Deploying ATM Residential Broadband Networks Mark Laubach, Chief Architect Com21, Inc., 1991 Landings Drive, Mtn View, CA 94043 laubachQcom2 1 .com http://www.com2 1 .cornl ABSTRACT 1964 [8]. Since then, the size of packets has been debated, as well as variable vs. fixed size. ATM residential broadband technology is Packets are transmitted over any media these rapidly entering commonplace discussion. The days however, the next economic and technical capabilities provided by ATM network inter$ace frontier is moving packets over Cable TV devices promise data bandwidth speeds far in (CATV) networks. The driving push is to excess of those provided by traditional twisted deliver IP datagrams over cable TV networks. pair public telephone networks. Cable TV oper- There are several datalink methodologies for ators and Regional Bell Operating Companies delivering IP datagrams via cable modems, This (RBOCs), e.g. PacBell, are preparing for this paper overviews the notion of sending small, integrated broadband future by installing or fixed sized packets over the CATV plant. These rebuilding existing all-coaxial cable plants into small packets are 53-octet Asynchronous Trans- two-way Hybrid-Fiber Coaxial plants and by fer Mode (ATM) cells [ 11. In addition to meet- offering a wide range of both data and interac- ing this Internet rush, ATM provides an tive services which they feel will be most attrac- integrated services base on which to offer addi- tive to their subscriber base. Initially these tional services. services will only provide Internet access and access to the major information service (e.g., The term cable modem has been associated Compuserve, AOL, and Prodigy). These service mostly with a description of a CATV Internet offerings will quickly advance to support multi- access device located in the home. ATM based player gaming and collaborative services such systems allow the deployment of other services as voice and desktop video teleconferencing. beyond basic Internet access. In this latter use, the ATM-based cable modem can be viewed as As an introduction to some of the issues sur- a broadband services interface for the home, or rounding ATM residential access technology, an ATM Network Interface Device (NID) or an this paper summarizes two of the standardiza- ATM Premises Interface Device (PID). These tion efforts: the ATM over HFC definition work terms are synonymous in their use in this paper. taking place in the ATM Forum’s Residential Broadband Working Group and the standards Numerous standards organizations are gear- progress in the IEEE P802.14 Cable TV Media ing towards producing cable modem standards. Access Control and Physical Protocol Working The ultimate goal of each is to drive cable Group. Delivering ATM-based integrated ser- modem availability to commodity status and vices via a Cable TV has its own set of deploy- made available via consumer ofs the shelf pur- ment issues and beneJts that are briejly chases at computer boutiques and electronic overviewed and summarized. supermarkets. The minor problem with the commodity process is that these numerous stan- dardization activities are competing and largely INTRODUCTION uncoordinated and there are about a dozen cable modem manufacturers producing product, some Packet technology has been around since 1996 NCTA Technical Papers - 166- of whom wish to establish defacto standard sta- processes. This also has the effect of reducing tus by being first to market. complexity in the subscriber unit and by cen- tralizing the allocation intelligence in the net- The IEEE P802.14 Cable TV MAC and work. Communications between the head-end PHY Protocol Working Group is chartered with controller and each subscriber unit is important providing a single MAC and multiple PHY as permission to use the upstream channel is standard for cable TV networks. The efforts of granted by the head-end controller whose allo- P802.14 must support IEEE 802 layer services cation algorithm must take into account the (including Ethernet) and must also be ATM needs communicated to it by each subscriber. Compatible. ATM residential broadband work is currently taking place in the ATM Forum. ATM in the Residential Broadband Network The customer network interface du jour is The selection of ATM cells as the data-link Ethernet 1OBaseT. There is a mandate for a layer protocol data unit for Cable TV networks 1OMbps Ethernet interface in the home. Sub- has the advantage in that it provides a suitable scriber access equipment can be a personal integrated multiplexing platform capable of computer, X-Terminal, or any such device supporting a mix of guaranteed (predictive) which support the TCPAP protocol suite. traffic flows with best-effort (reactive) traffic flows. In addition, the nature of ATM allows Engineerinp Challenpes of ATM over Cable other multimedia applications to be added in the future without