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A DYNAMIC RESERVATION PROTOCOL FOR INTEGRATING CBR/VBR/ABR TRAFFIC OVER IEEE 802.14 HFC NETWORKS Henry C. B. Chan and Victor C. M. Leung Department of Electrical and Computer Engineering The University of British Columbia Vancouver, B. C., Canada, V6T 1Z4 {chunc, vleung}@ece.ubc.ca ABSTRACT We propose and analyze a novel dynamic reservation (DR) protocol for integrating constant bit rate (CBR), variable bit rate (VBR) and available bit rate (ABR) traffic over Hybrid Fibre/Cable (HFC) networks. This protocol allows a VBR station to capture isochronous channels on a bandwidth on demand basis. To enhance channel utilization, both the capturing probability and the bandwidth available for reservation requests are adjusted dynamically. We also propose an effective method to provide ABR service such that non-time sensitive data traffic can only be transmitted over the residual bandwidth of the time-sensitive CBR and VBR traffic. Simulation results indicate that the proposed protocol gives performance close to the ideal results, is efficient in terms of bandwidth utilization, and provides the desired ABR service characteristics. 1. INTRODUCTION In recent years, there has been a growing interest to utilize HFC networks to provide a wide range of interactive and multi-media services (e.g., video on demand, telephony, and Internet access) to residential subscribers [1]. As HFC networks have been primarily designed for providing broadcast television service from a headend controller (HC) to the subscriber premises, an effective media access control (MAC) protocol is required for subscriber-to-HC communications. Apparently, none of the MAC protocols in existing IEEE 802 standards work well for a HFC network. Due to its tree-based topology, the existing IEEE 802.6 (DQDB) and IEEE 802.5 (Token Ring) access protocols are not suitable because these access protocols only work in the dual bus and ring architectures, respectively. Although the IEEE 802.3 (CSMA/CD) MAC protocol can work in a tree-based architecture, it cannot be applied efficiently because HFC networks have high propagation delays and users are unable to directly detect the transmissions of other users. Consequently, the IEEE 802.14 committee has been set up to define a new MAC protocol for the HFC networks

This work was supported by the Natural Sciences and Engineering Research Council of Canada under grant OGP0044286.

[1],[2], and a number of MAC protocols have been proposed [3],[4],[5]. In general, the current proposals focuses on supporting asynchronous data services. The objective of this paper is to address the integration of CBR, VBR and ABR services over HFC networks. Motivated by the PRMA [6] and DRMA protocols [7], we propose a new Dynamic Reservation (DR) protocol for integrating CBR, VBR and ABR services over HFC networks. This protocol allows a VBR source to acquire isochronous channels by capturing mini-slots. To enhance bandwidth utilization, both the capturing probability and the number of mini-slots per frame are varied dynamically. ABR stations acquire data slots using random access and piggybacks data slot requirements in the transmitted data packets in order to minimize access contention. We propose an effective method to ensure that ABR traffic can only be sent over the residual bandwidth of the CBR and VBR traffic. The rest of the paper is organized as follows. Section 2 gives the basic architecture of HFC networks. Section 3 explains the DR protocol. Section 4 presents the simulation results and discussions. Section 5 gives the conclusion. 2. ARCHITECTURE OF HFC NETWORKS The basic architecture of a HFC network, as shown in Fig. 1, includes 50-2000 stations in subscriber premises connected to the HC in a tree topology. For performance analysis purpose, an abstraction of the HFC network as shown in Fig. 2 is often used. With a coverage area of around 80-100 Km in diameter, the signal propagation delay from the HC to a station can be as high as D = 200 µs (i.e., a round trip delay of 400 µs). Signals broadcasted from HC to the subscribers are first sent as optical signals through fibre links to fibre nodes, where the optical signals are converted to electrical signals before distributing to the subscribers via coaxial cables. The paths from the HC to the subscribers and subscribers to HC are known as the downstream paths and the upstream paths, respectively. In both types of paths, unidirectional amplifies are installed at each station to restrict the flow of information in the appropriate direction only. Due to these unidirectional amplifiers, it is

