MULTIPLE
PRIORITY
DISTRIBUTED
ROUND ROBIN MAC PROTOCOL
FOR SATELLITE
ATM
William M. Shvodian Lockheed Martin Federal Systems 9500 Godwin Drive Buildingl 10/012 Manassas, VA 20110 E-mail: bill.
[email protected] ABSTRACT This paper
introduces
Multiple
Priority
Distributed
Round
Robin (MPDRW, a new Media Access Control (MAc) or Demand Assignment Multiple Access (DAMA) protocol for traffic with more than one priority level. It was designed for, but not limited to, use in geostationa~ (GEO) broadband communications satellites using ATM or similar fast packet switched transport with Multi Frequency – Time Division Multiple Access (MF-TDM) uplinks. This protocol provides a robust mechanism for fair, c$mamic allocation and assignment of uplink slots to user terminals with multimedia traffic. OPNEF~ simulation results show that MPDRR provides flexible, eficient access for bursty trajlc in a geostationa~ satellite system with MF-TDMA uplinks. 1
INTRODUCTION
A new generation of broadband communications satellites, which will employ spot beams and digital on board processing and fast packet switching, is currently being planned and developed. Today’s technology will give these satellites greater capabilities than previously possible. The satellites will be able to provide service to a large number of small inexpensive terminals. The desire for low power terminals with small antennae makes MF-TDMA attractive for the satellite uplinks, while the downlinks will likely use Time Division Multiplexing (TDM). These satellites will provide users with services including voice and video telephony, video conferencing, LAN interconnection, telemedicine, and Internet access. Many of these satellite systems will use ATM because of its ability to provide transport for various types of traffic with guaranteed Quality of Service (QoS). ATM originally was designed for point to point terrestrial links with no need for a MAC layer to share bandwidth. However, a MAC protocol is required to allow a large number of satellite terminals with bursty traffic to efficiently share limited bandwidth. Most existing satellite MAC protocols are inappropriate for ATM traffic. For instance, circuit switched DAMA protocols do not provide the flexibility needed for bursty traffic. Random access protocols are inappropriate for satellite ATM because they cannot provide necessary QoS and they limit throughput.
Even though ATM MAC protocols have recently become an issue for Wireless ATM and Residential Broadband Access, the characteristics of the satellite environment require the development of a unique MAC protocol. For instance, a satellite ATM MAC needs to be compatible with propagation delays that are much greater than the delay in terrestrial Wireless ATM. The delay poses an even greater challenge for satellites in geostationary orbit than for those in Low Earth Orbit (LEO) or Medium Earth Orbit (MEO). Satellites also have mass and power limitations that restrict the amount of centralized on-board processing. Additionally, the use of MFTDMA satellite uplinks imposes unique challenges on the design of the satellite MAC protocol. Several MAC protocols have been proposed specifically for satellite ATM, but the methods for making MF-TDMA slot assignments have not been addressed in the literature. In a TDMA system, any slot can be assigned to any terminal. In a MF-TDMA system, a terminal can only transmit on one frequency at a time in order to maintain the advantages of using MF-TDMA. This limits flexibility in making slot assignments and increases the complexity of the slot assignment processing. Since a slot assignment scheme could require significant processing either on-board the satellite or in the terminals, it is an issue that needs to be addressed. This paper gives a brief overview of some proposed ATM MAC protocols and then presents MPDRR, a candidate satellite MAC protocol for ATM traffic that provides a computationally efficient scheme for allocating and assigning MF-TDMA slots. 2
PROPOSED
SATELLITE PROTOCOLS
ATM MAC
While several satellite ATM MAC protocols have been proposed, a recent paper concludes that new satellite multiple access schemes are necessary [1]. This section discusses some of the proposed ATM MAC protocols. 2.1
CFDAMA
The Combined Free/Demand Assignment Multiple Access (CFDAMA) protocol has been described by Le-Ngoc, et al. [2], [3]. Uplink slots are assigned first based on DAMA requests, and the remaining slots are freely assigned according to an CFDAMA can work with centralized allocation strategy. control, either on board the satellite or on the ground.
