Characteristics of Multichannel Switch * Multichannel Switching The switch has the ability to group channels arbitrarily and support super-rate switching of any bandwidth. * In-Order Cell Sequencing Guarantee in-order cell sequencing without the use of resequencing mechanism * Simplicity Multichannel switching is facilitated without increasing the complexity of the switch * High Performance Internal fabric is nonblocking and bufferless

Characteristics of Multichannel Switch * Multirate Switching The Switch can arbitrarily assign the bandwidth to match different rates of the ATM network. * Multicast The Switch supports multipoint connections which is considered an essential function. * Fairness Ensure fairness among different I-O pairs by share buffering(recirculation paths). * Multiple Performance Requirement Supports two classes of traffic by channel grouping.

Group Switching (a) (b) (c) Suppose cells had been processed by the input processor (a) If the two cells are belong to the same group, 0 upward, 1 downward. If the two cells are belong to different group, 1 upward, 0 downward. (b) If the two cells belongs to group X 1 upward, 0 downward. Else, 0 upward, 1 downward (c) The cell belong to group X goes upward, and the other goes downward

ATM Multichannel Switch Multi-channel Switch Contains Following Components: * Input Processor---Detect input cell information, sorting, buffering,attributing... * Multi-Cast Stage---Maintain the multicast function of this switch. Contains (R)NBGN/AE/(R)NBGN/CN/Feedback Ckt * Routing Stage---Maintain the basic or advanced routingfunction of this switch. Contains RNBGN/NBGN/BR/Feedback Ckt * Buffering & Traffic Control---Provide share buffer and some other logics for controlling the entire switch.

Running Adder The running adder can automaticaly generate the packet(cell) output channel address according to the group it belongs to. The IP i (input processor) assign 1 to its output line if the targetd channel group is X, and the other is 0. Then, after the operation of summing elements, the AG i (address generator) generate the accurate output channel ID by doing the following operation: If the packet target group X, the output equals SUM i-1. If the packet target group X, the output equals N+SUM i-i-1. (N=Total channel no.)

Nonblocking Binary Group Network Input Channel Number Channel Group Output Channel Destination Number The NBGN (Nonblocking Binary Group Network) is made of the combination of running adder and flip network. * Note that NBGN can only separate cells into two groups. If more than two groups are desired, we must use casacde NBGN or Recursive NBGN (RNBGN).

Recursive NBGN * The RNBGN is the combination of NBGN and the buffer-feedback ckt. The switch with 2^r channel need r+1 recursive channels. * By the control of buffer-feedback recursion ckt, this switch can provide the variable channel grouping ability. * When the size of the switch increase, the use of RNBGN can effectively reduce the logic complexity rather than adding more and more NBGN. * The feedback mechanism also provide Fairness and cell output ordering.

Physical Structure of Multicast Stage * Multicast stage is made of PP (Port Processor), AE (Address Encoder), CN (Copy Network), NBGN, and NBGN. * The stage need memory device to work properly.

Details of Port Processor and NBGN * The PP i is used to: 1. Determine the VPI/VCI and the required number of copies from the input cell. 2. On the other hand, it has to transform the cell list into monotonic form for further operation. 3. Augment the cell by three additional field: Broadcast Connection ID, Cardinality, and Sum. These fields are dedscribe follows. Broadcast Connection ID is used to determine the required information for all functions of switching. Cardinality records the required number of copies. Sum identifies the cell as active one (assign 1) or just an empty one (assign 0). The Broadcast Connection ID is maintained throughout the entire switch. * The first NBGN is used to: 1. This NBGN has (n+Rm) inputs, Rm is the number of recirculation. 2. According to the SUM field, this NBGN perform binary grouping, active cells are concentrated at the upper group, while the empty ones are at the lower group. 3. The relative ordering is not changed, because of the characteristic of NBGN.