requiring iterative changes to the The standardization and implementation of basic ATM protocol. Cable operators can two-way interactive services on Hybrid Fiber- deploy ATM systems as part of an evolutionary Cable (HFC) TV networks is fraught with many path to a fully integrated multimedia bearer ser- engineering problems which must be overcome: vice offering. l CATV systems are inherently asymmetric in An ATM data-link protocol can be layered nature, i.e. there is more downstream band- in a straightforward manner for both the down- width available than upstream and interac- stream and upstream segments of a cable tive services such as voice or video modem network, The challenges are that telephony require symmetric data rates. upstream traffic management and resource ATM based services will create an environ- management must be creatively controlled to ment where symmetric virtual circuits will support the guaranteed and best effort Quality be opened and closed frequently. of Service (QOS) needs of the cable modem. A residential ATM bearer service easily supports . High utilization of the upstream bandwidth Internet access to the home via the Classical IP is necessary to be cost effective and accom- over ATM standards of the Internet Engineering plished by sharing bandwidth between sta- Task Force [3] or by providing an IP over Ether- tions with the access based on dynamic net adaptation overlay service. assignments within a slotted regimentation. While ATM in the home is desired as a The choice of the allocation protocol and future interconnection method by some HFC the placement of the bandwidth ownership operators, the cost burden of the ATM interface intelligence is important. A straightforward is not economically feasible today. It is approach is to place the ownership of the expected that ATM network interface control- upstream bandwidth under the direction of the lers will be decreasing in cost quickly over time head-end controller which is tightly coupled so planning a cable modem bear service now to with the ATM traffic and signaling management 1996 NCTA Technical Papers -167- GENERAL: ANI = Access Network Interface TII = Technology Independent Interface TE - f -TII HFC SPECIFIC: ADT = ATM Digital Terminal ADAPTER AIU = ATM Interface Unit ANI NOTES: 1. The NT may be NULL in non-HFC access networks. 2. Interfaces at the reference points may be the same or may be different. L---J 3. The HAN contains physical media and passive devices. It may also contain active devices; e.g. bridges or switches 4. There will be more than one PMD and MAC layer specified for the UNI reference points Work in Progress - Based on RBB Reference Configuration - August 1995 - Version 3.0 Figure 1. ATM Forum Residential Broadband Reference Model support both Ethernet and ATM home interfaces ing a full function UN1 interface to the home can be viewed by some as prudent. via an active Network Interface Unit (NIU) termed an ATM Interface Unit (AIU). Control- ling the system is a ATM Digital Terminal ATM FORUM’S RESIDENTIAL BROAD- (ADT) located at the cable system head-end BAND WORKING GROUP (see Figure 1) The discussion of ATM within the home is beyond the scope of this paper. The ATM Forum is focusing attention on delivering ATM over residential broadband dis- The ATM Network Interface (ANI) defines tribution systems. This work is being carried the connection between the ADT and the ATM out in the Residential Broadband (RBB) Work- WAN network. This interface may either be ing Group (WC). The material presented in this specified as a Network-Network Interface section represents work in progress in the ATM (NNI) or as a UNI. The AN1 will be based on Forum and is offered as an example of the cur- existing ATM standards and the WG expects rent thinking on the subject. At some time in the complete compliance with existing physical future, the ATM Forum will be producing a (PHY) interface standards. published specification which includes the ATM over HFC UN1 details. The ATM Forum is a The HFC access network is in effect a black closed industrial consortium requiring member- box to the ATM Forum’s design activities. It ship dues for participation. was decided early in the RBB charter process, that the RBB WG will rely on the efforts from The two goals of the RBB WG are to 1) IEEE P802.14 Cable TV MAC and PHY Work- deliver ATM to the home and 2) deliver ATM ing Group for the transport of ATM cells over within the home. These can be euphemistically the HFC network. The UNI-HFC will define an termed as the last mile problem and the last RF interface for the ADT and AIU. A possible yard problem. The current proponents of ATM protocol stack representation of the relationship over HFC systems are concerned with deliver- 1996 NCTA Technical Papers - 16% 4 b i PHYHFC 4 - PHYHFC : < e I ANI UNiHFC I HUNI ‘. .__-_____-____-_____---------,P802.14 MAC & PHY i Figure 2.