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CFDAMA can also reportedly work in a distributed mode where the terminals listen to all of the requests and use an algorithm to calculate their assigned slots. While it is possible that CFDAMA could work in a MF-TDMA system, these papers do not describe how the MF-TDMA slots would be assigned. 2.2
Distributed Access Protocol for an A TM User-Oriented Satellite System
Priscoli, et al, [4] describe a satellite MAC protocol that separates the bandwidth into slots assigned statically on a call basis and slots assigned dynamically on a per-cell basis each frame. This allocation scheme allows for mixing continuous stream oriented traffic that requires guaranteed Quality of Service (QoS) with bursty packet oriented traffic. This protocol works well for a Time Division Multiple Access (TDMA) system, but does not appear to be compatible with a MF-TDMA system because of the statically assigned slots. 2.3
Hierarchical
Round Robin
Hung, et al., [5] propose a satellite ATM MAC layer compatible with MF-TDMA uplinks. A centralized scheduler uses a scheduling algorithm called Hierarchical Round Robin to allocate the MF-TDMA uplink slots. Their paper describes an algorithm for allocating the number of slots to each terminal, but the issue of assigning the MF-TDMA slots is not covered. 2.4
Round Robin Reservation
DAMA
Kota and Kallaus [6] proposed the Round Robin Reservation DAMA protocol. This protocol supports TDMA uplinks, but not MF-TDMA links. 3
MPDRR
DESCRIPTION
Our goal is to develop a satellite MAC protocol compatible with MF-TDMA uplinks that allows for efficient, fair sharing of the uplink bandwidth by multiple priorities of traffic, while minimizing delay and imposing reasonable processing requirements on board the satellite and in the terminal. The following is a description of the MPDRR protocol. 3.1
Uplink Frame Structure
MPDRR uses an uplink frame/superfiame structure. A data frame consists of N ATM cell slots. A superframe consists of There are F M data frames and one overhead frame. frequencies that the terminals can use on the uplink. The total number of data slots to be allocated is N x F slots. The slot assignments are the same for all M data frames of the superframe. We assume that in a MF-TDMA system, the terminals will be able to transmit on more than one fi-equency during a flame, but only on one frequency during a single slot time. In other words, slots assigned to a terminal must not overlap in time.
Each active terminal is assigned a fixed request slot in the overhead flame. Requests for data slots are made every superframe in the terminal’s request slot. FREQUENCIES f-l
E!
f3
f4
slot 1 slot 2 slot 3 slot 4 slot 5 Slot6 slot 7 Slot8 slot 9 SlotN
F R A M E
[[[[[
Figure 1 MF-TDMA Data Frame Structure 3.2
Prioritization
of Traffic
The MPDRR protocol provides bandwidth allocation based on one or more levels of priority or QoS. Each type of ATM traffic, Constant Bit Rate (CBR), Variable Bit Rate – real time (VBR-rt), Variable Bit Rate - non real time (VBR-nrt), Available Bit Rate (ABR), Unspecified Bit Rate (UBR) maybe assigned a separate priority level. Guaranteed traffic like VBR Sustainable Cell Rate (SCR), ABR Minimum Cell Rate (MCR) and UBR+ Guaranteed Rate traffic may each have a separate priority. A simpler option may be to use only two levels of priority. The higher priority would be used for guaranteed rates like real-time voice or video, VBR SCR, ABR MCR or UBR+ guaranteed rate. Lower priority requests would be for nonguaranteed bursty traffic. The best solution may lie somewhere in between with 3 or 4 levels of priority. 3.3
Slot Requests
Each terminal communicates its current bandwidth needs by making a request for slots in the overhead frame. The requests contain explicit number of slots requested for each of the levels of priority. Slots are allocated based on request priority. The satellite will gather slot requests and transmit them down to the terminals via the downlink. Slot requests will be identified either by position in the downlink overhead fi-ame or by explicit labeling such that each terminal knows how many slots it requested and how many slots were requested by each of the other terminals. 3.3.1
Request Calculation
Each terminal must calculate the number of slots to request for each priority level once per superframe. The algorithm that the terminal uses to determine the number of slots to request will depend on whether the traffic is real-time or non-real-time, and whether the satellite is LEO, MEO or GEO.
