of an IMA frame, the IMA sender maintains a continuous stream of filler on the physical layer by adding a filled letter (Cells), which is discarded at the receiving end. Transport Convergence Sublayer TC (Transmission convergence Sub-layer):


(1) Generate and authenticate the head error control of HEC (header error controls) sequence.


(2) Adapter transmission frame


(3) generating and resuming transmission frames


(4) at the top of the physical layer PMD, look at the head of the letter, through 8 HEC to do 1-bit error correction, if there are many errors will be discarded


(5) The delimitation of the letter element, which usually observes 7 valid 53-byte elements before the processing and exchange of the signal is initiated.


(6) The TC sub layer encapsulates the ATM signal into a frame structure that can be received by a particular physical layer application.


ATM Layer of ATM


Basic functions of ATM layer:


(1) The ATM layer is located on the physical layer and is responsible for the exchange and multiplexing of the signal through ATM network.


(2) To enter the ATM layer is 48 bytes of net load, the 48-byte net load is called the segmented and Reorganized Protocol Data unit SAR-PDU (segmentation and reassembly)


(3) The ATM Layer plus 5 bytes of the letter header and the new source checksum to the physical layer 53 bytes of the letter, for transmission.


(4) It also transmits the virtual circuit number Vpi/vci, carries on multiplexing/divide to the signal element, the general flow controls GFC (Generic flows control)


ATM Three virtual Links: The goal of each virtual link is to get a forwarding post. When the ATM switch receives a letter via the Vpi/vci link, it first checks that the local set Vpi/vci is matched, and if so, the output port of the connection is determined according to the forwarding, and the value of the Vpi/vci corresponding to the output link is determined, and then the signal is used for this Vpi/vci forward on this virtual link.


The permanent virtual circuit PVC:PVC is statically configured by the administrator and cannot be released once the connection is connected. It is appropriate to always use a connection with advanced requirements.


Switched Virtual Circuit SVC (switched virtual Circuit): Established by signaling, you can set up connection and release by command mode. Each node receives a build request from another node and, if satisfied with the configuration requirements, sends the connection response information to the node, and waits for the connection to be established before sending a connection request to the next target.


Soft VC (Soft virtual circuit):


This type refers to an ATM network that is based on SVC, while peripheral devices are connected to the ATM network via PVC. It is established as an SVC, but the peripherals and ATM switch entrances and exits must be manually configured with PVC. The advantage of this method is that the PVC is connected to the user to facilitate the management of the user, and SVC can ensure the reasonable use of the link.


Vpi/vci:


(1) in ATM, a combination of a pair of Vpi/vci is used to identify a logical connection. The value of the Vcpi/vci is only meaningful on the local interface.


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


(3) In ATM communications, the letter is in the Vpi/vci logo to enter the switch, and then the switch according to the routing table, at the exit to replace the new Vpi/vci, the completion of VP Exchange or VC Exchange. In an ATM switch, there is a corresponding form, similar to the MAC address table. ATM Vpi/vci corresponds to the form containing (input-vpi/vci end value-out of the letter port-need to replace VPI? VCI value).


ATM Header format: ATM has user network interface Uni (users to network interface) and network network interface Nni (network to network interface) two types of letter header format. The UNI header is used for communication between ATM terminals and ATM switching nodes in ATM private networks. The NNI header is used for communication between ATM exchange nodes.


The adaptation layer of ATM


AAL hierarchy: AAL is the interface between the high-level protocol and the ATM layer, which is responsible for transferring information between the ATM layer and the high-level Protocol. There are 5 types corresponding to 5 kinds of business, of which AAL5 to support the data communication business. The AAL layer is above the ATM layer and corresponds to the data link layer of the OSI Reference Model.


(1) Aggregation Sublayer CS (Convergence Sub-layer)


It contains the business-specific aggregation sublayer SSCS (Service special Convergence Sub-layer), which is responsible for the various business characteristics associated with the common part of the aggregation sublayer cpcs (Common parts convergence sub-layer) , error detection is done by adding variable-length fill characters to the frame before and after the frame.


(2) Grouping and reorganizing the child layers:


The role of packet and recombination sub-layer SAR (segmentation and reassembly) is to divide the aggregated frames into equal 48-byte net loads when the peripheral devices send out data, and reassemble the 48-byte net load into frames when the peripheral device accepts the data.


AAL type:


AAL1: for CBR (constant bit rate) type, sending data at fixed intervals.


AAL2: Can transmit compressed voice, can practice universal Channel signaling in ISDN.


AAL3/4: The first attempt to implement the signaling delay technology, which provides for connection-oriented and connectionless data transfer.


AAL5: Ability to handle connection-oriented and connectionless data.


Top of ATM:


The ATM high-level accepts the user data, rows it into packets, and then gives it to the ATM adaptation layer for processing.