Interference, even with MCCA channel access method in IEEE 802.11s mesh networks Artem Krasilov IITP RAS Moscow, Russia Email:
[email protected]
Andrey Lyakhov IITP RAS Moscow, Russia Email:
[email protected]
Abstract—IEEE 802.11s draft defining mesh networking includes two channel access methods: mandatory EDCA and optional MCCA. It is known that random access with EDCA, designed for ad hoc and infrastructure modes, is weak in multihop case because of the hidden stations effect. In contrast, by means of channel reservation and advertisement of set up reservations MCCA combats against the effect, provides more coordinated channel access and comes as a solution when better QoS is desired. In this paper, we draw readers attention to the hidden stations effect again and show that the interference caused by transmissions of ACK frames by hidden stations is not combated with MCCA. We evaluate the effect with a simulation model and propose a solution which is compatible with IEEE 802.11 and may be implemented in the framework of MCCA. Also, we discuss the issue of the interference coming out of MCCA advertisement horizon and propose a solution which appear to require more flexibility than provided by the current IEEE 802.11s draft. Keywords-IEEE 802.11s; MCCA; interference; QoS;
I. I NTRODUCTION Being an extension of IEEE 802.11 specification, IEEE 802.11s [1] inherits the mandatory channel access method, EDCA, for operation in multihop scenarios. Many papers, e.g. [2]–[4], show that the performance of EDCA degrades dramatically in multihop networks because of the hidden stations (STAs) effect. One may say that IEEE 802.11 introduces the RTS/CTS mechanism to avoid this effect. Unfortunately, this mechanism works effectively in infrastructure mode only as all STAs hear CTS frames transmitted by the AP and collisions of data frames are completely avoided. In multihop networks, due to their nature, there is no guarantee that all STAs hear CTS frames, so the RTS/CTS exchange is not enough to protect transmission [5]. In 2006, a novel channel access method, called MCCA (in early IEEE 802.11s drafts, the method was called Mesh Deterministic Access (MDA), specifically designed for operation in multihop networks was introduced. MCCA allows mesh STAs to reserve time intervals, called MCCAOPs, for data transmissions in a periodic manner. The transmitting and receiving STAs advertise each MCCAOP reservation to their neighbors which then re-broadcast the advertisement to ensure that all STAs in the two-hop neighborhood of
Alexander Safonov IITP RAS Moscow, Russia Email:
[email protected]
the transmitting and receiving STAs become aware of this reservation. During an MCCAOP, the transmitting STA obtains access to the channel with the highest priority, while other STAs which track this reservation defer from starting their transmissions to escape interference. The MCCA looks like a solution for Quality of Service (QoS) provisioning, especially for periodic multimedia traffic, because it gives an opportunity of controlled access to the channel with virtually no interference from neighboring STAs. Multimedia traffic, such as a VoIP or Video traffic, usually requires low and stable end-to-end delay and low packet loss probability. As reservations provide some control on the delay, the packet loss probability is the most critical factor. In this paper, we show that transmissions in MCCAOPs are not fully protected from interference and therefore packet loss ratio requirements may not be met. We reveal the reasons of packet losses, additional to random noise and interference from MCCA-incapable STAs, and propose solutions which guarantee reliable transmission in MCCAOPs and accomplishment of QoS requirements. The rest of the paper is organized as follows. Section II gives a short description of MCCA. In Section III, we describe a phenomenon which we call the ACK-induced interference, study its roots and magnitude and propose a solution to combat the phenomenon which is in the framework of IEEE 802.11s amendment. In Section IV, we discuss another phenomenon which we call the two-hop interference. A solution is also proposed, which unfortunately cannot be implemented as easy as the solution for the ACKinduced interference, but being implemented provides strong protection of MCCAOP reservations from any interference. Section V concludes the paper and discusses further research on flexible protection schemes of MCCAOP reservations for balancing between the quality of protection and the network capacity. II. MCCA D ESCRIPTION In this section, we adapt MCCA description for the purpose of this paper. For details, please refer to the original IEEE 802.11s specification [1].