Details of Address Encoder and NBGN * The Address Encoder functions as follows: 1. Determine the input cells are requested to multicast or not. 2. Determine the multicast interval of the requested cells. 3. Deciding the multicast request is going to be done in the Copy Network or not. 4. The SUM field used by the first NBGN will be assign a new value here. By the running adders in the AE, these cells are calcuted in Top-Down order to determine whether the output channel no. is exceed n or not. If exceeded, the cell is remarked 1 in the SUM field, other wise remark 0 instead. * The second NBGN functions as follows: 1. After processed by AE goes next into the NBGN. These cells are being passed to CN or the recursive loopback. If the SUM field is 0 after AE, the cell is passed to CN, otherwise, the cell loopback to the starting of multicast stage, waiting for next time slot to re-enter again. 2. The re-entered cells have higher priority than the new-entered cells because of the Top -Down process order of AE. 3. In order to keep the fairness, the feedback twisted. By the way, it also helpful to keep the relative ordering of the recursive cells.

Multiple Performance Requirement To support two service classes, cells are separate into three groups by the first RNBGN –group 0: active class 0 cells (delay sensitive) –group 1: active class 1 cells –group 2: empty cells Cells are separate into four groups by the second RNBGN –group 0: cells to be passed to the CN –group 1: class 0 cells to be recirculated –group 2: class 1 cells to be recirculated –group 3: empty cells

Details of Copy Network and BGT * The functions of the Copy Network: 1. The basic function of Copy Network is to generate two or more identical cells. 2. The copies made by the CN must be in an interval of several continuous output channels. In other words, the output channels of copies can’t be aribitary assigned. If not, the CN may encounter contention problem. * The functions of the BGT (Broadcast and Group Translator): 1. The function of BGT is used to assign a new group ID to the copies made by the CN. Then the cells can be switched to proper channel groups in the routing stage. 2. Assign of the new channel group is based on the Broadcast Connection ID generated by PP. This field is not changed after the replication of CN. 3. This part of work is done in table which is stored in the memory device of the switch.

Summary of Multicast Stage * Fairness is achieved by virtue of the recirculation path. Cells are delayed in one time slot, and they gain higher priority in next one. * Multichannel & Multicast functions are obtained. * In-order cell sequencing is garenteed by mark the cell index of time. This index is maintained throughout the entire switch to keep the cell relative ordering. * No matter are duplicated by the Copy Network or not, the BGT will give them a group ID. Mulit-rate switching can be done when the routing stage grouping the channels according to it. * Multi-performance requirement is discuss in the next page.

The Routing Stage * The routing stage is made of RNBGN, NBGN, and BR (Banyan Router). * The over all function of this unit is 1. The cells enter the RNBGN and targeted for various channel groups. After they completing the output channels, the excess cells and empty ones were grouped at the bottom of ouput. ( They are listed according to a Gray code sequence of channel group address.) 2. Next, the listed cells enter the NBGN, the excess and empty ones are marked for recirculation. The effective ones ges next into the BR. ( Now, the cells entering the BR are in Monotonic Ascending Order.) 3. At last, the BR route the cells to the right place where they should be.

Simulation Results of the switch In Multicast Stage, we are interested in following parameters: the normalized load ( p ) the cell lost probability the output channel group N In multicast stage: 1. The number of the recirculation channels (Rm) is the most efficientive parameter. 2. If Rm is large enough (say Rm>64), the cell lost proability can as low as 10e-6 and the p is as high as If the Rm is small (say Rm<8), the p droped to 0.6 as the cell lost proability maintained as 10e-6. The amount of the recursive path is very improtant when the cell lost proability & p is assured in demands. In Routing Stage: 1. Under N output channel groups, the Rr here is also efficientive as the Rm in the multicast stage. 2. If N is reduced, i.e., the channels are grouped, the required number of Rr is also reduced under the same quality of cell lost proability and p.

Conclusion 1. Because the optical transmission speed increasing in recent years while the electrical processing speed remains in the MHz degree, multichannel grouping becomes more and more important. 2. When the number of users increased, the multicast switching is achieved without increasing the complexity of the entire switch. Mearly change the output grouping numbers, the switch can deal with this easily. 3. The switch garentee the cell ordering, switching delay, and high throughput performance. These connection-oriented characterisitcs are very important for high speed ATM network. 4. The switch can act according to the different characteristics of the cells. In the ATM network, there are several kinds of cell like “real time image”, “non-real time image”, “file transfer”.....and so on. Switching them depend on different proirities is a real way to deal with high speed ATM network. 5. The multicast, multirate, and multichannel switching functions provides the ATM network with more variable useage. Especially, time require for the multicast function can be reduced as compared to the single channel switch.