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       INTERNET-DRAFT                    October 1, 1996
       
                    IP Over Cable Data Network Service
       
                  draft-ietf-ipcdn-ipcabledata-spec-00.txt
       
                              October 1, 1996
       
       
                               Masuma Ahmed
                             mxa@terayon.com
                           Terayon Corporation
       
                               Guenter Roeck
                             groeck@cisco.com
                                  Cisco
       
       
       
       
       1.  Status of this Memo
       
       This document is an Internet-Draft. Internet-Drafts are
       working documents of the Internet Engineering Task Force
       (IETF), its areas, and its working groups. Note that other
       groups may also distribute working documents as Internet-
       Drafts.
       
       Internet-Drafts are draft documents valid for a maximum of
       six months and may be updated, replaced, or obsoleted by
       other documents at any time. It is inappropriate to use
       Internet-Drafts as reference material or to cite them other
       than as "work in progress."
       
       To learn the current status of any Internet-Draft, please
       check the "lid-abstracts.txt" listing contained in the
       Internet-Drafts Shadow Directories on ds.internic.net (US
       East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West
       Coast), or munnari.oz.au (Pacific Rim).
       
       
       2.  Abstract
       
       This document describes the application of IP over a cable
       data network service environment configured as a logical IP
       subnetwork (LIS). Specifically, this document describes the
       cable data network interfaces to support IP, IP service
       features, IP address assignment using Dynamic Host
       Configuration Protocol (DHCP), Address Resolution Protocol
       (ARP), and other service-specific issues relating to
       supporting IP over cable data network service. This document
       considers only directly connected IP end-stations and the
       router operating in the conventional LAN based paradigm over
       a cable data network. As background information, this
       document also provides an overview of the cable data network
       
       
       
       
       
       
       
       
       
       
       
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       service, the architecture and the related network
       interfaces. This document does not specify an Internet
       Standard of any kind. It is presented for discussion
       purposes only.
       
       
       
       3.  Conventions
       
       The following language conventions are used in the items of
       specifications in this document:
       
           *   MUST, SHALL, or MANDATORY - this item is an absolute
               requirement of the specification.
       
           *   SHOULD or RECOMMEND - this item should generally be
               followed for all but exceptional circumstances.
       
           *   MAY or OPTIONAL - this item is truly optional and may
               be followed or ignored according to the needs of the
               implementor.
       
       
       
       
       4.  Introduction
       
       The goal of this specification is to allow compatible and
       interoperable implementations for transmitting IP packets
       over cable data network service.  This memo defines only the
       operation of IP over cable data network service and is not
       meant to describe the operation of cable data network
       service.  Note that the cable data network service described
       in this document is referred to as high speed cable data
       service (HSCDS) in the Request For Proposals [1] (RFP) issued
       by CableLabs.
       
       
       The cable data network service is a public carrier service.
       Therefore, supporting IP over a public carrier service has
       issues such as security, scalability, fairness, charging
       based on service tiers, traffic management and should be
       dealt with appropriately. This document tries to address
       some of these issues.
       
       In this document, the cable data network is defined as an
       end-to-end network consisting of three overlaid networks; IP
       routed network, a data link layer subnetwork and the
       physical HFC access networks. A functional diagram of the
       end-to-end cable data network is shown in Figure 1. In this
       configuration, IP packet data service is supported using the
       
       
       
       
       
       
       
       
       
       
       
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       packet data bearer service capabilities of the data link
       layer cable sub-network which in turn is supported using the
       physical transmission medium and the Medium Access Control
       (MAC) protocol of the physical layer in the Hybrid Fiber
       Coax (HFC) access networks. Note that the data link layer
       subnetwork is a routable network unlike a bridged network
       and may support functions such as Address Resolution
       Protocol (ARP) filtering.
       
       
       
        ________             IP Routed Network
        |Router|--------------------------------------------|PC|
        |______|          Data Link Layer Subnetwork
                |headend|----------------------------|modem|
                |equip. |                            |     |
                             HFC Access Network
                         ----------------------------
       
       
       
       Note: The Data link layer subnetwork shown here is
             a routable network and is not a bridged network.
       