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MCCA is an channel access method that allows STAs to access the channel in predefined time intervals, called MCCAOPs, with lower contention than it would otherwise be possible. Each MCCAOP reservation (or simply reservation) represents a regular set of MCCAOP intervals (or simply MCCAOPs) arranged in the DTIM interval and is defined by three parameters: Offset, Duration and Periodicity. MCCAOP Periodicity shows how many MCCAOPs of length indicated with MCCAOP Duration are arranged in the DTIM interval. The MCCAOP Offset specifies the position of the first MCCAOP with respect to the beginning of the DTIM interval. So, MCCAOPs in the DTIM interval are separated by the time interval equal to the DTIM interval divided by the value of MCCAOP Periodicity. An example of a reservation with MCCAOP Periodicity equal to 2 is shown in Fig. 1. To set up a reservation, which may be for either individually or group addressed transmissions, a STA, called MCCAOP owner, transmits an MCCAOP setup request specifying parameters of the planning reservation. The receiver(s), called MCCAOP responder(s), checks the requested reservation for any conflict with reservations it is involved in and reservations of neighboring STAs it is aware of, and transmits a response with accept or reject code. To reduce the probability of reservation conflicts a STA advertises its own reservations and reservations of neighboring STAs it is aware of. For this purpose, the STA periodically generates and transmits via beacons or management frames (a) an MCCAOP Advertisement Overview Information Element (IE) and (b) a set of MCCAOP Advertisement IEs which contain all reservations tracked by this STA. The MCCAOP Advertisement Overview IE contains values of Medium Access Fraction (MAF), MAF limit and Accept Reservations bit. The MAF at a STA is the ratio of the time reserved for MCCAOPs in the DTIM interval of this STA to the duration of the DTIM interval. The MAF value is upper bounded by the MAF limit which indicates the maximum amount of channel resources which may be used for MCCAOP reservations. A STA creates and accepts new reservations and signals this capability with the Accept Reservation bit only if its own MAF value and the MAF value of its neighbors is less than MAF limit. Each MCCAOPs Advertisement IE contains three reports: • TX-RX times report includes individually addressed
reservations for which the STA is the MCCAOP owner or MCCAOP responder; • Broadcast times report includes group addressed reservations for which the STA is the MCCAOP owner or MCCAOP responder, and also may include known beacon transmission times; • Interfering times report includes reservations for which neighbors of this STA are the MCCAOP owners or MCCAOP responders (i.e. TX/RX and Broadcast reservations reported by neighboring STAs). At the beginning of an MCCAOP, the owner gets access to the channel with the highest priority by setting it EDCAF parameters to the minimum values (AIF S = P IF S, CWmin = 0). In turn, neighbors of both MCCAOP owner and responder provide a Reservation Allocation Vector (RAV) mechanism to indicate busy medium until the receipt of a frame transmitted by either the MCCAOP owner or the MCCAOP responder. EDCA TXOPs of a mesh STA is not allowed to extend across any of MCCAOP reservations tracked by the STA, i.e. reservations included in TX-RX, Broadcast and Interfering times of the STA. III. ACK- INDUCED I NTERFERENCE Let us explain the phenomenon which we call the ACKinduced interference with a simple example of four STAs arranged in a line as shown in Fig.2. STAs A and B set up an MCCAOP reservation, and STA D transmits EDCA traffic to STA C. Even if STA D is MCCA-capable, it is two hops away from STA B and may start its EDCA transmissions at any time, including those times when the transmissions cross the MCCAOP of STAs A and B. According to base IEEE 802.11 standard, if a STA receives an individually addressed Data frame, it responds with an ACK frame (we assume Immediate-ACK policy). If such an ACK transmission from STA C crosses the MCCAOP of STA A, a collision on STA B occurs despite prior advertisement of the MCCAOP. To evaluate the magnitude of the phenomenon in various scenarios, we develop an MCCA module for NS-3 simulator [6] which implements the physical and link layers of IEEE 802.11 STAs relatively accurate, comparing with other simulation tools. In particular, we simulate networks of STAs transmitting at a power level of 16 dBm (40 mW) in 5 GHz band using 802.11a PHY. The calculation of
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Figure 2.
