CATV Mounted Wi-Fi WAN Public Service: Realization and Prospective. Abstract

Transcription

1 CATV Mounted Wi-Fi WAN Public Service: Realization and Prospective. E. Biton, A.Raichel, M. Zussman, D. Shklarsky PassOver Inc. 33 Bezalel St. Ramat-Gan Israel {berez, adir, Abstract The Wi-Fi family of standards (IEEE wireless-lan, existing a, b and g and the incoming e and n) is emerging as the leading tool for the access of high rate wireless data. Basically it is the wireless version of the old and well known Ethernet protocol (IEEE 802.3). The MAC of all the versions above is using TDD (Time Domain Duplex) and CSMA (Carrier Sense Multiple Access) method as their access control tools; polling is also an alternative within the standards, but it is rarely used in existing implementations. By definition, as a LAN tool, the Wi-Fi standards limited the distances between users to very short ranges (~200 meters). The motivation behind the research described here is to devise a way to enable Wi-Fi modems to communicate with their access points at longer distances (WAN!). It is an essential capability that enables the creation of mass markets public services. Considering the rate of grows in the distribution of Wi-Fi modems it is a very attractive opportunity. The match with CATV is attractive since: (i) CATV service operates at WAN architecture (about 2000 homes per node); (ii) coaxial cables are the best physical medium to carry radio waves: absolute fidelity and low attenuation; (iii) CATV networks have enormous unused latent capacities that can be use for other purposes in addition to the distribution of TV signals. Definition of new available bandwidth along the cables is relatively simple, it is implemented by the addition of two ways by pass amplifiers to the CATV amplifiers and the addition of frequency converter/transmitter at the CATV outlets. The adaptation of the MAC is much more complicated. Since the polling provision is not a real alternative we had to find ways at which ordinary CSMA-TDD modems would be able to operate through the long distances of the CATV network. The solution was found by the introduction of switching elements into all the active components that are dedicated to the Wi-Fi layer. These switches are centrally controlled from the node through a dedicated cabled control channel. Based on the switching, a pseudo polling concept was devised; at this concept ordinary CSMA-FDD modems are manipulated to act in polling order while being unaware to it. The result is a very efficient centrally managed broadband wireless access system that is very similar to wired Ethernet networks that are managed via switched hubs. There are many advantages to our system compared with the regular Wi-Fi networks, the majors are: Robustness: It operates with all the versions of Long distance (WAN).

2 Central handling of large number of users enables complex value added services. Central control. High bandwidth utilization. Enormous growth potential of capacity at each node. 1 Introduction The Wi-Fi family of standards (IEEE wireless-lan, existing [1], a [2], b [3] and g [4] and the incoming e [5] and n) is emerging as the leading tool for the access of high rate wireless data. The IEEE WLAN standard has been widely accepted and rapidly deployed both in enterprise and home environments. The motivation behind this research is to devise a way to enable dispersed Wi-Fi modems to communicate with their Access Point (AP) at longer distances (WAN!). It is an essential capability that enables the creation of mass markets public services. Indeed, a similar motivation led to the development of the emerging IEEE [6] standard for fixed broadband wireless access system, also known as Wi-Max. However, considering the rate of grows in the distribution of Wi-Fi modems (which are not Wi-Max compatible), establishing Wi-Fi access system at long distance seems a more attractive alternative. Carrying Wi-Fi signals at long distances has already been considered in the context of outdoor IEEE cellular networks [7]. Nevertheless, the authors of [7] only considered the implication a high propagation delay on the MAC layer, and did not consider the PHY layer. The potential of data delivery over Community Antenna Television (CATV) networks was realized as early as the 1960 s. Since then much attention has been paid to the development of new technologies for CATV networks that will provide broadband services for residential users (e.g., DOCSIS 1.1 [9] and DOCSIS 2.0 [10]). The majority of the CATV networks are based on the branch-and-tree and tree-and-bush topologies. Modern cable networks use both coax and fiber optic cables for transmission and are called Hybrid Fiber/Coax (HFC) access networks. The HFC network architecture has emerged as the preferred CATV architecture because it can readily support bidirectional communication and signaling between the subscriber unit and the Head-end (HE) by deploying amplifiers that work in both directions. In this research we establish a new concept of a CATV mounted Wi-Fi. We call this system Wi-Fi via Cable (WVC). With the WVC system Wi-Fi signals are carried out over the CATV infrastructure from the AP, which is located at the HE 1, to the customers premise. Thus, by placing the AP at the HE, wireless broadband access can be provided to end-users. Obviously, some obstacles must be solved to enable the new hybrid wireless cable architecture. First, some frequency band must be allocated on the CATV to the Wi-Fi signals. Obviously, the new Wi-Fi transmission must not interfere 1 The Access Points are located at the Head-End; however each Fiber-Node is connected to an independent set of Access Points.