       
              Figure 1: End-to-end Cable Data Network
       
       
       The rest of the document details the support of IP, Address
       Resolution Protocol (ARP), and IP address assignment over
       cable data network service.  As background information, a
       brief overview of the cable data network service along with
       the cable data network architecture is provided in
       Appendices A and B.
       
       
       
       5.  Cable Data Network Architecture
       
       As mentioned earlier, a cable data network consists of three
       overlaid networks; IP overlaid network, data link layer
       subnetwork and HFC access network.  This section describes
       the requirements associated with the IP network over the
       cable data network service only.  Specifications of the data
       link layer and the physical layer networks are beyond the
       scope of this document.  As background information, a brief
       description of the physical and data link layer networks is
       provided in Appendix B.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       5.1  IP Routed Network
       
       The end-to-end cable data network MUST provide the
       internetworking capabilities by using IP as the network
       layer protocol technology. A router MUST be used to provide
       the layer 3 connectivity between different customer
       equipment and the wide area network.  Therefore, the
       interface provided by the router to the customer equipment
       MUST be a network layer interface and the data transferred
       MUST be a routable protocol which may be routed to the
       backbone network belonging to the same carrier network or to
       the Wide Area Network (WAN). The router MUST provide all
       internetworking between customer equipment (e.g., PCs)
       attached to the cable data modems and between cable modem
       users and the WAN.
       
       
       
       6.  Cable Data Network Interfaces
       
       The network components of the cable data network and the
       related interfaces are shown in Figure 2. We followed the
       conventions as much as possible with a few exceptions used
       in the HSCDS RFP issued by CableLabs to name the network
       elements and the associated interfaces of the cable data
       network.  The network components of the cable data network
       include:
       
       
           *   cable data modem (CDM) and PC/WorkStation at
               the subscriber premise
       
           *   cable data modem termination system (CDMTS) at
               the distribution hub or headend
       
           *   router, Dynamic Host Configuration Protocol (DHCP)
               server and local web servers at the headend
       
       
       In the sections below, the router interfaces to support IP
       over cable data network are described.  Other relevant
       interfaces of the end-to-end cable data network are also
       briefly described.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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                  Headend or
                  Distribution Hub
                |------------------------|
                |________ ---------      |
                ||Local | |Manage-|      |
                ||WWW   | |ment   |      |
                ||Server| |System |      |
                |---^---- -^----^--      |
                |   |      |    |        |
                |   |   |--|----|---|    |
                |   |  ---I/M1    ---I/M2|
                    |   |           |    |
                |___v___v_        |-v---|| I/F3  |-------| I/F2  |---|I/F1 ____
           I/F7 ||Router |<---|-->|CDMTS|<---|-->|HFC    |<--|-->|CDM|<-|->|PC|
       To <--|-->|_^_____|   I/F4 |-----||       |Access |       |---|     |__|
       WAN      |  |   |                 |       |Network|                   |
                |  |   |<----------------|----|----------------------------->|
                | ---I/F5                |    I/F6
                |  |                     |
                ||-v----|                |
                ||DHCP  |                |
                ||Server|                |
                ||______|                |
                --------------------------
       
          I/F: Network Interface
          I/M: Management Interface
       
       
       
                Figure 2: Cable Data Network Interfaces
       
       
       
       6.1  IF/1 Interface
       
       
       The I/F1 is the interface between the CDM and the PC at the
       subscriber premise. The I/F1 interface supports native
       Ethernet and IEEE 802.3 Medium Access Control (MAC)
       protocols over 10Base-T physical interface. The I/F1
       interface carries transparently the higher layer protocols
       (e.g., IP) above the data link layer protocol to the PC (or
       workstation). The specification of this interface is beyond
       the scope of this document.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       6.2  I/F2 Interface
       
       
       The I/F2 is the interface between the CDM and the HFC access
       network. The I/F2 supports an RF digital transmission
       interface between the CDM and the HFC access network and
       performs upstream RF channel signal modulation and
       downstream RF channel signal demodulation functions. In
       addition, I/F2 supports a data link layer interface to the
       HFC network providing network access control and data
       delivery functions.  The specification of this interface is
       beyond the scope of this document.
       
