Dynamic Collision Resolution and Traffic Scheduling for DOCSIS Systems with QoS Support Wen-Kuang Kuoa, Sunil Kumarb and C.-C. Jay Kuoa
a
Department of Electrical Engineering-Systems and Integrated Media Systems Center University of Southern California, Los Angeles, CA 90089-2564 Email:
[email protected],
[email protected] b Electrical and Computer Engineering Department Clarkson University Box 5720, Potsdam, NY 13699 Email:
[email protected] Abstract-The Data Over Cable Service Interface Specifications (DOCSIS) of the Multimedia Cable Network System (MCNS) organization intends to support IP traffics over HFC (hybrid fiber/coax) networks with significantly higher data rates than analog modems and Integrated Service Digital Network (ISDN) links. The availability of high speed-access enables the delivery of high quality audio, video and interactive services. To support quality-of-service (QoS) for such applications, it is important for HFC networks to provide effective medium access and traffic scheduling mechanisms. In this work, we consider an HFC network that has a shared upstream channel for transmission from stations assigned with different service priorities to the headend. We first present a multilevel priority collision resolution scheme with adaptive contention window adjustment. The proposed collision resolution scheme separates and resolves collisions for different classes of critically delay-sensitive and best effort traffics, thus achieving the capability of preemptive priorities. To enhance the performance of the proposed scheme, we adopt a novel methodology in which the headend dynamically selects the optimal backoff window size according to the estimate of the number of contending stations for each priority class. A traffic scheduling policy with multiple priority queues is also employed in the headend to schedule data transmission. This scheduling strategy is used to satisfy bandwidth requirements of higher priority traffics. A set of simulation scenarios is conducted by using OPNET to demonstrate the performance efficiency of the proposed scheme.
I. INTRODUCTION Recently, the rapid growth of the number of residential Internet users and the increased bandwidth requirements of multimedia applications have necessitated the introduction of an access network that can support the demand of such services. Community Antenna Television (CATV) networks seem to be one of the most potential solutions from an economic perspective for two major reasons. First, the CATV infrastructures already connect a majority of homes. Second, the Hybrid Fiber Coax (HFC) used in CATV networks can be used to deliver broadband services without requiring costly upgrade of existing CATV network systems. We can thus foresee that HFC networks will be in an important position for broadband access networks in the near future. However, CATV networks are traditionally used to provide analog audio and video broadcast programs from the headend (HE) to subscribers. As a consequence, CATV networks are characterized by a tree and branch topology with the broadcasting node at the root and recipients at leaves. The
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bandwidth is divided into several channels, most of them dedicated to the downstream transmission (from HE to cable modems) while only a few are for upstream transmission (from cable modems to HE). Since all users have to share the upstream channel, it is necessary to identify an effective Medium Access Control (MAC) protocol to make efficient use of CATV networks. The current CATV standard activities include Multimedia Cable Networks Systems (MCNS), IEEE 802.14, Internet Engineering Task Force (IETF), Digital Audio Visual Council (DAVIC), Digital Video Broadcasting (DVB), ATM Forum Residential Broadband Working Group (RBWG) and the Society of Cable Telecommunications Engineers (SCTE). The Data Over Cable Service Interface Specifications (DOCSIS) has been defined by MCNS to support IP traffics over HFC networks. RBWG investigates the provision of ATM for media distribution in the CATV networks. IETF is contributing to IP delivery on top of CATV networks. More details of this specification can be found in [1]-[5]. Among all the effort mentioned above, DOCSIS driven by CableLabs and its vendor companies was the first set of specifications to be finalized. SCTE has adopted DOCSIS and submitted it to ITU for approval. Hence, DOCSIS will be the first set of CATV network specifications adopted by the international standard body. It is also accepted by most major vendors. DOCSIS is therefore expected to be the most widely used protocol to provide high-speed residential access. One of the challenging issues for DOCSIS is to provide QoS required by delay sensitive media applications. In this research, we investigate the ability of the DOCSIS protocol to provide preemptive priority access to users. An effective priority mechanism is necessary to provide QoS in CATV networks for services such as video on demand, interactive computer games, video telephony, and so on. Priority mechanisms have been implemented in other MAC protocols, such as DQDB and the token ring. But these priority mechanisms are not suitable for the contention-based CATV networks. Lin and Gampbell [6] proposed an amendment, the Extended Distributed Queue Random Access Protocol (XDQRAP), which adds an extra slot to each frame to identify priorities. However, this scheme only supports two priorities with a fixed frame format. Citta and Lin [7] implemented a priority scheme with variable probabilities incorporated with the p-persistence random access protocol.