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Figure 1. Architecture of HFC network impossible for the stations to communicate directly with each other; instead, all communications between stations must pass through the HC. As the HFC networks are originally designed to support high quality television broadcasts from the HC to subscribers, the bandwidth of the downstream path is very abundant (54-750 MHz). The spectrum of the current televisions channels are between 54 and 450 MHz, so that approximately 300 MHz of bandwidth is available for introducing new services to subscribers. In the upstream path, the bandwidth (5-40 MHz) is relatively narrow, so that communications from the stations to the HC are very limited. The main purpose of the IEEE 802.14 standard is to develop efficient MAC protocols for providing various communication services to subscribers while utilizing this upstream bandwidth efficiently. Due to various operational reasons (e.g., ease of management, power constraints), the upstream and downstream bandwidths are further divided into 2 MHz and 6 MHz channels, respectively. By employing different modulation techniques, each upstream and downstream channel can support a data rate of about 2--10 Mbps and 30--40 Mbps, respectively. This paper addresses the access protocol for providing CBR/VBR/ABR services over an upstream channel of B bps. To provide various communication services to subscribers, a special cable modem is installed in each user’s premise. Functioning as an intelligent device to bring multimedia communication services to residential subscribers, this modem is responsible for signaling interaction with the HC for various call management functions such as connection establishment and disconnection, and while a connection has been established, converting analogue signals into packets (cells) for transmission over the HFC network by means of the DR protocol described below. To extend communications over a wide area, the HC will be connected to existing

Figure 2. Abstraction of HFC network public networks, other HFC networks, and the emerging ATM-based broadband integrated services digital networks (BISDNs) through appropriate interworking units. As information transfer from the HC to the stations is relatively simple due to the abundant bandwidth in the downstream path and its broadcast nature, we shall focus on the protocol design in the upstream path, which is also the main concern of the IEEE 802.14 standard. 3. THE DYNAMIC RESERVATION PROTOCOL Each upstream channel is slotted and framed. A slot consists of a 48-byte payload, consistent with that of an ATM cell, and a 6-byte header, based on the header size proposed for the IEEE 802.14 standard. The above slot format facilitates internetworking with ATM networks as an ATM cell can be easily formed at the HC by removing the 6-byte header and inserting the corresponding 5-byte ATM cell header. Each slot payload can either hold a packet, or be divided into 6 mini-slots of 8 bytes each for carrying reservation request from the stations to the HC as explained later. Based on the traffic situation, the HC broadcasts/piggybacks the slot-type of each upstream slot (i.e., whether it is a reserved slot or a slot of mini-slots) via the downstream channel. Details of the slot allocation method will be explained later. Note that the timing of the broadcast has to take into account the propagation delay. If slot s of frame k becomes effective at t for the station furthest away from the HC, the HC needs to broadcast the status at t-D, where D is the maximum propagation delay, in order to ensure that all stations receive the message. CBR and VBR stations are assumed to have quantized bit rates of 64 Kbps. A periodic frame structure with a frame duration of 6 ms is employed such that when a VBR station increases its bit rate by 64 Kbps, an additional slot (48 bytes) per frame is required. We assume that each station can synchronize its upstream transmissions with those of other stations (e.g., using the ranging process in [4]) such that they appear to be situated at the same distance from the HC.