3 with current CATV transmissions. Next, in order to prevent oscillations, the 802,11 TDD access requires total separation between up and down links at any active element of the network. Finally, some special consideration should be taken in the MAC layer. Nevertheless, we show that the system is feasible with only - some minor manipulation of the AP MAC protocol, addition of two ways by-pass amplifiers at the CATV infrastructure and addition of frequency converters at the CATV outlets. We note that no modifications are required at the customers stations (STAs). The paper is structured as follows. First, in section 2 we provide some detailed description of the Wi-Fi via cable infrastructure. Next, in section 3 we describe the MAC protocol. Then, in section 4 we provide some performance analysis results. Finally, we conclude in section 5. 2 Wi-Fi via Cable - Infrastructure The system is composed of four main blocks: (i) a central unit composed of: a scheduler, a management system, a router, and an Access-Point (AP), located at the CATV head-end; (ii) modified CATV infrastructure with by-pass elements and Up-Down converters at the cable outlets; (iii) a switching system controlled by the central unit through a dedicated control channel; and (iv) standard network cards (STAs). According to the concept, the frequency range of the coax network is expanded to 1200 MHz (wider expansion is possible but not needed at this stage). The network's amplifiers are modified by the addition of bidirectional by-pass linear amplifiers operating in the frequency range of 900 to 1200 MHz. The modification enables bidirectional free linear flow of the Wi-Fi signals through the network, pending to frequency matching through proper frequency conversion. A device is located at the cable outlets and acts as the interface between the cables and the air; it separates the Wi- Fi signals from the CATV signals, converts them into the original frequencies and transmits/receives them to/from the air. In order to prevent oscillations TDD access requires total separation between up and down links at any active element of the network; at the by-pass amplifiers this separation can be achieved either by TDD to FDD conversion or by the introduction of switches at the amplifiers (note that the TDD to FDD solution doubles the bandwidth consumption); as for the output device only the switching solution is viable. The essential switching process dictates central discrimination between uplink and downlink and control capability to operate the switches. The control capability includes control center at the HE and a two way service channel between the center and the switches (at the by-pass amplifiers and at the outlet's devices). Additional benefit of the switches is the elimination of accumulated noise at the uplink such accumulation can limit the number of users. The combination of the controlled switches enables the separation of single users or groups of users by blocking their reception and/or transmission capability.

4 3 MAC Protocol in WVC Transmitting Wi-Fi signals over the CATV infrastructure allows extending the reach and expanding the coverage of the network by utilizing the low-loss and deterministic coax path. However, the CATV topology introduces some new obstacles which are not accounted for by the legacy standards. First, an immediate outcome of carrying the WiFi signals from the HE to the customer premise is increase in the propagation delay. The IEEE standard considers propagation delay less than 1 µsec which is associated with a distance that is less than 200 m. However, WVC introduces a transmission range of up to 10km; that is a propagation delay up to 50 µsec. High propagation delays decreases dramatically the throughput of the CSMA/CA random access mechanism. Next, the CATV tree topology carries signals only downstream from the HE (the root) to the users (the leaves) and upstream from the users to the HE. Signals from one branch to the other are isolated. Thus, all STAs are hidden in the WVC. The standard do consider the problem of hidden terminals and resolve this problem through the RTS/CTS handshake. However, the RTS/CTS handshake is inefficient with high number of hidden terminals. Thus, it is clear that the DCF (CSMA/CA) protocol with the RTS/CTS handshake is inadequate for our system. Finally, WVC is basically an access system, thus most of the traffic is transmitted downstream from the AP to the STAs. Furthermore, the WVC should support many STAs. On the other hand, wireless LAN is basically designed for symmetric traffic between few STAs. In other words, the CSMA/CA protocol is efficient only with low number of STAs. A high number of STAs leads to a high number of collisions, which results in a low throughput. Furthermore, the AP and the STAs have the same priority in accessing the medium. Thus, upstream traffic from all STAs might control the medium and impede downstream transmission from the AP (which get transmit opportunities as a single STA). Due to the above considerations resulting from carrying the Wi-Fi signal over the CATV infrastructure it is clear that the PCF protocol is more adequate than the DCF protocol. However, although all STAs inherently obey the medium access rules of the PCF, it is an option for a STA to be able to respond to a CF-Poll received from a PC. A STA that is able to respond to CF-Polls is referred to as being CF-Pollable. In practice most STAs are not CF-Pollable. Therefore, a new DCF based polling scheme is called for. Indeed, such a scheme has been proposed in [8]. However, the ack-based polling strategy in [8] has some serious flaws. First, it assumes that the More Data bit in the frame control field indicates that a STA has more data to transmit. However, according to the IEEE standard [1] the More Data bit indicates to a STA in a power-save mode that more data are buffered for that STA at the AP. The More Data bit is set to 0 in all frames transmitted by any STA. Thus, the AP cannot obtain any information from this field as suggested in [8]. Next, according to the procedure (9.2.8 in [1]), delaying and transmitting any other frame at the shared medium will be interpreted as a failure of the previous frame (which is pending for ).