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However, this scheme cannot be applied to DOCSIS since the collision resolution procedure is not random p-persistence. We employ two mechanisms to implement an effective priority system in this study. First, HE uses a multiple priority queues scheduler to allocate the bandwidth to cable modems (CM) of different priorities. Second, the MAC protocol uses a dynamic backoff window scheme to solve collisions so that higher priority CMs are able to transmit bandwidth requests without interference from lower priority CMs. The proposed scheme can be easily integrated with DOCSIS. The remainder of this paper is organized as follows. Section 2 provides an overview of DOCSIS specifications and CATV networks. Section 3 describes the proposed dynamic backoff window scheme and the proposed scheduling algorithm with multiple priority queues. Section 4 discusses the simulation model for DOCSIS and presents simulation results and discussion. Section 5 provides the conclusion.
window range. Once a value is chosen, the station must let existing allocation request minislots pass before it makes its request. If a CM has made a request but does not receive a response before a timeout value, it assumes a collision. In such a situation, the CM increases its window size by a factor of 2, as long as that size is less than the Data Backoff End. It then retries the request by using the new window value. This process continues until a maximum of sixteen tries. The CM knows the transmission is successful when HE sends it a data grant.
Network
Upstream
HE
II OVERVIEW OF DOCSIS MAC PROTOCOL
CM
CM
Downstream
The logical topology of a CATV network is shown in Fig. 1. The downstream path flows from HE to all CMs and resides in a 6 MHz TV channel selected by the cable operator. Since HE is the exclusive transmitter in the downstream direction, no downstream MAC mechanism is needed. While the downstream channel is broadcast in nature, each CM is assigned an individual address that allows to filter out any data not addressed to it via unicast, multicast, or broadcast transmission. Each upstream channel is shared by a number of stations and divided in time into individually numbered allocation units called minislots. The shared nature of upstream flows requires a MAC mechanism to coordinate transmissions. The physical equipment of the cable plant requires isolation of signals in the upstream direction, which results in a scenario where upstream transmissions can only be heard by HE but not by CM. Thus, concurrent upstream station transmissions from CM to HE can collide, but individual CMs are unable to hear transmissions of other CMs. At the MAC layer, HE allocates bandwidth to CMs by reserving minislots in the upstream channel. HE sets an Allocation MAP for minislots that are broadcast to CMs. The upstream bandwidth is allocated for both the contention period and the reservation period in terms of minislots. Any station that desires to request an allocation must contend for access during periods that are specified in this MAP. If a request is successful, HE will allocate an upstream bandwidth for the station in a future allocation map. If the request results in a collision, MAC will initiate a collision resolution process. The contention resolution technique used in the DOCSIS protocol is based on a truncated binary exponential backoff scheme. The truncated binary exponential backoff algorithm requires two parameters for its operation. The first is the Data Backoff Start and the second is the Data Backoff End. These values are set as a part of the Allocation Map. To begin a request, a CM sets its window to the size of the Data Backoff Start. It then chooses a random value that is within the
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CM
Fig. 1: The logical topology of a CATV network The DOCSIS protocol does not specify how to provide QoS guarantees. The specification of the MAC layer suggests the implementation of different classes of services for integrated services over the Internet. III MULTI-PRIORITY ACCESS SCHEME FOR MAC PROTOCOL Although the current MCNS DOCSIS protocol supports a set of upstream flow scheduling services that HE can offer to CMs, some problems still exist. First, the main problem is a high delay bound for delay sensitive traffics in the contention resolution mechanism. The time it takes a CM to send a successful request to HE must be kept as low as possible for a real-time flow even during a high contention period. The time spent in the contention process includes that of collisions, retransmissions, etc. To satisfy the delay-constrained property of real-time traffics, it is necessary to assign a higher priority to real-time traffics as compared to non-real-time traffics. However, DOSCIS treats all CMs equally irrespective of their traffic priority, and new real-time flows can be easily blocked to result in a large delay period. If a high priority (real-time) request is blocked from accessing the channel or suffers a high number of collisions from a lower priority (non-real- time) flow, its QoS cannot be guaranteed. Second, DOCSIS does not provide a mechanism to give high-priority CMs immediate access to the channel, nor does it separate and resolve collisions in a priority order. To solve these problems, we introduce a scheme that supports multi-priority access for DOCSIS. In our scheme, higher priority CMs use contention slots assigned to them according to their priority for initial access as well as for retransmissions. A CM with a new request waits for a group of contention slots (i.e. windows) with a priority that matches its own priority, and transmits the request with probability 1 when a slot with the same matching priority becomes
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available. The station randomly selects a contention slot in the window. As described in Section 2, the DOCSIS protocol uses the binary exponential backoff to resolve collisions. We propose modifications to the collision resolution backoff scheme by giving a different backoff value to CMs of different priorities. The backoff value in our scheme is equal to the number of contention slots reserved for high priority CMs. Besides, the backoff value must be properly selected according to traffic conditions to achieve the optimal operation. In particular, the fact that the optimal value of backoff depends on the number of contending CMs suggests that the binary exponential backoff scheme can be improved by dynamically selecting the backoff value according to the estimate of the number of contending CMs based on the measurements of the channel activity performed by HE. The details of the proposed dynamic backoff window scheme is described below. Consider that n stations are contending for requested minislots. For a contention window of size W, the backoff value b(t) is randomly chosen in the range (0, W-1). Furthermore, b(t) is decremented in each slot and can be modeled by the following Markov Chain, Pr{b(t ) = 0} Pr{b(t + 1) = k } = Pr{b(t ) = k + 1} + , 0≤ k
