underlying multiple access scheme. The majority of satellite access schemes have been designed to support video and telephony. Internet traffic is highly bursty ...
IMPROVED MEDIUM ACCESS CONTROL FOR DATA TRAFFIC VIA SATELLITE USING THE CFDAMA PROTOCOL P. D. Mitchell, T. C. Tozer, D. Grace
Abstract This paper explores an efficient Medium Access Control protocol for carrying data traffic via satellite. The Combined Free/Demand Assignment Multiple Access (CFDAMA) protocol combines free assignment of data slots with demand assignment, providing a minimum delay of one round trip time at low loads and the statistical multiplexing gains of demand assignment at high loads. The delay/utilisation performance of CFDAMA is explored for both Poisson and selfsimilar traffic models. It is shown for Poisson traffic that CFDAMA provides both extremely low end-to end delay and variance in delay of packet transmissions over a wide range of loads up to 85% of the channel capacity. For self-similar traffic, it has been found that the scheme is limited to a lower maximum channel loading, but performs better at low channel loads. The importance of an accurate traffic model in network performance evaluation is highlighted. Comparisons are made between CFDAMA and the Slotted ALOHA protocol, and it is shown that CFDAMA outperforms Slotted ALOHA over virtually the entire range of channel utilisation.
Introduction Satellite systems are likely to carry an increasing proportion of multimedia traffic including Internet based applications, as well as continuing to provide global communications services such as broadcast television, video services and telephony. Currently satellite bandwidth is a limited and expensive commodity that must be used efficiently to support a large number of users and provide high returns for the service provider; efficient bandwidth utilisation is governed by the underlying multiple access scheme. The majority of satellite access schemes have been designed to support video and telephony. Internet traffic is highly bursty in nature and requires the development of new Medium Access Control (MAC) schemes to effectively support it. This paper examines the performance of one proposed scheme, the CFDAMA protocol, for carrying data traffic via satellite and compares it with the Slotted ALOHA scheme. The scenario of interest is shown in figure 1 below. GEO Satellite with OBP Slot Data Allocations IGSàVSAT
Multiple Access Uplink Channel Space Segment Ground Segment
Data VSATàIGS
+
Slot Requests Internet Gateway Station (IGS)
300 Ground Stations (VSATs)
Figure 1 – The Satellite Scenario Communications Research Group, Department of Electronics, University of York
The VSATs communicate with the Internet Gateway Station (IGS) via an on-board processing satellite. The outbound link (IGSàVSAT) is a broadcast channel (one to many), and is of little interest here. The inbound link (VSATàIGS) consists of a multiple access uplink channel (many to one) that will employ a MAC protocol such as the CFDAMA or Slotted ALOHA protocols investigated in this paper.
Overview of Medium Access Control (MAC) Medium Access Control is a software function and corresponds to the lower half of layer two of the ISO-OSI reference model. The MAC layer regulates and controls user access to a common (shared) link by: • •
Coordinating user packet transmissions by allocating resources (time slots in TDMA or frequency channels in FDMA). Detecting and coordinating packet retransmissions during periods of contention.
There are many possibilities in the choice of bandwidth allocation strategy. In a TDMA based scheme, time slots can be allocated to users on a fixed assigned basis (FA), demand assigned basis (DA) or they may be open to random access (RA) or freely assigned (FreeA). Fixed assignment of time slots is simple to achieve and useful for some applications e.g., heavy route telephony, but inefficient for bursty traffic such as Internet traffic. Demand Assignment of time slots is designed to provide for the instantaneous requirements of users by allowing users to request bandwidth as and when they need it, providing increased capacity through statistical multiplexing gains; the effectiveness of demand assignment may be reduced over long propagation delay links such as a GEO satellite link. Allowing users access to the satellite link on a random basis can achieve low delays if the traffic requirements are low but it is impossible to achieve high utilisation by random access alone. A number of transmission slots may go unused with these techniques and hence bandwidth is wasted. Unused slots can be freely assigned to ground terminals, usually in a round robin fashion, to pre-empt any traffic demands minimising the delay associated with a pure demand assigned system. Ground stations use free assigned slots if they happen to have packets of data to send at the instant of the free assigned slots. For highly sporadic (bursty) traffic, most schemes will employ some form of demand assignment. Combined schemes may incorporate random access or free assignment with demand assignment. Issues such as the proportion of each type of slot then becomes an important factor in the performance of the scheme, and some schemes are adaptive where the proportion of each type of slot adapts to the current traffic pattern on the channel. Demand assignment schemes need a suitable slot request strategy. Requests can be made by assigning fixed request slots (one per terminal), allowing random access to request slots, or by piggybacking requests onto data packets. In the latter case, issues such as the number of times that terminals can make request in a given time frame and the maximum number of slots that can be requested are important factors. The performance of a MAC protocol has a large impact on higher layer functions, particularly on the applications themselves. The MAC layer is responsible for ensuring that the applications receive a good Quality of Service (QoS) in terms of delay and packet loss whilst making efficient use of the available spectrum.