       
       6.3  I/F3 Interface
       
       
       The I/F3 is the interface between CDMTS and the HFC access
       network. The I/F3 interface performs almost the same
       protocol functions as the I/F2 interface with a few
       exceptions. The I/F3 interface at the CDMTS is used to
       control and manage a number of CDMs in the HFC access
       networks. Therefore, one of the primary functions of the
       I/F3 interface is to manage and control the usage of
       upstream and downstream RF channel resources by the
       subscriber modems. Also, at the physical level, the
       following differences exist between the I/F2 and I/F3
       interfaces:
       
       
           -   upstream and downstream channel frequencies
              (e.g., I/F3 upstream and downstream frequencies are
               opposite to those at the I/F2)
       
           -   receive and transmit power levels
       
       
       In addition, it is possible that the I/F3 may aggregate more
       than one fiber nodes and as such the I/F3 interface may have
       different Bit Error Rate (BER) and Signal to Noise Ratio
       (SNR) than the I/F2 interface. The specification of this
       interface is beyond the scope of this document.
       
       
       6.4  I/F4 Interface
       
       The I/F4 is the interface between the CDMTS and the router
       located at the headend or the distribution hub. Separation
       of the router and the CDMTS may be an implementation issue
       and as such the I/F4 interface is vendor implementation
       specific.  Therefore, the specification of I/F4 interface is
       
       
       
       
       
       
       
       
       
       
       
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       beyond the scope of this document.
       
       
       6.5  /F5 Interface
       
       The I/F5 is the interface between the router and the IP
       address server which in this case is the Dynamic Host
       Configuration Protocol (DHCP) server. The I/F5 interface is
       a traditional IP routed network from the headend router to
       the DHCP server(s). As the data transmitted across this
       network is native IP, the choice of LAN and WAN media is
       extremely flexible. It is possible that the router or the
       CDMTS itself may contain the DHCP server functions and thus
       the I/F5 interface may support a proprietary interface
       depending on a specific vendor's implementation.  Therefore,
       the specification of I/F5 interface is beyond the scope of
       this document.
       
       
       6.6  I/F6 Interface
       
       The I/F6 is the IP interface between the router located at
       the headend or distribution hub and the PC located at the
       subscriber premise. The I/F6 interface MUST support the IP
       network layer interface between the router located at the
       distribution hub/headend and the PC (or workstation) located
       at the subscriber premise. This interface MUST support
       dynamic assignment of network layer address, i.e., the IP
       address to the PC on PC power up using DHCP [4]. This
       interface is described in detail in Section 8 below.
       
       
       6.7  I/F7 Interface
       
       The I/F7 is the Wide Area Network (WAN) interface between
       the router and the public backbone network. This interface
       supports all of the required standard WAN interfaces
       supported in a public carrier network. Specification of the
       I/F7 interface is beyond the scope of this document.
       
       
       
       7.  IP Service Features
       
       The types of IP service features that may be supported over
       cable data network service include:
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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           *   Guaranteed and best effort IP service delivery
               (e.g., by using RSVP and Integrated services protocol)
       
           *   Packet/protocol filtering (e.g., packet access,
               filtering, forwarding, and control)
       
           *   Subscription based service provisioning
              (e.g., access to the IP service via a service order process)
       
           *   Dynamic and static configuration of IP addresses
               to subscriber's end systems (using DHCP)
       
           *   Different tiers of IP service (e.g., using IP access list)
       
           *   IP multicast service
       
       
       
       
       8.  Logical IP Subnetwork Configuration
       
       In the Logical IP Subnetwork (LIS) configuration, each
       separate administrative entity configures its hosts and
       routers within a closed logical IP subnetwork. Each cable
       data network can be considered to be under one
       administrative entity, i.e., under the jurisdiction of one
       cable data network service provider.  The cable data network
       can be configured as a single or multiple IP subnetworks
       depending on the geographic span and physical architecture
       of the cable data network configuration and the number of
       hosts supported in the network.
       