ACK-induced interference
Packet Error Rate (PER) is based on received Signal-toInterference-plus-Noise Ratio (SINR) and used Modulation and Coding Scheme (MCS), as in [7]. As IEEE 802.11s only defines MCCA behavior for singlehop, we develop a simple Multi-hop Reservation Protocol (MRP). For a particular data flow using MCCA at each hop, MRP sets up reservations along a multi-hop path chosen by the path selection protocol and controls reservations changes if the path changes. As the path selection protocol, we use default IEEE 802.11s HWMP protocol with Airtime link metric. Detailed description of MRP is out of scope of this paper due to space limitation, but it is important to mention that in some cases no multi-hop reservation can be set up along the path found by the path selection protocol due to either (i) reaching MAF limit or (ii) inability to insert the requested MCCAOP reservation between already set up ones at some hop. Background traffic transmitted with EDCA is modeled as TCP-sessions for transmission of 2-MB files, with TCPsegment size of 512 bytes. The traffic is transmitted with access category AC BK. To model MCCA traffic, we use voice codec G.729 which generates constant bit rate flows of 20-byte packets with inter-arrival time of 20 ms. Packets of a flow are only generated when MRP succeeds in setting up a multi-hop reservation along the path found by path selection protocol. Otherwise, no packet of the flow is generated. According to ITU recommendation [8], the quality of a voice flow transmission is calculated based on the average end-to-end packet delay, jitter and packet loss ratio and may be estimated with an integral quantity, called R-factor, which may take any value from the interval 0 ≤ R < 100. As also recommended by ITU in [9], the voice flow quality is acceptable if R > 50. Since voice flows are transmitted in MCCAOP reservations, the voice flows quality at the destination STAs indicates the quality of MCCAOP protection from interference. During simulations, we calculate the average number of voice flows with R > 50, i.e. the number of flows delivered
Figure 3. Manhattan scenario. STAs connected with the lines are neighbors
with acceptable quality. In this paper, we show the scenario which we call ’Manhattan’ when mesh STAs are arranged in a regular grid with buildings in the cells as in Fig.3. STAs connected with the lines are neighbors, and diagonal STAs do not interfere, as buildings attenuate the signal by 100 dB. We run two series of experiments to evaluate the voice flows quality in the best and the worst cases. In the first series reflecting the best case, the network is loaded by voice flows only. Varying the number of initiated flows, we randomly choose the source-destination pairs and let MRP to set up a multi-hop reservation for each pair, one-by-one. In the second series, after the initialization phase when all possible voice flows are set up, source-destination pairs for background traffic are chosen and TCP sessions open. When a TCP session closes, new source-destination pair is chosen and new session opens, so that the rest of the channel resource not occupied by MCCAOPs for voice flows is always loaded by background traffic, i.e. the network works in saturation. Such an experiment is considered to evaluate the voice flows quality in the worst case. As we mention, MRP may fail in setting up a multi-hop reservation due to several reasons, so to calibrate the model we observe the number of actually established reservations in each run and average results. Fig.4 plots the numerical results in the case of 5x5 grid. The curve “established” shows the number of actually established flows vs. the number of initiated flows. When the number of initiated voice flows is relatively small, less than 25 with the implemented MRP, it equals to the number of established flows. As expected, when the number of initiated flows increases, MRP fails to set up multi-hop reservations for some of them. So, the curve “established” shows the maximum number of flows which may be transmitted with acceptable quality in the considered network till we use the implemented version of MRP. Two other curves in Fig.4 show the average number of voice flows transmitted with acceptable quality as the function of the number of initiated voice flows, in the worst and the best cases. The curve “no-background” corresponds
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to the first series of experiments when voice flows are only transmitted in the network, while the curve “sat-background” corresponds to second series when in addition to voice flows saturated background traffic loads the network. The curve “no-background” coincides with the curve “established”, meaning that all established flows are transmitted with acceptable quality when no EDCA background traffic exists in the network. When the background traffic is present, the quality of voice flows considerably degrades, up to 50% when the number of flows is less or compared with the number of STAs in the network, which is probably the practical range of network load. It is interesting to note that the relative value of the gap between the two curves decreases when the number of flows grows. The quality degradation is caused by packet losses. Fig.5 plots the end-to-end Packet Delivery Ratio (PDR) of established voice flows vs. the number of initiated flows. As the number of set up reservations increases with the number of initiated flows, the amount of channel resources used for EDCA transmissions reduces, which, in turn, leads to lower collision probability, higher PDR and, in the end, better voice flows quality.