To provide multi-media communication services to home users, HFC networks need to support CBR, VBR and ABR services. We first describe the protocol operation for each type of service separately before presenting the method for integrating them over HFC networks. CBR station protocol Due to the use of a 6 ms frame, CBR services, which emulate existing circuit switching services, can be supported easily. During call setup, a CBR station simply requests the HC to reserve the required number of slots per frame. This ensures isochronous transport service with bounded access delay and access delay variations. VBR station protocol VBR services allows a station to vary its bit rate to enable statistical multiplexing. In general VBR services can be further divided into real-time and non-real-time. In this paper, we consider that all VBR services are real-time in nature and assume that each VBR station can be modeled by an on-off traffic source. Each VBR source alternates between active and idle states. The active and idle periods are independent and exponentially distributed with means

frame. •

The capturing probability pv is adjusted dynamically based on the following equation Nv – Nr – 1

pv =

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Nv – Nr – 1 – i 1 N – N r – 1 i -----------  v r (1 – r ) 1 + i  i

where Nv is the number of VBR stations, Nr is the number of reserved slots, and r is the activity factor for a VBR station. The formula for the capturing probability is explained as follows. It is not difficult to show that if a station is active and there are i other capturing stations, the station should use a capturing probability of 1/(1+i) in order to maximize the chance of success. Unfortunately neither the HC nor the stations know the exact value of i. However, given that there are Nr stations with reservations, the probability that there are i other capturing stations is N – Nr – 1 – i N – Nr – 1 i  v  r (1 – r ) v   i

1 1 of --- and --- , respectively, for an activity factor of r. β

α

During an active period, a VBR source generates a burst of M 48-byte packets every frame. The packets are stored in a buffer before transmission. The buffer size is M packets so that if a packet cannot be transmitted within one frame, it will be overwritten by a new packet and hence lost. This gives a bounded access delay of 6 ms for all successfully transmitted packets. A VBR station does not transmit any packet during idle periods. When a VBR station starts an active period, it first acquires a mini-slot in the upstream channel to signal the HC that it wants to reserve M slots in the upstream direction. Contention will occur if more than one stations capture a mini-slot, in which case the mini-slot is wasted. If the HC successfully receives a captured minislot, the HC registers the slot request in a queue and grants available slots to the VBR stations in each frame accordingly so that isochronous transport service (i.e., zero time jitters) can be provided. When a VBR station completes its active period, it informs the HC to release the reserved slots, through its last packet transmission. To enhance bandwidth utilization, we propose that both the request bandwidth (i.e., the number of mini-slots per frame) and the capturing probability be adjusted dynamically as follows. •

The number of mini-slots per frame is adjusted dynamically by turning any un-reserved slot to carry 6 mini-slots in the payload. This follows a similar concept as proposed in [6] where mini-slots float within a

noting that an active station without a reserved slot is a capturing station. By taking into account all i, the average capturing probability an active station should use can then be found by the above equation. ABR station protocol ABR services are designed to support asynchronous data applications such as file transfer and email through the residual network bandwidth. The station protocol used in this paper is based on the protocols proposed in [3][5]. Our major contribution is to propose an effective method for integrating the ABR station protocol with the above VBR station protocol. Basically when a data station becomes active, it first captures a mini-slot with probability pd=0.2. In the mini-slot, it informs the HC its slot requirement based on the content of its buffer. Upon receiving a successful mini-slot, the HC registers this slot requirement in a first-come-first-served queue and allocates the slots accordingly based on the requests in the queue. In all subsequent packet transmissions, the data station also piggybacks the additional slot requirements. By using this piggybacking method, a data station does not need to contend for slots for all data cells so that bandwidth efficiency can be enhanced. To provide ABR service, the HC disables the data transmissions whenever it detects that there are capturing VBR stations in the network by the following method. Service integration We now discuss how to provide integrated services. As it

•

RI = 1 - Assign the slot as mini-slots for the VBR stations only,

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RI = 0 and data request queue is not empty - Assign the slot for the next ABR station in the data request queue,

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RI = 0 and data request queue is empty - Assign the slot as mini-slots for all (VBR and ABR) stations.