       In general, the router in the cable data network MUST
       support at least one subnetwork configuration (referred to
       as `router LIS configuration').  The hosts within the same
       subscriber premise MUST have direct access to the other
       hosts belonging to the same host subnet configuration but
       MUST not have direct access to the other cable data network
       service hosts supported in the same router LIS. All hosts
       within the same host LIS MUST have the same IP
       network/subnet number and address mask, i.e., all of the IP
       devices on each of the Ethernet interfaces of the subscriber
       CDMs MUST be on the same IP router subnet.
       
       Depending on the cable data network service requirements, it
       is RECOMMENDED that the router providing LIS functionality
       over the cable data network service be able to support more
       than one LIS.  Therefore, the router SHOULD be configured as
       a member of one or more LISs.  All members within a router
       LIS MUST have the same IP network/subnet number and address
       mask.
       
       
       
       
       
       
       
       
       
       
       
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       As mentioned in Appendix A, RF channels are used as the
       physical transmission medium in the HFC access networks to
       support cable data network service. In addition, separate RF
       channels at different RF frequency spectrum are used for
       upstream and downstream transmission. Also, depending on the
       CATV network lay-out, two-way CATV data transmission may be
       supported using a single downstream RF channel and multiple
       upstream RF channels. For the purpose of this document, the
       downstream RF channel and the associated upstream RF
       channels used for two-way data transmission are considered
       as a single two-way RF transmission entity.
       
       Depending on the span of the cable data network and the
       number of hosts supported per RF transmission entity, a
       router LIS MUST be configured to support all hosts connected
       to a single or multiple RF transmission entities.  The
       router providing interconnection of differing LISs MUST be
       able to support multiple sets of parameters (one set for
       each connected LIS) and be able to associate each set of
       parameters to specific IP network/subnet number. The router
       MUST be able to provide multiple LISs support with a single
       physical I/F4 interface between itself and the CDMTS.
       Similarly, a router MUST be able to support a single LIS
       that spans over multiple CDMTSs. Also, the router MUST be
       able to provide a single LIS support to more than one RF
       transmission entities with a single physical I/F4 interface
       between itself and the CDMTS.  Note that, as mentioned
       earlier, the router and the CDMTS functions may be combined
       into a single entity.  In such a case, the I/F4 related
       requirements described here do not apply.
       
       Hosts that are not within the same subscriber premise but
       within the same IP router subnet as well as of different IP
       router subnets MUST communicate via the IP router.
       Therefore, the hosts within the same router LIS MUST not
       have direct access to each other.  The router MUST support
       sending IP packets to any and all hosts within the same
       router LIS as well as of differing router LISs but the hosts
       within the router LIS MUST send packets to the router only.
       Since it is expected that only a small amount of the cable
       data network service traffic will be from one host to
       another, this will not cause excessive relay traffic, but
       does have significant impact on the IP subnet model.
       
       8.1  Address Resolution Protocol
       
       The hosts and router had the same subnet mask for the large
       router subnet and the hosts that happened to talk to many
       other hosts on the same router subnet may be required to
       support very large (e.g., 10,000 entries) Address Resolution
       Protocol (ARP) tables.  Therefore, the router MUST view a
       
       
       
       
       
       
       
       
       
       
       
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       single or multiple RF transmission entities in the cable
       data network as one subnet (e.g., 1,000 to 10,000 hosts).
       
       Normally, ARP [5] is used between hosts and the router, and
       between hosts.  ARP used in the cable data network for each
       of these cases is described below.
       
       * Router to Host
       
       To avoid scaling and security problems with use of ARP over
       a large IP router subnet (e.g., 1,000 to 10,000 hosts), the
       router MUST not ARP for the MAC address of the host.
       Instead, the router MUST assume that DHCP is used by the IP
       hosts. In the process of relaying the DHCP requests between
       the hosts to the DHCP server, the router MUST capture the
       MAC address of the host and the host's IP address assigned
       by the server. The router MUST bind this information
       together into its ARP table.  The entry in the ARP table
       MUST be flagged to prevent it from aging out normally.
       Unicast ARP MAY be used to validate the entry and refresh
       it.
       