To confirm the hypothesis that the packet losses and consequently the voice flows quality degradation are caused by the ACK-induced interference, we force STAs in the simulation model to suppress ACK frame transmissions if they cross any of tracked MCCAOP reservations. The results of the experiments with ACK suppression show that the curve “sat-background” coincides with the curve “established”, i.e. all set up flows are transmitted with acceptable quality. The drawback of ACK suppression, besides the fact that it degrades the EDCA performance as a STA missing an ACK for data frame doubles its contention window and retries the data frame, is that it violates IEEE 802.11 base standard and may not be implemented. We propose another way to solve the ACK-induced interference problem, which we call Directional RAV (DRAV) by analogy with DNAV [10]. Let us get back to the example in Fig.2. Though STA D does not include the MCCAOP reservation of STAs A and B in its tracked reservations list, STA D is aware of the reservation as it receives the Interfering times report from STA C. The ACK-induced interference may be combated by forbidding a STA (STA D in the example) to start its EDCA TXOP if planned ACK transmission from the receiver crosses any MCCAOP reservation included in the receiver’s Interfering times report. An open issue is further actions when the backoff counter of a STA reaches zero while the STA is forbidden to start an EDCA TXOP. Anyway, the STA may have the same behavior as when it is forbidden to start an EDCA TXOP which extends across any of MCCAOP reservations tracked by the STA, as defined by IEEE 802.11s. So, the proposed solution lays in the framework of IEEE 802.11s. The results of simulation with DRAV MCCAOP protection method confirm that all established flows are transmitted with acceptable quality, as if no background traffic exists in the network. IV. T WO -H OP I NTERFERENCE The MCCA operation rules implicitly assume that only direct neighbors of MCCAOP owner and responder may interfere with the MCCA transmission. However, many papers, e.g. [4], [11], [12], show that in some cases the interference comes from two-hop neighbors. We refer to this phenomenon as to two-hop interference. Our investigation with the simulation model confirms that two-hop interference takes place and in some scenarios, e.g. outdoor operation, may significantly degrade MCCA performance. Transform the scenario in Fig.3 into outdoor by removing buildings from the grid cells. Consider two overlapping transmissions, no matter with EDCA or MCCA, as shown in Fig.6. In the simulation model we use, the received signal strength between STAs is determined by two-ray path loss model [13]. The SINR-based interference model predicts in such a case that STA B receives a frame from STA A
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Figure 7. Outdoor scenario. The number of voice flows with acceptable quality vs. the number of initiated flows. DRAV protection is used
incorrectly with considerably high probability, because of the interference from STA C two hops away from STA B. To investigate how much the two-hop interference influences MCCA performance we repeat the experiments described in Section III. To separate the ACK-induced and two-hop interference from one another, DRAV protection method is applied. The results in Fig.7 show that even in the absence of background traffic, the quality of many set up flows is unacceptable. The difference between “nobackground” and “established” curves increases with with the number of flows, which may be explained as follows. In Fig.6, if STAs A and B set up an MCCAOP reservation which overlaps with an MCCAOP reservation of STAs C and D, as allowed by MCCA rules, the interference from C will always, in a regular manner, corrupt the transmissions from A and B. When the number of flows increases the probability of reservation conflict increases too, forcing the PDR “no-background” curve in Fig.8 to decline from 100% line, compare with Fig.5. When background traffic loads the network, the PDR become worse: in addition to MCCAOP reservation conflicts the interference from EDCA transmissions appears. How can one protect MCCAOP reservations from the twohop interference? In [11], Cicconetti et al. propose to shift MCCAOPs by choosing another MCCAOP Offset value if any interference is detected. This method is only effective if the interference is caused by MCCAOP reservations conflict and there is room in DTIM interval for a shift. When the interference is caused by EDCA traffic the method
