Should Ethernet or ATM be used as primary cable protocol for HFC?

Transcription

1 Should Ethernet or ATM be used as primary cable protocol for HFC? 1. Introduction Eric D. Dennis, Chiou-Ping Wang, Yi-Chen Lin, Qian Zhao This paper will be a comparative analysis of those two leading data link layer protocols, Ethernet and ATM, which are currently being used in local cable systems worldwide. These two protocols will be evaluated to determine technical (network architecture), management (network management) and the financial costs (economics) of implementing each. Which protocol will emerge as the primary protocol for cable systems in the twenty-first century? This very question regarding the development of a primary cable protocol is currently being addressed by two standards setting bodies IEEE (Institute of Electrical and Electronic Engineers) and MCNS (The Multimedia Cable Network System). The current development status regarding each of these two standards setting bodies will be key drivers in deciding which protocol best benefits cable systems and cable system vendors. 2. Background of HFC 2.1. Description of HFC Before the addition of fiber optics, cable companies established their networks by implementing coaxial cables. In the late 1980 s, fiber optics became a valuable transmission medium that was being added to cable system networks. Today, those systems are known as HFC networks. HFC stands for Hybrid Fiber Coaxial, which is the combination of fiber optics and the coaxial cable used together to create a network system which can transmit both digital and analog signals [1]. It can provide residential video, voice telephony, data, and other interactive services to subscribers homes 2.2 Architecture of HFC Figure 2-1: Architecture of HFC 1

2 Headend The headend is the most important location of HFC. It is responsible for the interconnection, switching, and transmission of the HFC network. For this reason, the headend has to be equipped to receive signals from consumers, broadcasting satellites, and local programming networks, as well as be able to intelligently route signals to proper destinations. Node Node is used as a conversion point, converting signals from the optical distribution plant to the coaxial distribution cable and, in the opposite direction, from the coaxial to the optical. Subscriber's home After the coaxial cable enters the subscriber s home, it ends at the Coaxial Terminal Unit (CTU). The CTU is used to interface with the end users equipment like the telephone, computer, TV set, etc. The most familiar CTUs are the cable modem and the set top box. The fiber optics is between the headend and the node, and the coaxial is between the node and the subscriber s home. Each fiber node can serve an area of homes (See Figure 2-1). The following discussion will focus on the part of the network from the headend point to the subscriber point Spectrum allocation of upstream and downstream A general two-way HFC architecture utilizes a 750 MHz spectrum. A Spectrum from 50 MHz to 550 MHz is allocated for analog cable TV signals, a spectrum between 5MHz to 50 MHz is used to carry digital upstream signals, and a spectrum between 550 MHz to 750 MHz is designed for a digital downstream signal (See Figure 2-2). Upstream signals including Telephony CATV Analog Signals Video Digital and Downstream Telephony MHz Figure 2-2: Typical HFC Spectrum Allocation 2.3. IEEE and MCNS group The IEEE (Institute of Electrical and Electronic Engineers) and the MCNS (Multimedia Cable Network System) are the groups who are working on establishing standards for data transmission over the HFC system. 1 The IEEE Working Group is a Committee of engineers representing the vendor community that has developed a specification for data over cable networking. The group, which was formed in the early 1990's, had intended to develop a specification that would be recognized as an international standard [2]. It standardizes the physical and Media Access Control (MAC) layers for HFC systems, which provides support for ATM. MCNS, a consortium of North American MSOs 2, CableLabs 3, and Arthur D. Little, developed a specification known as DOCSIS (Data over Cable System Interface Specification) for transmitting data over a cable network. The specification was embraced by the SCTE 4 and was submitted to the ITU 5 in March of 1998, where it was approved as an DAVIC/DVB, IPCDN, The ATM Forum Residential Broadband Working Group and Packet Cable are the other groups who are working on establishing the HFC standards. 2 Multiple System Operator, a company that operates more than one CATV system. 3 Cable Television Laboratories, Inc. 4 Society of Cable Television Engineers, a training, standards, and certification organization for "broadband communications." 2