       * Host to Router
       
       The DHCP MUST communicate the default IP gateway address to
       the host. Through configuration in the DHCP server, the IP
       address of the router MUST be supplied to the host. The host
       MUST issue a normal ARP for the IP address of the router.
       The subscriber CDM MUST encapsulate this packet to send it
       upstream.   The router MUST answer this ARP normally.
       
       * Host to Host
       
       Hosts ARPing other hosts attached to the same I/F1 interface
       MUST not leave the I/F1 interface. However, for hosts ARPing
       other hosts within the router LIS, the router MUST use the
       proxy ARP capability to answer these ARP requests.
       
       
       8.1.1  ICMP
       
       Data from one host to another on the same
       router subnet MUST be sent via the router. When two hosts
       are on the same subnet, the router would normally send an
       ICMP Redirect to inform the first host that a better (in
       this case, direct) path exists. However, since the cable
       media does not support direct host to host communications
       within the same router subnet, the router MUST do the
       forwarding and MUST suppress the ICMP messages.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       8.2  IP Address Assignment
       
       A host attached to the CDM at the subscriber premise MUST
       use DHCP to obtain its configuration and IP address.  The
       router MUST participate in all DHCP exchanges between the
       host and the DHCP server.  For example, upon power-up, the
       host may broadcast a DHCP message on its local Ethernet
       segment. The host may optionally include any host
       configuration parameters that it may need. The subscriber
       modem transmits this packet upstream to the router.
       
       Upon receiving the packet, the router adds its IP address to
       the gateway IP address field in the DHCP packet and may
       forward the packet to one or more DHCP servers. The DHCP
       servers send DHCP packets to the router with each packet
       containing offered IP addresses available for use which the
       router forwards to the host. The host selects an offered IP
       address and sends back a DHCP request message for a lease on
       that address to the router which forwards the packet to the
       DHCP server. The DHCP server sends an acknowledgement
       indicating a successful lease of the address. The router
       adds an ARP entry, binding the IP address to the Ethernet
       MAC address of the host and forwards the DHCP
       acknowledgement to the host.
       
       8.2.1  IP Broadcast Address
       
       It is RECOMMENDED that the
       router and the hosts within the IP subnet of the cable data
       network be able to receive and transmit IP packets with any
       of the four standard IP broadcast addresses as specified in
       RFC1122 [6]. Members upon receiving an IP broadcast or IP
       subnet broadcast packets for their LIS, MAY process the
       packet as if addressed to that station.  However, depending
       on the cable data network service requirements, the router
       SHOULD have the capability to suppress packets received with
       broadcast IP address.
       
       8.2.2  IP Multicast Address
       
       The IP multicasting method
       specified in RFC1112 [7] requires a Network Service Interface
       which provides a multicast-like ability to provide dynamic
       access to the local network service interface operations:
       
       
         - JoinLocalGroup (group-address)
       
         - LeaveLocalGroup (group-address)
       
       
       Security, subscription and subscriber billing related
       implications associated with dynamic subscription and
       removal from group address lists of any host in a router IP
       subnetwork require further study. Also, methods to support
       
       
       
       
       
       
       
       
       
       
       
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       IP multicasting over data link layer protocol of the cable
       data network service require further study and will be
       addressed in the future.
       
       
       8.3  IP Service Tiers
       
       Cable data network service providers may support different
       tiers of IP service using different charging schemes.
       Depending on the service tier subscribed to, a host can have
       access to different servers and application services such as
       premium web pages, guaranteed bit rate packet, multicast,
       etc. Different tiers of IP service MAY be supported using
       the IP access list. By arranging the IP address assigned to
       fall into one of several ranges, the number of access lists
       required may be reduced to a very small number.  The router
       MAY support such capability by modifying the DHCP Address
       Assignment packet to include the subscriber's cable modem ID
       in the DHCP  `client identifier' field. Note that the
       subscriber's hosts MUST not know the cable modem ID. This
       will be done transparently to them.
       
       8.4  Security
       
       The IP security issues such as supporting authenticated
       end-to-end IP transmission, e.g., using data encryption are
       beyond the scope of this document.
       