becomes ineffective, as EDCA traffic appears randomly, but not periodically. Moreover, the traffic generated by a TCP session may occupy all available bandwidth. To protect MCCAOP reservations from the two-hop interference we have to forbid all STAs in the two-hop neighborhood of both MCCAOP owner and responder to transmit during the MCCAOPs. STAs may escape the twohop interference caused by MCCA transmissions by rejecting MCCAOP requests which conflict any reservation from the list of tracked reservations, as already defined by the specification, extended with the reservations reported by neighboring STAs as their Interfering times. Such an extension effectively enlarges the coverage of MCCA advertisement without extra traffic overhead. EDCA transmissions may cause the two-hop ACKinduced interference. In the example in Fig.6, if STA D transmits to STA C, it may interfere with an MCCAOP of STAs A and B. STA D is three hops away from STA B and four hops away from STA A, so it is absolutely unaware of the MCCAOP between STAs A and B and absolutely free to start its EDCA transmission at any time. Probably, the only effective solution is the ACK suppression at STA C based on the same extended list of tracked reservations. We repeat the experiments with the proposed two-hop protection method and confirm that it combats the twohop interference regardless of the background traffic in the network. Everything comes at a price, and the twohop protection reduces the maximal possible number of reservations in the network, as one can see from Fig.9. V. C ONCLUSION Hidden stations which are known to be Achilles’ heel of random access in multi-hop networks may be combated by means of STAs coordination via advertised channel reservations. In this paper, we analyze MCCA channel access method appeared in IEEE 802.11s amendment and show that some types of interference, though quite specific, but causing considerable effects, are not combated by MCCA.
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[1] IEEE P802.11s/D10.0. Draft STANDARD for Information Technology – Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment: Mesh Networking [Electronic resource], 2011.
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[2] Xu S. and Saadawi T., “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?” IEEE Comm. Magazine, vol. 39, no. 6, pp. 130–137, 2001.
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Figure 9. Outdoor scenario. The number of established flows vs. the number of initiated flows. Two-hop and DRAV protection types are used.
We propose solutions for the ACK-induced and two-hop interference, which may be partially implemented in the framework of IEEE 802.11s. With that, full implementation of the proposed MCCAOP protection methods brings QoS provisioning even in saturated networks. So, it makes sense to consider the methods at least for some niche scenarios. Keeping this in mind, we would like to draw the readers attention to FLexible Architecture for Virtualizable wireless future Internet Access (FLAVIA) project [14] which proposes an architectural solution for making the medium access protocol programmable. By changing the role of the wireless network interfaces, from standardized and constrained service access points, to programmable wireless-processors (with standardized instruction sets) for composing specific access operations, the FLAVIA flexible architecture may facilitate fast test of fullyimplemented MCCAOP protection methods proposed in this paper. Even if IEEE 802.11s adopted the MCCAOP protection methods, which would hardly be possible even if the amendment was not at the ratification phase now, it would allow using only one method at a time. Taking into account that the two-hop protection considerably reduces the network capacity, while DRAV only protects from the ACK-induced interference, we suggest the protection type for an MCCAOP at each hop to be dependent on a number of factors such as the network topology and channel conditions. To implement the adaptive choice of protection type, we propose to add one more MCCAOP reservation parameter, which may be called MCCAOP Protection Type, to the existing three parameters (MCCAOP Offset, Duration and Periodicity), by including a corresponding field in MCCAOP signalling frames, which is possible with flexible FLAVIA architecture. ACKNOWLEDGMENT This work is supported by FP7 FLAVIA project.
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