3 international standard for data over cable networking [2]. The MCNS differs from the IEEE , in that it uses the IP protocol as a Network Layer: Ethernet as Data Link Layer. In order to provide security, MCNS inserts a Link Security Layer between the LLC and the MAC Layer. In the sections that follow, each protocol will be discussed to evaluate which techniques will be the primary protocol for HFC. 3. ATM over HFC cable network: An ATM broadband infrastructure challenge provides support for a family of integrated bearer service products supporting a range of service from guaranteed bit rate applications such as voice telephony, to elastic bit rate applications such as Internet access. Each of these services may be mixed in the same downstream and upstream frequency allocations (channels). In this section, we would like to describe some general aspects of how ATM works on HFC: 3.1. ATM Access Reference Architecture Figure 3-1 shows the Reference Architecture for ATM over HFC cable television (CATV) transmission systems. Figure-3-1 The UNI w, UNI X and UNI H interfaces are specific to the Access Network technology, Access Network termination, Home Network and the ATM End System [3]. These interfaces support a cell-based UNI for ATM transport between these elements. In an HFC transmission system, modulated digital signals are frequency division multiplexed onto the optical and coaxial physical medium along with analog television signals. In the upstream direction, the physical medium is shared among subscriber equipment (which are called stations), which transmit signals that are received only by the headend. A Media Access Control (MAC) layer protocol is accessed by stations to the upstream medium. In addition, the ATM transport protocols at a UNI HFC, figure 3-2, consist of a physical layer and a MAC [4]. The physical layer includes the modulation schemes for both the upstream and downstream channels. There can be more that one type of physical 5 International Telecommunication Union.

4 layer in a single direction. Figure Medium Access Control (MAC) Protocol Components An HFC MAC protocol can be broken into the following set of components [5]: ranging or acquisition process, frame format, support for higher traffic classes, bandwidth allocation, bandwidth request, and contention resolution mechanism. First, ranging is the phase during the initial setup, which determines the round-trip correction (RTC) parameter. After the ranging process, the station can be synchronized with headend. In addition, the frame format element of the MAC defines the upstream and downstream frames and describes their contents. In order to support the ATM, it also needs to differentiate between different classes of traffic. Further, the bandwidth allocation represents an essential part of the MAC and controls the granting of requests at the headend. Finally, the contention resolution mechanism, which is perhaps the most important aspect of the MAC, consists of a backoff phase and a retransmission phase MAC Protocol Overview The upstream data are segmented into fixed-size minislots. Several minislots can be concatenated in order to form a data slot while one-contention slot maps into one minislot. Data slots containing subscriber data packets are assigned to stations by reservation. The MAC layer prepends octets to each ATM cell in order to form a MAC packet data unit (PDU) that is carried in a data slot. Since more than one station can transmit a request at the same time, resulting in a collision, a contention resolution algorithm must be implemented as part of MAC. The downstream data are segmented into a fixed slot size of MAC PDUs. The MAC request/feedback cycle is defined as the time elapsed between request transmission and feedback reception at the station that is farthest from the headend. This insures that all stations have the same opportunity of transmission in any given slot and prevents unfairness due to the relative location of the stations with respect to the headend. The bandwidth allocation algorithm running at the headend controls the number of contention slots and data slots contained in a MAC cycle. Further, the headend also decides on the distribution and location of the contention slots and data slots on the upstream channel. Contention slots can be either grouped together in clusters or distributed over the request/feedback interval. The information about the contention/data slot location is sent to the stations on the downstream channel.