       
       
       9.  Issues
       
       Issues associated with cable data network service
       configurations to support capabilities such as IP
       multicasting, IP tunneling and Virtual Private Network (VPN)
       configuration include:
       
        - procedures for performing routing updates between the
          headend router and the modem router (in this case, the
          modem at the subscriber premise supports routing
          functions)
       
        - ability to create virtual private IP routed network
       
        - filtering of IP packets from outgoing routing protocol
          updates
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       10.  Acknowledgements
       
       Special thanks to Jim Forster and Dennis Picker for their
       valuable suggestions and critical review of the document.
       In addition, the author would like to thank Amir Furhman
       and Steve Lin for helpful discussions on the topic.
       
       
       
       
       
       
       11.  Appendix A: CATV Data Network Service
       
       Examples of CATV data network service capabilities include:
       
       
           *packet data delivery to subscriber cable data modem (CDM) with
            minimum peak bit rate of 500 kbps in the downstream direction.
            The maximum peak bit rate can be up to 40 Mbps.
       
           *packet data delivery to subscriber cable data modem (CDM) with
            maximum peak bit rate of 10 Mbps in the upstream direction.
            The minimum peak bit rate can be as low as 28 kbps.
       
       
       Various implementations of cable data network service
       supporting a number of data link layer protocols are
       available today.   Most of these implementations support
       data link layer protocol for the cable data network service
       using slot and frame approach in both upstream and
       downstream directions.  In the HFC access network, the
       downstream direction is described as the transmission of
       data flow from the network to the subscriber and the
       upstream direction is described as the transmission of data
       flow from the subscriber to the network.   In the downstream
       direction, usually broadcast mode is used to distribute
       traffic to the subscribers from the cable headend equipment.
       In the upstream direction, the network resources are shared
       and subscribers have to contend for it.  As an upstream
       resource arbiter, the cable headend equipment allocates and
       manages upstream bandwidth to the subscribers using data
       link layer bandwidth management algorithm.
       
       Radio Frequency (RF) channels in the upstream and downstream
       directions over HFC access networks are used as the physical
       medium to transport the cable data network service.  Various
       combinations of the modulation techniques are used for
       digital transmission of the cable data network service over
       
       
       
       
       
       
       
       
       
       
       
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       analog transmission medium of the HFC access networks.
       Examples of different modulation techniques include:
       
       
            * Spread spectrum modulation technique such as
              Direct Sequence Spread Spectrum
       
            * Quaternary Phase Shift Keying (QPSK) technique
       
            * Quadrature Amplitude Modulation Technique (QAM) with modulation
              order of 16, 64, and/or 256
       
            *Orthogonal Frequency Division Multiplexing (OFDM) technique
       
       
       The RF channels are configured to run between the cable
       modem at the subscriber premise and the channel controller
       at the headend. Upstream channel is shared among all the
       subscribers in the HFC networks and various physical layer
       access algorithms in addition to data link layer bandwidth
       management algorithms are used to access the upstream
       resources. One or a combination of the following physical
       layer access algorithms is used to support cable data
       network service in the upstream direction.
       
       
           * Synchronous Code Division Multiple Access (S-CDMA) method
       
           * Time Division Multiple Access (TDMA) method
       
           *Frequency Division Multiple Access (FDMA) method
       
       
       
       
       
       12. Appendix B: Cable Data Network Architecture and Interfaces
       
       
       The physical and data link layer portion of the cable data
       network architecture is described below.
       
       B1. HFC Access Network
       
       The physical HFC access network is a a shared-media, tree
       and branch architecture with analog transmission over fiber
       used for trunks and coaxial cable used for accessing the end
       systems. The majority of the existing HFC access networks
       support sub-split systems where the upstream frequency
       spectrum is supported from 5 to 30 MHz (and 42 MHz in the
       upgraded systems) and the downstream frequency spectrum is
       
       
       
       
       
       
       
       
       
       
       
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       from 50 to 550 MHz (and 750 MHz in the upgraded systems).
       There are also systems that support mid-split (5 to 108 MHz
       in the upstream direction, and 162 MHz and above in the
       downstream direction) and high-split (5 to 174 MHz in the
       upstream direction, and 243 MHz and above in the downstream
       direction) systems, however, these systems are primarily
       used in institutional networks.
       