A Simple Medium Access Control Protocol The ALOHA protocols are traditional, straightforward MAC protocols, designed for transmission of sporadic messages with short duration [1]. In the Slotted ALOHA scheme, the transmission time is divided into equal sized time slots that are accessed randomly by any user in the system. At the start of each slot, if a user has a packet to send then he will transmit it, maintaining a copy of the packet in case of corruption. If more than one user transmits in the same slot then a collision will occur and the packets will require retransmission after a randomised delay.
The Combined Free/Demand Assignment Multiple Access (CFDAMA) Protocol The CFDAMA MAC protocol, proposed by Le-Ngoc et al [2], is one of a new breed of protocols designed to provide significant improvements in the delay/utilisation performance of satellite channels. Three versions of CFDAMA have been described in the literature, differing in the way that users make slot reservation requests. The specific protocol examined in this paper is CFDAMA with piggybacked request slots (CFDAMA-PB) [3]. CFDAMA-PB combines demand assignment with free assignment of time slots. It is designed primarily for a finite number of users with bursty (variable bit rate) traffic such as Internet traffic. The combination of free and demand assignment provides a minimum delay of one satellite hop at low loads with the high channel utilisation of demand assignment at high loads. The protocol operates with a centralised scheduler node, either a hub station or an on-board processing satellite. Satellite on-board processing is preferred as reservation requests will be acknowledged with half the delay than if the scheduler was located on the ground with communication via a transparent transponder.
The uplink frame structure (ground station to satellite) is shown in figure 2 below: 128 Slots per Frame
F/D
F/D
F/D
F/D
212ìs
F/D
F/D
F/D
F/D
27.136ms
Figure 2 - The uplink frame structure of the CFDAMA-PB protocol The uplink frame consists of a series of data slots, allocated to users either as free assigned slots (F) or demand assigned slots (D). The satellite scheduler holds a free assignment table and a reservation request table. The free assignment table contains a list of the ID numbers of all active terminals in the system. The reservation request table queues user requests and holds the ID numbers of the users making the requests along with the number of slots requested. At the beginning of each frame, the scheduler will assign slots to users in the first instance based on the requests queued at the satellite, in a first come first serve (FCFS) manner. If the terminal at the top of the reservation request table has requested x slots then the terminal will receive x slots in succession and be removed from the table. This process continues until the reservation request table becomes empty. When the reservation request table is empty, the scheduler assigns successive slots one-byone to the terminal at the top of the free assignment table, moving the terminal from the top to the bottom of the table each time. This equates to free assignment of time slots on a round robin basis, with each terminal receiving an equal share of the available bandwidth. To give stations who have not requested bandwidth for a while a better chance of obtaining a free assigned slot, when an entry is removed from the reservation request table, the corresponding terminal is also moved to the bottom of the free assignment table. The ground stations piggyback requests for data slots onto data packets and will make a request for a number of slots based on equation 1 below. A request may be made in every data slot allocated to the terminal. Number of slots requested
=
(Number of packets queued – 1) – Number of expected slots
Eqn 1
Data Traffic Modelling Data traffic such as Internet traffic is traditionally modelled as a Poisson process with exponentially distributed time between individual packet generations. The accuracy of this model for simulating data traffic is, however, being questioned in the literature [4], [5]. These papers present the findings of studies carried out on a large amount of Ethernet traffic (which consisted primarily of TCP/IP traffic) at Bellcore Morristown Research and Engineering Centre. They found that Ethernet traffic exhibits self-similarity. Self-similar traffic is characterised by long-range dependence; the pattern of packet generation appears similar when viewed over a wide range of time scales. They concluded that traditional Poisson modelling can severely underestimate the burstiness of Ethernet traffic and hence over-predict network performance. Two traffic models have been developed in a protocol simulation tool for use with the CFDAMAPB and Slotted ALOHA models. The first is a Poisson generator with exponentially distributed time between individual packet generations (see figure 3). The mean of the distribution (1/λ) is derived from the overall activity level of the source.
Exponentially Distributed Time Between Individual Packets Pr(Tx = t) = λe-λt T1
T2
T3
T4
T5 time
Figure 3 - Poisson Traffic Model The second model is a self-similar traffic generator, modelled as an ON/OFF source where the time spent in each state fits a Pareto distribution [6] (see figure 4) with parameters α = 1.2 and k = 1. Packets are generated during the ON periods only, at regular intervals at a rate determined by the specified source activity level. The drawback of the Pareto distribution is that it exhibits infinite variance, so there is a non-trivial probability that the ON time may be longer than the simulation run time.