5 3.4 Security in ATM/HFC Network By using ATM over the HFC network, virtual paths (VP) and virtual channel (VC) can be established between the users and headend or distribution point of the network. Even by assigning a different VP/VC, which is a kind of addressing and not encryption, it would be rather easy to manipulate the VP/VC filter in the user equipment in order to receive information destined for other users. Consequently real signal encryption is needed. Encryption on the ATM cell level [6] In case of encryption on the cell level, only the payload of ATM cells is encrypted. As the ATM cell headers are not encrypted, the data stream appears like a normal ATM cell stream, and so it can be routed via ATM network elements like multiplexers or demultiplexers without first decrypting. As the ATM cell header of all cells remains visible, the header error check mechanism can be used for synchronization and cell delineation Traffic Management Unlike point-to-point physical media, the MAC layer dynamically allocates resources among all the users of the MAC layer service. As a result, upstream cell scheduling is done both at the ATM layer and at the MAC layer. The ATM layer in a station that is an ATM send system: has local knowledge of the state of ATM virtual connections, may shape CBR and VBR virtual connections to meet the traffic contract, implements the source reference behavior for ABR virtual connections, and passes ATM cells to the MAC layer in accordance with traffic contract. On the other hand, the ATM layer in a station that is an ATM node: has local knowledge of the state of ATM virtual connections, may enforce the traffic contract or shape connections to meet the traffic contract, may implement virtual source/virtual destination for ABR virtual connections, and passes ATM cells to the MAC layer in accordance with traffic contract [7] Collision Resolution A collision resolution standard is needed within the ATM/HFC type of broadcast type network in order to support ATM cell transfers. The following is the standard which is a combination of the priority plus first-in-first-out (FIFO) first transmission rule, the n-ary tree plus p-persistence collision resolution. Minislot Allocation algorithm [8] The headend runs a minislot allocation algorithm to decide the number of minislots in a cluster for the purpose of sending requests. Further, from time to time, the headend sends an allocation map describing the location and usage of a minilost cluster. Specifically, an allocation map protocol data unit (PDU) indicates the collision resolution engine this map comes from, the number of the first request minislot this map specifies, and the division of the request minislot cluster into group with different resolution queue (RQ) values. If the group whose RQ is 0 is further divided into subgroups with different priorities and admission time boundaries, which are used to enforce the priority + FIFO transmission rule. First transmission rule: priority and FIFO [8] A new arriving request can only use the request minislots of the group with RQ value equal to 0. The request can only use the subgroup whose priority is the same as its priority. Retransmission Rule: n-ary tree and p-persistence [8] If a collision has occurred to a transmitted request, the station, which sent the request, saves the RQ value provided to it in the headend.

6 Acknowledgement message. When the headend announces the arrival of the next minislot cluster by an allocation map, the station checks the RQ values of the groups to see if any of them is smaller than or equal to the saved RQ value. If one or more exists, the station will try to contend in the group by selects a randomly number from 1 to the split value, for example m. If m is smaller than or equal to the number of allocated minislots of the group, it transmits the request on the m th minislot of the group. Otherwise, it waits for next allocation map and retries. 4. Ethernet (DOCSIS 1.1) over HFC cable network 4.1. DOCSIS The wide range of Ethernet components and cabling systems that are available today provides enormous flexibility, low cost and makes it possible to build an Ethernet to fit just about any circumstance. In two way hybrid-fiber/coax (HFC), Data Over Cable Service Interface Specification 1.1 (DOCSIS 1.1) is the certification created by the vendor community and Cable Television Laboratories Inc. (CableLabs) which can provide users with high-speed internet, packet telephony, video conferencing and telecommuting (i.e., remote access to enterprise networks) access from the region headend to home through a broadband HFC. In DOCSIS specification, the Network layer protocol is the IP, and the Data link layer is Ethernet. The purpose of the DOCSIS is to transport IP traffic transparently through the HFC system. DOCSIS enables cable operators to build multiservice IP networks. It also has a capability of supporting a full range of carrier-class data, voice and video applications. DOCSIS consists of a group of specifications that cover operations support systems, management, and data interfaces, as well as MAC and PHY transport MAC Figure 4-1 The DOCSIS MAC provides a protocol service interface to upper-layer services. The MAC Service interface defines the functional layering between the upper layer service and the MAC. Firewall and policy based filtering service may be inserted between the MAC layer and the upper layer service, but such a service is not modeled in this MAC service definition