       A physical lay-out of the HFC access network is illustrated
       in Figure 3. As shown, a typical HFC access network consists
       of fiber nodes and cascaded amplifiers with remote
       distribution hubs centrally controlled from a central cable
       headend system.  Depending on network configurations, a
       single headend in the cable data network can support from 40
       to 200 or larger number of fiber nodes and each fiber node
       can support from 500 to 2000 or even larger number of
       households.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       To other<--//---|
       DH or  --//--->||<----SONET Ring           ________    |<------>
       HE             ||    (digital)             |      |    |Co-axial
                      ||                   |------|Fiber |----|Distribut(500/
                      ||       (analog)    |      |      |    |-ion     2000
                 _____||______ Fiber Optics| |--->|Node  |    |<------> homes
                 |            |<-----//----| |    |______|              passed)
                 |Distribution|------//------|
                 |Hub (DH)    |                            |<---------->
                 |or          |                 ________   |
                 |Head End    |   Fiber Optics  |      |---|<-Co-axial  (500/
                 |(HE)        |<-------//-------|Fiber |    Distribution 2000
                 |____________|------//-------->|Node  |---|<---------->homes
                       ||                       |______|   |<---------->passed)
                       || 20,000/100,000
       To other <--//--|| homes passed          40 to 200
       DH or    --//--->|                       Fiber Nodes
       HE
       
       
                        Figure 3: An Example HFC Access Network
       
       
       
       RF channels usually 6 MHz wide are used to transport analog
       services such as NTSC video, and digital services such as
       cable data network service, in the HFC access networks. An
       RF channel is the physical layer parameter of the HFC access
       network that extends from the physical layer interface of
       the cable data modem (CDM) located at the subscriber premise
       to the cable data modem termination system (CDMTS) located
       at the headend or distribution hub.  Separate RF channels in
       different frequency spectrum are used for upstream and
       downstream transmission.  Distribution hubs are remotely
       located from the headend and are configured to support one
       or more fiber nodes. These remote hubs are interconnected
       back to a centralized headend via digital transmission
       medium such as SONET ring.
       
       
       
       13.  Terminology
       
       
       In this document, the following terminology is used
       consistent with the Cablelabs HSCDS RFP.
       
       
        * CDM is the cable data modem at the subscriber premise.
        * CDMTS is the cable data modem termination system
          at the headend or distribution hub.
       
       
       
       
       
       
       
       
       
       
       
                                  - 17 -
       
       
       
        * Customer equipment is the equipment at the subscriber premise
          such as a PC or workstation.
        * HE is the cable head end.
        * DHE is the Distribution Hub Equipment.
        * Carrier equipment is the equipment such as CDM, CDMTS, HE
          that belongs to the public carrier network.
        * I/F refers to the network interface in the CATV data network.
        * I/M refers to the management interface in the CATV data network.
       
       
       
       14.  Authors' Addresses
       
       
                               Masuma Ahmed
                           Terayon Corporation
                          2952 Bunker Hill Lane
                          Santa Clara, CA 95054
                          Phone: (408) 486-5207
                           Fax: (408) 727-6205
                          Email: mxa@terayon.com
       
                               Guenter Roeck
                                  Cisco
                              174 Tasman Drive
                          Santa Clara, CA 95054
                          Phone: (408) 527-3143
                           Fax: (408) 727-6205
                          Email: groeck@cisco.com
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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                                References
       
       
        1. "High Speed Cable Data Service Request for Proposals",
           Cable Television Laboratories, April 1995.
       
        4. Droms, R., "Dynamic Host Configuration Protocol",
           RFC1531, Bucknell University, October 1993.
       
        5. Plummer, D., "An Ethernet Address Resolution Protocol -
           or - Converting Network Addresses to 48 bit Ethernet
           Address for Transmission on Ethernet Hardware", STD 37,
           RFC826, MIT, November 1982.
       
        6. Deering, S., "Requirements for Internet Hosts -
           Communication Layers", STD 3, RFC1122, USC/Information
           Sciences Institute, October 1992.
       
        7. Deering, S., "Host Extensions for IP Multicasting", STD
           5, RFC1112, Stanford University, August 1989.
       

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