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While a LEO or MEO satellite may be able to dynamically change the bandwidth allocation for real-time traffic, the propagation delay for a GEO satellite may prohibit dynamic bandwidth allocations for real-time traffic regardless of the MAC protocol. The 270 ms or more to request a dynamic bandwidth change on top of the 270 ms propagation delay for the actual traffic will probably not be acceptable for real-time applications like voice. Most likely, the terminal will need to request a fixed amount of bandwidth continuously for the realtime traffic in a GEO system. For non-real-time traffic, terminals request slots based on the number of cells queued minus the number of unused cell slots (allocated slots times remaining frames). If there are more cells queued than cell slots allocated in the current superframe, the terminal requests enough slots to service the remaining queued cells. This algorithm was chosen to minimize the amount of wasted slots. Each terminal must request less than or equal to N total slots, since N is the number of slots in a fi-ame. The slots requested at the start of a superframe are received by the terminals and processed according to the allocation and assignment algorithms. The new slot assignments take effect at the start of the next superframe. 3.3.2
Request Identification and Ordering
Terminals receive their requests and the requests of all the other terminals in the downlink overhead frame. Since MPDRR requires coordinated terminal request ordering for slot allocation and assignment, a method is required to allow the terminal to identify which requests belong to which terminals and to order the requests of all of the terminals. One way to coordinate the ordering is to identifi requests for each terminal based on an assigned position of the request in the downlink overhead frame, but this may unnecessarily complicate the Another way to facilitate request satellite processing. identification is to assign each active terminal a virtual terminal number, which will be included in the slot request message. Terminals can use the virtual terminal numbers to sort the requests that are received on the downlink, then allocate and assign slots in order based on the virtual terminal numbers. In order to perform the MPDRR allocation and assignment calculations, each terminal must know exactly how many terminals are active to make sure all requests are received. To keep all terminals in sync, the satellite must broadcast the number of active terminals. Because of transmission errors, error detection and correction must be used to minimize errors in the slot requests. If an uncorrectable error occurs in a request slot on the uplink, the satellite will replace the downlink message with zero (or a default value) to avoid ambiguity. A terminal that receives an uncorrectable request message for any of the incoming slot requests on the downlink (ffom the satellite) will not be able to calculate the slot assignments correctly. In order to avoid
collisions, such a terminal is not allowed to transmit data for the entire next superframe. This makes the minimization of slot request errors crucial. A more powerful code may be needed for the request messages than for the ATM traffic. 3.4
Slot Allocation
and Assignment
Every terminal processes all requests. Slots are allocated and assigned based on the requests. The slot allocation algorithm determines how many slots each terminal will get. The slot assignment algorithm resolves which of the slots (frequency and position in the frame) are assigned to each terminal. In MPDRR, the number of slots allocated to each terminal is first calculated. For each slot that is allocated and assigned to a terminal, the terminal can transmit M cells, since there are M data frames in a superframe. All slots are reallocated and reassigned every single superframe. One benefit of this process is that terminals can quickly synchronize because state information is not carried from one superframe to the next. Also, reassigning all slots every superframe simplifies the MF-TDMA slot assignment. 3.4.1
Slot Allocation
