Handbook of Local Area Networks, 1998 Edition:Advanced LAN Interconnectivity Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Cell-Based ATM Inverse Multiplexing The very nature of public carrier networks demands a stable, standardized mechanism for transport. The ATM Forum’s evolving standard for inverse multiplexing for ATM (IMA) will play a crucial role in the acceptance and implementation of ATM inverse multiplexing. IMA is a new user-to-network interface (UNI) being specified by the ATM Forum. The physical interface (PHY) committee of the ATM Forum defines standard mappings of ATM cells onto existing physical layer media; UNIs and PHYs are usually inseparable. In this case, the IMA UNI rides on top of existing T1 or E1 ATM PHY, performing inverse multiplexing via a cell-based control protocol, which is a major departure from the normal PHY definition. (See Exhibit 4-5-1.) Exhibit 4-5-1.  Cell-Based Inverse Multiplexing for ATM (IMA) IMA is expected to be widely accepted in both the user and equipment vendor communities. Within the carriers’ networks, IMA can be used instead of T1 ATM for point-to-point trunking between frame relay/ATM switches, greatly improving bandwidth without upgrading to DS3 or OC-3c. On the customer premises, the IMA specification promises vendor interoperability, giving users maximum flexibility in the equipment selection process. However, the definition of a specification is a long and painstaking process. The IMA specification is still being formulated. Once the standard is defined, the normal maturation process for a new technology will set in. Early adopters of IMA will have to accept a number of adjustments as new hardware, software, and protocols are rolled out. Those customers for whom the risk in cost and reliability of deploying a new technology is too high will have to wait until the dust settles. Multivendor, interoperable NxT1 ATM may take a while to become a reality. There are users who have network requirements that cannot wait either for stabilized IMA solutions or for high-speed ATM (i.e., DS3 and OC-3c) to become available or more cost-effective in the public network. They need solutions to their networking challenges sooner rather than later. Until the new IMA specification is finalized, they see themselves as having no stable means of interconnecting their ATM networks via inverse multiplexing. NXT1 ATM INVERSE MULTIPLEXING—CLEAR CHANNEL ATM An alternative to IMA allows users to take full advantage of inverse multiplexing’s NxT1 (i.e., multiple independent T1 connections) bandwidth for their ATM networks. ATM cells can be inverse multiplexed bit-by-bit, meaning that ATM traffic can be transported transparently over traditional fractional T3/E3 and T1/E1 circuit facilities. This technique is called clear channel ATM (CCA). Unlike IMA, clear channel ATM is a specification already approved by the ATM Forum. Clear channel ATM is another way of referring to the ATM Forum’s transmission convergence sublayer (cell-based TC) specification, also known as ATM over HSSI (high speed serial interface). Cell-based TC specifies a standard format for transmitting cells over any clear-channel bitstream interface. In other words, ATM transports at the bit level, instead of at the cell level. Connectors, clocking, modem control, and status are not addressed by cell-based TC, but are defined by other standards. At the physical layer, V.35, HSSI, or any other type of connector that could accept the ATM bitstream can be used. What is defined by the cell-based TC specification is the bit order and how “start-of-cell” is determined; then, cells are simply placed bit-by-bit onto the transporting technology (see Exhibit 4-5-2). Exhibit 4-5-2.  Clear Channel (Bit-by-Bit) ATM Inverse Multiplexing Clear channel ATM means that an ATM bitstream can be carried over any WAN data circuit, including inverse multiplexed data circuits. Having an HSSI port available for the traffic flow is not necessary; a V.35 port will do, and it is even possible to use an ATM DS3 or OC-3c UNI directly from an ATM switch. By using cell-based TC in conjunction with a readily available UNI (e.g., on an inverse multiplexer with a direct UNI connection to an ATM switch), the difficulties and long maturation created by the process of defining yet another UNI are eliminated. It is usually time for a new user-to-network interface to migrate into customer premises equipment, appearing most often first in ATM switches, and then in other devices such as routers and network interface cards (NICs). There are no guarantees that a vendor will even choose to support it. In the ATM switch, a new UNI means that the switch must handle a new interface, address the conversion from one rate to another, and buffer cell traffic moving between old and new UNIs. Despite the UNI deployment taking place in the ATM switch, actual switching is not required in this instance. Because ATM switches tend to be expensive, the addition of a new UNI can mean interface upgrades and reconfigurations. Alternatives to Adding a New UNI The functions of a cell-based TC imux need not be deployed in a switch because only cell buffering and rate conversion are required. By leaving the switching functions where they belong—in the ATM switch—a more inexpensive and lower-risk solution can be deployed. In the case of clear channel ATM inverse multiplexing, a readily available ATM interface such as a DS3 or OC-3c becomes an ATM “DTE port,” which then transports the data stream via multiple T1 or E1 circuits. By converting the traffic from DS3 or OC3 to lower NxT1 rates, and by providing the necessary buffers, the need for a “new” UNI is eliminated. Clear channel ATM lets users keep their options open with regard to WAN access and ATM transport. They can use the DS3 and/or OC-3c UNI interfaces they have available on their backbone ATM switches without having to deploy a new UNI. Users do not even need to use an ATM switch. With a clear channel ATM inverse multiplexer, a single device such as a high-speed workstation or server equipped with an OC-3c or DS3 ATM NIC can connect directly into the CCA imux. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.