7 (see Figure 4-1). The following data services are provided by the MAC service interface: Classifying and transmitting packets to MAC service flows. Receiving/Transmitting packets from MAC flows. Packets may be received with suppressed headers. The headers of transmitted packets are suppressed based upon matching classifier rules. The headers of received suppressed packets are regenerated based upon a packet header index negotiated between the CM (Cable Modem) and CMTS (Cable Modem Termination System). Synchronization of grant timing between the MAC and the upper layer service. This clock synchronization is required for applications such as embedded Packetcable VOIP clients in which the packetization period needs to be synchronized with the arrival of scheduled grants from the CMTS. Synchronization of the upper layer clock with the CMTS Controlled Master Clock. The MAC of DOCSIS defines a single transmitter for each downstream channel from the CMTS. All CMs listen to all frames transmitted on the downstream channel upon which they are registered and accept those where the destinations match the CM itself or CPEs (Customer Premises Equipment) reached via the CMCI (Cable Modem Termination System) port. CMs can communicate with other CMs only through the CMTS. The upstream channel is characterized by many transmitters (CMs) and one receiver (the CMTS). Time in the upstream channel is slotted, providing for Time Division Multiple Access (TDMA) at regulated time ticks. The CMTS provides the time reference and controls the allowed usage for each interval. Intervals may be granted for transmissions by particular CMs or for contention by all CMs. CMs may contend to request transmission time. To a limited extent, CMs may also contend to transmit actual data. In both cases, collisions can occur and retries are used QoS: DOCSIS 1.1 defines new functionality that allows cable operators to provide guaranteed bandwidth, i.e., quality of service (QoS) for both upstream and downstream traffic through the CM and the CMTS. The Resource Reservation Protocol (RSVP) is used by a host to request specific QoS from the network for particular application data streams or flows. It is defined at the network layer to work alongside IP and on top of Ethernet. DOCSIS embeds advances which enable the delivery of services requiring QoS and VPN (virtual private network) capabilities across the HFC infrastructure. The enhancements include features that optimize scheduling and resource reservation for voice and video delivery while supporting embedded chip-level support for things like hardware-assisted packets or frame-based fragmentation and reassembly [9] Privacy Baseline privacy employs the 56-bit data encryption standard (DES) block cipher for encryption/decryption of user data to secure the privacy of the connection. The encryption is integrated directly within the MAC hardware and software interface [10]. Privacy of user data is achieved by encrypting link-layer data between cable modems and CMTS (Cable Modem Termination system). Cable modems and CMTS headend controller encrypt the payload data of link-layer frames transmitted on the cable network and MAC Management Messages are not encrypted. A set of security parameters including keying data is assigned to a cable modem by the Security Association (SA). All of the upstream transmissions from a cable modem travel across a single upstream data channel and are received by the CMTS. In the

8 downstream data channel a CMTS will select an appropriate SA based on the destination address of the target cable modem [11] Signaling H.323 is the preferred choice for signaling in DOCSIS. H.323 is a standard for audio and video conferencing over shared data networks adopted in 1996 by ITU (International Telecommunications Union). H.323 [12] specifies protocols for call signaling and control, plus several compression and decompression algorithms (codecs) ranging from 8 Kbps to 64 Kbps [13]. In addition to H.323, cable operators and vendors plan to support Simple Gateway Control Protocol (SGCP) and Media Gateway Control Protocol (MGCP), a more centralized packet telephony architecture which may prove to be a scalable solution Congestion Control/Bandwidth Utilization The Carrier Sense Multiple access/collision Detection (CSMA/CD) access method has been used to resolve contention for the shared media data going upstream. Payload Header Suppression has been used in DOCSIS 1.1 to allow the suppression of unnecessary Ethernet/IP header information for improved bandwidth utilization. 5. Comparison of Ethernet/ATM over HFC The next generation cable systems will need to offer reliability, true scalability, high performance, distributed and efficient network management, and support for customer selfcare [14]. These industry expectations are the drivers toward a better cable system for both residential and consumer subscribers. How do Ethernet and ATM perform in the technical, network management, and economic arenas as we approach the twenty-first century? 5.1 Network Architecture Ethernet is the MAC solution supported by MCNS (DOCSIS 1.1) members. This solution supports a variable length implementation, and also allows for greater flexibility in future cable systems. The Ethernet variable length packets also favor an Internet Protocol (IP) type traffic. Security is addressed by providing encryption at the link layer level and payload level. Signaling is addressed by the H.323, SGCP and MGCP protocols. These signaling methods allow for a scalable packet telephony architecture. Congestion is addressed by CSMA/CD. The Ethernet solution is highly supported because it is the embedded infrastructure of the majority of North American cable systems. It is easy to maintain and is quickly approaching ATM-like quality with the recent increases in development of Gigabit Ethernet speeds. The IEEE committee touts the ATM MAC protocol, which has a fixed cell format and is used from the headend to the cable modem. There is no signaling encryption for ATM; however, security is available for the payload of ATM cells only. Congestion resolution is based on a backoff and retransmit methodology. The reason ATM is regarded as superior is because it allows for the combined delivery of guaranteed bandwidth services and classic internet. There are several similarities between the two protocols. At the physical layer, which defines modulation formats for digital signals, the IEEE and MCNS specification are similar. Additionally, both cable modem protocol solutions call for a 10Base-T Ethernet connection from the cable modem to the PC. So even with an ATM-based solution, Ethernet is a key component.