Pr(Tx = t) = αkαx-α-1 T1
Pareto Distributed ON and OFF Times
T2
T3
OFF Time
ON Time
T4
T5
time
Figure 4 - Self-Similar Traffic Model
Simulation Models and Implementation A model of CFDAMA-PB has been designed and developed based on the operational description, request strategy, and frame structure described previously. A model of Slotted ALOHA has also been developed to mirror the CFDAMA-PB scheme to allow effective comparison of the two schemes. As such, a frame structure has been introduced to the Slotted ALOHA model. The frame duration and the number of slots per frame are identical to the CFDAMA-PB model. The satellite acknowledges correctly received packets on a frame by frame basis. If a ground station does not receive an acknowledgement within a specified time, the terminal will timeout, and the collided packet will enter a prioritised retransmission queue. The packet will then be retransmitted in one of the next k slots (chosen at random) where k is a simulation parameter. Terminals will continue to retransmit collided packets until successful, after which any newly generated packets will be transmitted onto the channel. The CFDAMA-PB and Slotted ALOHA models have been simulated for the scenario shown in figure 1. The parameters of the simulation are shown in table 1 below: Number of active user terminals 300 Traffic Model Poisson and Pareto (αON=αOFF= 1.2, kON=kOFF= 1) Channel Data Rate 2 Mbits/s Frame Duration / Slot Duration 27.136 ms / 212 µs Number of slots per frame 128 Slot Size 424 bits (53-bytes) Satellite Altitude 37,500 km Channel Utilisation for CFDAMA-PB (Erlangs) 0.05 TO 0.90 in steps of 0.05 Channel Utilisation for Slotted Aloha (Erlangs) 0.025 to 0.350 in steps of 0.025 Retx Parameter k (Slotted ALOHA only) 30 Simulated Time 240 s Table 1 - The Simulation Parameters
Results Figure 5 below shows the mean end to end delay experienced by packets as a function of channel load for both the CFDAMA-PB and Slotted ALOHA schemes for Poisson and self-similar traffic.
Figure 5 - Mean End To End Delay of Packets as a Function of Channel Load
Comparison of Slotted ALOHA and CFDAMA-PB The results highlight a major performance advantage of the CFDAMA-PB scheme over the Slotted ALOHA scheme. The Slotted ALOHA scheme has a maximum throughput in the region of 32.5% of the channel capacity whereas the CFDAMA-PB scheme can be loaded up to 85% of the channel capacity for Poisson traffic and 75% for self-similar traffic. The CFDAMA scheme exhibits very low end to end delay over virtually the entire channel utilisation range, and only at channel loads below 7% does the Slotted ALOHA scheme exhibit better delay performance. This behaviour can be explained as follows. At extremely low channel traffic levels, nearly all the Slotted ALOHA transmissions will be successful, resulting in end to end delays equal to a single satellite hop plus the waiting time between packet generations and the start of the next time slot, averaging out at half the slot duration. For CFDAMA-PB at very low traffic loads all the slots will be free assigned, the minimum end to end delay is then equal to one satellite hop plus the delay between packet generations and the arrival of a free assigned slot, averaging out at 150 slot durations. At low channel utilisation the mean end to end delay approaches the absolute minimum delay equal to one round trip time with a mean end to end delay of 1.2 round trip times at a channel load of 10%. At high channel loads, the end to end delay experienced by packets increases but even at a channel utilisation of 85%, the mean end to end delay is only 1.65 round trip times, still less than the minimum delay of 2 round trip times in a pure DAMA scheme. The performance of the Slotted ALOHA scheme is very poor due to the limited channel utilisation and long delays even at low channel utilisation, reaching 2 round trip times at a channel utilisation of 30%. Behaviour of Slotted ALOHA with Poisson and Self-Similar Traffic Models The performance of the Slotted ALOHA scheme is invariant to the traffic model. The reason for this can be explained by the decoupling of the packet arrival process and the packet transmission process. When nodes are in a state of retransmission, the end to end delay is a function of the retransmission strategy and not the arrival process at the source. Also, for a large number of nodes at low channel loads, each station is generating a small amount of traffic. Under these conditions, the regularity of the constant packet generation in the ON/OFF process will be low and so the packet trains will tend towards individually generated packets and will approach the Poisson model. Behaviour of CFDAMA-PB with Poisson and Self-Similar Traffic Models The comparative performance of CFDAMA-PB with the Poisson and self-similar traffic models is more interesting. The end to end delay is lower for self-similar traffic up to a channel load of around 50% after which the Poisson traffic experiences lower end to end delays. Above a channel load of 85% for Poisson traffic and 75% for self-similar traffic, the ground station queues build up in this implementation of the CFDAMA-PB scheme. This is due to a few nodes dominating the channel and inhibiting other nodes from making bandwidth reservation requests. The burstiness of the self-similar traffic is much greater than the Poisson traffic, the slot reservation requests are therefore larger, enabling some nodes to dominate the channel at a lower channel load than with the Poisson model due to reception of long runs of demand assigned slots. Other nodes then have to wait a long time for a slot and if they have been in the ON state generating