All stations monitor the status of the next upstream slot via the downstream channel and then apply the above station protocol accordingly. 4. SIMULATION RESULTS AND DISCUSSIONS To analyze the performance of the DR protocol, we have conducted simulations using Simscript II.5. We consider the worst case scenario where all stations have a propaga-

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utilization of the system. A slot is defined as utilized if it carries a voice packet in its payload. Note that while access delay and delay jitter are also important performance indicators for voice traffic, the DR protocol already gives a bounded access delay of 6 ms and offers isochronous service (i.e., zero delay jitters) once a reservation is secured. That means PLR and utilization are the main performance indicators. Fig. 3 shows the PLR of the DR protocol in comparison to that of the ideal DR protocol, which employs ideal values for the capturing probability. Results indicate that the DR protocol and ideal DR protocol can support about 155 and 157 voice stations, respectively, while maintaining a target PLR of less than 1% [9]. Therefore the DR protocol gives performance close to that of the ideal DR protocol. Fig. 4 compares the slot utilization of DR protocol and ideal DR protocol. It can be seen that the two results are quite close. In the case of the DR protocol, the slot utilization 100

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We first consider a HFC network with only voice stations. We evaluate the packet loss ratio (PLR) and slot

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M=1, --- =1.5 s and --- =2.25 s [6]. In general, this bi-state model simulates the process of requesting an incremental bandwidth of 64 Kbps and can be adapted for representing video traffic [8]. Note that because of the tree topology and the ranging process [4], access fairness is not a concern in HFC networks. For the ABR traffic, we consider that the data packet inter-arrival time follows an exponential distribution and each data packet consists of 8 cells. The size of each data station buffer is 64 cells. In each simulation, 1000,000 frames were sent. Results are discussed as follows.

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tion delay of 200 µs from the HC. As an example of VBR traffic, we consider the bi-state voice model with 1 β

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is straightforward to incorporate CBR service, we focus on the integration of VBR and ABR traffic. Unfortunately the above VBR and ABR station protocols cannot be integrated directly. The problem is that if the data traffic is heavy, the HC will grant nearly all slots to the data stations based on the data request queue and allocate very few mini-slots for the voice stations. To give priority access to the VBR traffic, we propose that a special minislot called the request-mini-slot be made available at the beginning of each slot. Each capturing VBR station acquires this request-mini-slot with probability one by setting all its bits to one. If the HC receives an idle request-mini-slot (i.e., no capture), it sets a request indicator (RI) to zero, otherwise it sets the RI to one. The RI is used indicate whether there are capturing VBR stations in the network. If so, ABR traffic is switched off until there are no more capturing VBR stations detected in the network. Based on the traffic situation and the RI, the HC assigns an upstream slot which has not been reserved by the VBR and CBR stations as follows:

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Figure 7. Data access delay for different aggregate data rates

is about 90% when the number of voice stations is 155 (i.e., the maximum number of voice stations as found above). This indicates that the protocol is very efficient.

bandwidths considered above. Results show that for voice bandwidths of 5 Mbps and above the curves are very close, indicating that the improvement in utilization due to higher voice bandwidth is not significant for voice bandwidths exceeding 5 Mbps. In these cases,

Fig. 5 compares the PLR of DR protocol for five different sets of voice channel bandwidth. To facilitate the comparison, the number of voice stations (x-axis) is normalized with respect to the number of voice stations that can be supported by full statistical multiplexing. That means N vn =1 is the ideal target. From the graph, it can be seen that the normalized number of voice stations that can be supported by a 1, 3, 5, 7 and 9 Mbps voice channel are 0.73, 0.86, 0.89, 0.91 and 0.92, respectively. This number approaches the ideal target of 1 when the voice bandwidth is increased. Fig. 6 compares the slot utilization of the DR protocol for the five different voice 100