9 5.2. Network Management The latest version of the Ethernet (DOCSIS 1.1) defines new functionality that supports Quality of Service (QoS) components through CM and CMTS for upstream and downstream traffic. QoS is important as cable companies push for multi-tiered service offerings. These multi-tiered service offerings will increase during the next 2-3 years as the Ethernet wire speeds continue to increase. Currently, Ethernet speeds are increasing from 10/100 Mpbs and headed towards Gigabit speeds. The continued increase in packet based throughput will close the gap between Ethernet and ATM. ATM presently provides QoS guarantees that are required for integrated delivery of video, voice, and data traffic to cable modems [15]. This QoS allows for good contol over scheduling and end-to-end latency, as well as a flexible configuration to support incremental growth and different scenarios. The ATM QoS is a more mature solution than Ethernet Economics Network Managers and Financial Analysts are key personnel in any businesses decision making process. The cable industry is not any different. The people making decisions appreciate the technical details and the system capabilities, but their underlying desire is to understand the cost. The MCNS coalition, which supports DOCSIS 1.1, represents eightyfive percent of US cable operators and seventy percent of Canadian cable operators. That suggests that North American cable systems today are comprised of seventy plus percent Ethernet-based systems. The ATM solution is supported by the IEEE, which is backed by cable modem vendors. From a technical standpoint, ATM is slightly superior to Ethernet because of its flexibility when working with multi-tiered cable system offerings. However, ATM-based systems cost significantly more, since they usually have to be introduced into existing systems. Economically, ATM will introduce significant infrastructure overhead costs to the implementation process. Many companies are not even considering changing out their existing infrastructure. What has happened in the cable industry is more of a merging of both protocols. 6. Conclusion The results of the comparative analysis between Ethernet over HFC and ATM over HFC indicate that both systems are attractive to cable industry vendors and cable system providers. While both have advantages and disadvantages, Ethernet-based systems make up a significant portion of the current cable system infrastructures. ATM from the headend to the cable modem is costlier to install and is generally only installed in new cable systems and as an augment to existing cable company current systems. Cable modem vendors have avoided a Beta Vs. VHS type situation by making their cable modems backward compatible. In other words, their modems will work with Ethernet based systems or ATM-based systems. This eliminates them from being locked out of any market due to a decision on any specific protocol. The cable operators, on the other hand, are committed to delivering high-speed internet services to consumers; however, they believe ATM would add unnecessary complexity and cost to their cable modem systems. Our research has found that cable system providers are remaining loyal to the Ethernet-based systems they own, and they are willing to wait for the advancements promised in the next 2-3 years by Gigabit Ethernet.

10 References [1] Richard A. Lawson, The Analysis and Performance of the End Equipment in a Two-Way Hybrid Fiber Coax (HFC) Cable Architecture., Thesis for Master of Science Degree in Interdisciplinary Telecommunications, pp. 27, University of Colorado, [2] Cable Modem University, Standards, [3] ATM Forum Technical Committee, Residential Broadband Physical Interfaces Specification, af-rbb-phy , January, [4] ATM Forum Technical Committee, Residential Broadband Architectural ramework, af-rbb-phy , July, [5] Nada Golmie, Sandrine Masson, Gerard Pieris and David Su, Performance Evaluation of MAC Protocol Components for HFC Network, SPIE, vol.2917, [6] Robert Wolters,Dietrich Boettle,and Chris Sierens, A Novel ATM Based Data Transport System for Hybrid Fiber Coax CATV Network, Research Division of Alcatel, D Stuttgart, Germany. [7] IEEE, IEEE Project /a Draft 3 : Cable-TV Access Method and Physical Layer Specification, August, [8] Nada Golmie, Sandrine Masson, Gerard Pieris and David Su, A Review of Contention Resolution Algorithms for IEEE Network, September, [9] Cable Datacom News, "Cisco Sees Cable As Key to IP Convergence", June, [10] Data-Over-Cable Service Interface Specifications. SP-RFIv1.1-I , CableLabs, http// [11] Security in DOCSIS-based Cable Modem Systems, Executive Summary, CableLabs, August, 1999 [12] David Lindbergh, H.323: Multimedia Conferencing for Packet Switched Networks, PictureTel Corporation, [13] Cable Internet TV & Telephony, Cable IP Telephony Primer, [14] Mark Komanecky, The New Paradigm for Delivery of IP-Based Services Over Cable, Telecommunications Magazine, pp. 37, July [15] Cable Modem Standards and Specifications,