The algorithm for allocating slots is similar to the Distributed Round Robin algorithm. Each terminal receives its requests and the requests of the other terminals and executes an algorithm to determine the slot allocations. The difference is that MPDRR allows more than one request priority level and multiple frequencies. The slots are allocated to terminals in order of request priority. The total number of slots to be allocated is known by each terminal to be N x F. Slots are fust allocated to all priority one requests, then priority two requests and so on. If there are more requests for a given priority level than available slots, slots are allocated one by one in a round robin manner until there are no remaining slots. If the total number of requests for a given priority is less than the remaining available slots, each terminal is allocated the number of requested slots and the number of available slots is reduced accordingly. Remaining slots are assigned to the next lower priority. Once the number of slot allocations equals N x F, all of the slots have been allocated. If the total number of slots requested by all terminals is greater than N x F, some of the slot requests will go unfulfilled. These requests are discarded instead of being queued so that state information is not carried over from one superfiame to the next. This keeps terminals from getting out of sync. If the total number of slots requested at all priority levels is less than (N x F), there will be extra slots remaining after all requests are serviced. In order to use the uplinks efficiently, extra slots are assigned to the terminals according to a chosen strategy. The algorithm used for assigning these slots will affect system performance. Slots may be allocated fairly to all of the terminals, or allocated only to the terminals with active requests
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in the current superframe. No more than N slots can be allocated to a single terminal since each terminal can only transmit on one frequency at a time. 3.4.2
Slot Assignment
After the number of slots allocated to each terminal has been calculated, the actual frequency and time slot assignments are made. Each terminal is assigned the number of slots that have been allocated to it. We chose a simple assignment algorithm that limits a terminal’s data slot assignments to at most two frequencies in an effort to simplify terminal design and minimize processing requirements. The request slot may be in a third frequency because it is a fixed assignment. Data slot assignments for a terminal will be contiguous in time, but because slots are re-allocated and reassigned every single superframe this protocol does not incur the fi-agmentation inefficiencies described in [7]. During any slot interval, a terminal will only transmit on a single frequency. Slot position and tlequency assignments can and will change from superframe to superframe. The first terminal is assigned the number of slots that it has been allocated, starting from Slot 1 of Frequency 1, then Slot 2 of Frequency 1, up to it’s total allocation. Next, Terminal 2 is assigned the next slot in Frequency 1 if the first terminal was assigned less than N slots. Once all of the slots in Frequency 1 are assigned, slots are assigned in Frequency 2 and so on. 4
MPDRR
PPOTOCOL
DETAILS
rate (128 kbps), mean burst length (16250 bytes), activity factor (.1) and the equations used to determine mean on time (1.01s) and mean off time (9. 14s) were derived from [8]. The number of terminals was incremented in steps to increase the uplink utilization. Simulations were performed with one, two, four and eight 384 Kbps MF-TDMA uplink channels. In the simulation model each frame consists of twenty-four slots, and the superframe contains eighteen data frames and one overhead (request) frame. The superframe duration is 423 ms. The superframe size was chosen to allow for 270 ms of propagation for the requests, plus time for processing before the start of the next superframe. This superframe structure allocates 5.3~0 of the uplink bandwidth to the request overhead frame. In this frame structure, one slot per data frame provides 16 Kbps of bandwidth for AALl traffic. A second superframe of 305.5 ms was simulated to study the impact of a shorter superframe. This second superframe structure consisted of 12 data frames and one overhead frame, with l’.’7°/0 overhead. 5.2
AND OPTIONS
3
The author has documented many protocol details and options. They are available upon request. 5
MPDRR
MPDRR Simulation Results
Figure 2, Figure 3 and Figure 4 show delay versus utilization for MPDRR with varying numbers of uplinks shared by terminals. The figures clearly show the benefit of MPDRR’s ability to share several channels instead of locking terminals to one channel when the traffic source is bursty.