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about 90% utilization can be achieved for N vn =0.9, which corresponds to the number of voice stations that can be accommodated while meeting the clipping probability criteria. Fig. 7 shows the access delay of the data cells for different aggregate data rates. The aggregate data rate is obtained by varying the packet inter-arrival time accordingly. We assume that the slot size is increased to 55 bytes due to the inclusion of a request mini-slot. The figure shows that an aggregate data rate of 0.1 Mbps can be well supported. Below 0.1 Mbps, it was found that no packet was lost during the simulations indicating that the system was quite stable. We have also evaluated the PLR of the voice stations for different aggregate data rates. It is found that the PLR stays almost constant and is close to the PLR achieved by the above voice-only system for the same number of voice stations. This shows the desirable result that time-sensitive VBR traffic is virtually unaffected by non-time-sensitive ABR traffic. Fig. 8 shows the slot utilization for different aggregate data rates. The interesting shape of the curve can be explained as follows. At low data rate, the slot utilization increases relatively slowly because of the bandwidth wastage caused by random access. At around 0.1 Mbps, the curve starts to increase sharply. This is because when data rate is high, each data station buffer is full of packets most of the time. In this case, the slot assignments are governed by the piggyback request rather than random access. That means slots not utilized

traffic is virtually unaffected by the amount of ABR traffic in the network.

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[1] J. W. Eng and J. F. Mollenauer, “IEEE project 802.14: standards for digital convergence,” IEEE Comm. Mag., vol. 33, May 1995.

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[2] IEEE Standards 802.14 Committee, IEEE Std. 802.14 Draft2R1, Jun. 1997.

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[3] D. Sala and J. O. Limb, “A protocol for efficient transfer of data over fiber/cable systems,” Proc. Infocom’96, San Francisco, pp. 904-911, Mar. 1996.

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Figure 8. Utilization for different aggregate data rates by the voice stations are likely to be filled by data cells causing the sharp increase in slot utilization. The curve flattens off very close to 100% utilization indicating that the integration method is very efficient. 5. CONCLUSIONS We have presented a DR protocol for integrating CBR, VBR and ABR services over HFC networks. This protocol allows VBR stations to capture isochronous channels while providing statistical multiplexing. It enhances bandwidth utilization by dynamically adjusting both the capturing probability and the request bandwidth, given by the number of mini-slots per frame. ABR stations capture data slots through random access and piggyback data slot requirements over the transmitted packets to minimize access contention. By using request mini-slots, VBR stations can claim access priority over ABR stations such that ABR traffic is only transmitted over the residual bandwidth of the CBR and VBR traffic. Based on the traffic situation, the HC allocates bandwidth dynamically for the integrated traffic. Simulation results indicate that the protocol performance is close to the ideal result, the protocol is efficient in terms of bandwidth utilization and the performance of the VBR

[4] C. Bisdikian, B. McNeil, R. Norman and R. Zeisz, “MLAP: A MAC level access protocol for the HFC 802.14 network,” IEEE Comm. Mag., vol. 34, pp. 114-121, Mar. 1996. [5] M. Ivanovich, M. Zukerman and R. G. Addie, “Performance investigation into an IEEE 802.14 MAC protocol for HFC networks,” Proc. Globecom’97, 1997. [6] D. J. Goodman, R. A. Valenzuela, K. T. Gayliard and B. Ramaurthi, “Packet reservation multiple access for local wireless communications,” IEEE Trans. on Comm., vol. 37, pp. 885-890, Aug. 1989. [7] X. Qiu and V. O. K. Li, “Dynamic reservation multiple access (DRMA): A new multiple access scheme for personal communication system,” ACM/Baltzer Wireless Networks, vol. 2, pp. 117128, 1996. [8] B. Maglaris, D. Anastassiou, P. Sen, G. Karlson and J. Robbins, “Performance models of statistical multiplexing in packet video communications,” IEEE Trans. on Commun., vol. 36, pp. 834-844, Jul. 1988. [9] J. G. Gruber and N. H. Le, “Performance requirements for integrated voice/data networks,” IEEE J. Select. Areas in Commun., vol. SAC-1, no. 6, pp. 981-1004, Dec. 1983.