a stream of packets, they will request a large number of slots themselves which will further inhibit other nodes from receiving a slot. At low loads, the improved performance of the scheme for self-similar traffic over Poisson traffic can be explained by consideration of the underlying operation of the CFDAMA-PB scheme. For the self-similar traffic model, packets are generated in the ON state at a constant rate which is determined by the overall activity level of the node. Slots are initially allocated on a free assigned basis, with each terminal receiving a regular slot. It turns out for the scenario described in this paper that up to a channel load of 50%, the time between each successive free assigned slot is less than the constant time between packet generations for the self-similar traffic generator the ON state. Up to channel loads of 50%, there is always an upcoming free assigned slot to transmit every generated packet in and so the end to end delay stays at a constantly low value. The performance of the scheme with the self-similar traffic model is scenario dependent. The reason why the Poisson traffic source exhibits larger delays at low channel utilisation is due to a probability of very small inter-arrival times between packets, resulting in a small amount of queue build up and requests for demand assigned slots. Analysis of the number of free assigned slots and the number of demand assigned slots as a function of channel utilisation confirmed these findings as there were no demand assigned slots for any channel utilisation values up to and including 50% for the self-similar traffic model. Figure 6 below presents the Cumulative Distribution Function (CDF) for the end to end delay for both schemes as a function of channel load for both Poisson and self-similar traffic.
Figure 6: Cumulative Distribution of End to End Delay as a Function of Channel Load CDF Comparison of Slotted ALOHA and CFDAMA-PB The CDF curves for Slotted ALOHA are stepped with packets grouped at a number of discrete values of end to end delay. This is a consequence of the fundamental operation of the Slotted ALOHA scheme. The end to end delay of a successful packet transmission is dominated by the number of retransmissions required. It is interesting to see that greater than 60% of the packets experience lower delays with Slotted ALOHA access than CFDAMA-PB at a channel utilisation of 30%. This is the proportion of packets with a successful first transmission attempt. The remaining 40%, however, experience much greater delay than with the CFDAMA-PB scheme due to one or more retransmissions. It can be seen that the CFDAMA-PB scheme exhibits a much lower variance in the end to end delay compared with the Slotted ALOHA scheme. CDF Behaviour of Slotted ALOHA with Poisson and Self-Similar Traffic Models The mean end to end delay of packets as a function of channel load for the Slotted ALOHA scheme has been shown to be invariant to traffic model. The distribution of end to end delay values also shows strong invariance to the traffic model. CDF Behaviour of CFDAMA-PB with Poisson and Self-Similar Traffic Models The range of values of end to end delay is similar for Poisson and self-similar traffic. At a low channel load of 30%, CFDAMA-PB performs better with self-similar traffic than with Poisson with a greater percentage of the packets within a particular delay bound. At a channel load of 30%, 90% of the packets experience an end to end delay less than 1.23 round trip times for self-similar traffic and less than 1.37 round trip times for Poisson traffic. This relationship results from the match between the regularity of free assigned slots and the rate of constant packet generation in the self-similar model. At the maximum level of channel load for efficient operation with each type of traffic source, the behaviour is reversed with Poisson traffic experiencing lower delays than the self-similar traffic at a higher channel load. The reason for this is due to the greater burstiness of the self-similar traffic. At a channel load of 85%, 83% of packets from a Poisson source experience end to end delays less than 2 round trip times. At a channel load of 75%, a smaller proportion of 73% of the packets from the self-similar source experience end to end delays less than 2 round trip times.
Conclusions The CFDAMA protocol offers excellent delay/utilisation performance for data traffic via satellite. The effectiveness if the free assignment strategy is clear as it provides end to end delays approaching the minimum bound of one satellite hop over modest channel utilisations. The random access of Slotted ALOHA is only able to offer better delay performance than free assignment at very low channel utilisation and suffers from low maximum throughput, rapidly increasing delays and inherent instability. The contentionless nature of free assignment is the major advantage of the CFDAMA-PB protocol. At high channel utilisation, the combination of free assignment and demand assignment enables the channel to be loaded up to 85% of its capacity, maintaining a delay performance that is far superior to a pure demand assignment scheme. The CFDAMA-PB scheme exhibits low variance in the distribution of end to end delay values. The importance of an accurate traffic model in network performance evaluation has been highlighted through differing performance of the CFDAMA-PB protocol with Poisson and self-similar traffic models.
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Acknowledgments This work is partially supported by BT. The use of OPNET modeller by OPNET technologies under the University Consortium (UC) agreement is acknowledged.