2.5 -+-1
channel
E
--w-2 channel
SIMULATION
--+4
An OPNETTM model was created to simulate the MPDRR protocol and to study its performance for bursty traffic. In a satellite system with MF-TDMA uplinks, it would be possible to restrict a group of terminals to a single uplink frequency. If the capacity of an individual channel is sufficiently high relative to the bandwidth requirements of the terminals, satisfactory performance may be achieved by treating each of the MF-TDMA channels as a separate TDMA link servicing a given number of terminals. However, in a system with MF-TDMA channels with capacity as low as 128 Kbps to 384 Kbps, the bursty nature of the traffic would cause large delays and inefficient use of the uplink bandwidth if the terminals were assigned to a single uplink channel in this manner. OPNETTMsimulation results show how sharing uplink channels allows MPDRR to provide lower delay for bursty traffic. Model Inputs 5.1 Two state MMPP sources were created to generate bursty ATM cell traffic with an activity factor of 0.1. The values for peak bit
-x-
0.5
channel
8 channel
0
~
o
1
0.5
1
Utilization
Figure 2 MPDRR with extra slots assigned to all terminals Figure 2 shows the delay performance when the extra slots are assigned fairly to all terminals. The mean delays are lower in Figure 3 where extra slots are assigned to active terminals first. We have implemented two different methods for allocating extra slots to active terminals. First we allocated all of the possible slots to the terminals with any active requests. This worked fairly well for bursty traffic, but worked poorly for less bursty sources. The active terminals would be assigned all of the slots while starving the terminals with no current cells queued. We improved the performance by allocating up to two times the number of slots that a terminal requested (instead of
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
up to N). This resulted in more stable performance and improved delay versus utilization, especially when utilization is below 80%. Figure 3 shows the results for active terminals assigned up to two times the requested slots. We speculate that allocating up to two times the requested slots may coordinate well with the slow start behavior of TCP, but this requires further testing. 3 2.5 +
2 > ; n
+2 1.5
1 channel channel
-A- 4 channel
1
+8 r
The low delay at 10’%utilization is due to the terminals always having enough spare slots assigned to them that they never have to wait for slots to be assigned when they get a burst. 6
CONCLUSIONS
An efficient, flexible, responsive MAC protocol is required for transporting multimedia traffic over satellites. Existing satellite MAC protocols have not been optimized for MF-TDMA uplinks, and most require centralized processing. MPDRR has been presented in this paper as a potential MAC protocol for satellite ATM using distributed request processing to provide multiple access to traffic with multiple levels of priority. The simulation results show the benefit of allowing terminals with bursty traffic to share multiple MF-TDMA uplink channels and the impact of superfiame duration on delay.
channel
0.5 0
0
0.5
1
Utilization
Figure 3 MPDRR with extra slots assigned to active terminals Decreasing the duration of a superframe reduces the mean delay, but will increase the overhead by raising the ratio of overhead ffames to data frames. Reducing the superframe duration from 423 to 305.5 ms produces the delay versus utilization shown in Figure 4. The reduced delay needs to be traded against the increased overhead of the smaller superframe. 2.5 2 +
1 channel
-w-2
channel
--A-4
channel
+8
channel
0.5
0.5
MILCOM ’94 [7] S. Kota, J. Kallaus, H. Huey, D. Lucantoni, “Demand
Assignment Multiple Access (DAMA) for Multimedia Services – Performance Results,” A41LCOM ’97 [8] A. Baiocchi, N. B. Melazzi, M. Listanti, A. Roveri, R. Winkler, “Loss Performance Analysis of an ATM Multiplexer Loaded with High-Speed ON-OFF Sources,” IEEE Journal on Selected Areas in Communications, Vol. 9, No. 3, April 1991
0
0
[I] I.F. Akyildiz, S.-H. Jeong, “Satellite ATM Networks: A Survey,” IEEE Communications Magazine, July, 1997. [2] J. I. Mohammed, T. Le-Ngoc, “Performance Analysis of Combined Free/Demand Assignment Multiple Access (CFDAMA) Protocol for Packet Satellite Communications,” ICCC ’94, pp. 869-873 [3] T. Le-Ngoc, S.V. Krishnamurthy, “ Performance of Combined Free/Demand Assignment Multiple Access (CFDAMA) With Pre-Assigned Request Slots in Integrated Voice/Data Satellite Communications”, International Conference on Communications (ICC), Seattle, June 1995. [4] F. D. Priscoli, M. Listanti, A. Roveri, A. Vernucci, “A Distributed Access Protocol for an ATM User-Oriented Satellite System,” Proceedings of the ICC ’89, June 1989, paper 22.1. [5] A. Hung, M.-J. Montpetit, G. Kesidis, “ATM via Satellite: A Framework and Implementation,” Wireless Networks, Vol. IV, No. 2, 1998 [6] S. L. Kota, J. D. Kallaus, “Reservation Access Protocol for Multiplanar ATM Switched Satellite Network (MASSNet),”
1
Utilization
Figure 4 MPDRR with extra slots assigned to active terminals and 305.5 ms superframe These delay versus utilization curves are for bursty data traffic. Delay sensitive applications like real-time voice will constantly be allocated slots and will only incur propagation delay, not queuing delay. Note that the delays shown in all figures only include queuing delay in the terminals. They do not include uplink propagation delay, the on-board queuing delay or the downlink propagation.
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