5 The MAC protocol in WVC is based on the two modes of operations supported by the MAC- the PCF and the DCF. However, as opposed to the legacy , here, the DCF mode is divided into two modes: a Pseudo-Polling Period (PPP) and a Contention Period (CP). The three modes are used alternately in time. That is a contention-free period by the PCF is followed by a pseudo-polling and a contention periods by the DCF (see). DCF PCF DCF PCF DCF B Contention-Free Period Pseudo Polling Period Contention Period B Contention-Free Pseudo Polling Figure 1: CFP/PPP/CP alternation A. Contention Free Period The CFP is based on the polling scheme controlled by the PC operating at the AP. During the CFP the AP polls the CF-Pollable STAs according to the PCF frame transfer procedure. The CFP begins with a beacon and ends with the CF-END message. According to the standard PCF protocol if a CF-Pollable STA does not respond to a CF-Poll within the period following a transmission from the PC, then the PC shall resume control and may transmit its next frame after a PIFS period from the end of the PC s last transmission. However, incorporating high propagation delay may lead the PC to mistakenly assume that the polled STA didn t respond within PIFS period. Thus, the WVC PC shall wait for a period of MaxPropagationDelay+PIFS before resuming control and transmitting its next frame. MaxPropagationDelay shall be determined according to the maximal radius of the CATV infrastructure (typically 10km, i.e., ~50 µsec). B. Pseudo Polling Period The PPP begins at the end of the CFP. During the PPP the AP gains control of the medium by waiting a shorter time between transmissions than the STAs using the DCF access procedure. That is, the AP waits a period between transmissions instead of the standard DIFS + Backoff Time. To poll a STA, say the n-th STA, the AP sends CTS message destined to the n-th STA with a duration field that accounts for a maximal size frame. Upon receiving the CTS message all STAs update their NAV and refrain from accessing the medium for the duration reported by the CTS message. Furthermore, the n- th STA receiving a CTS message destined to it may access the medium according to the RTS/CTS handshake. However, since the polled STA didn t initiate the handshake by transmitting RTS, it is unclear whether the STA proceeds with sending data frame upon receiving CTS. To solve this problem, we utilize the switching system as follows: before transmitting CTS the AP (through the dedicated control channel) close the DS switch

6 associated with the n-th STA. Thus, all STAs except the n-th STA that is polled receives CTS and update their NAV to indicate that the medium is busy. The n-th STA, which do not receive CTS, sense the medium as idle and embarks with the backoff procedure. If it has an impending frame, the frame is send whenever the backoff timer reaches zero. Otherwise no frame is sent. If this is the case, the AP waits for DIFS+CW till it turns to send data or poll the next STA. The above frame transfer procedure is illustrated in Figure 2. D1 CTS D2 CTS D3 CTS U1 U3 DIFS backoff DIFS CW DIFS backoff No Response Dx=Frames sent by PC Ux=Frames sent by polled stations Figure 2: Example of Pseudo Polling frame transfer C. Contention Period To allow new STA s to complete authentication and association we allow the STAs to send the authentication/association messages in random access mode during the contention period. 4 Performance Analysis Here we present some performance analysis results for a typical scenario. We assume that a CATV fiber node is connected 2000 subscribers. Next, we assume that the WVC system market penetration ratio varies from 1% to 50%. That is, the number of Wi- Fi subscribers at the fiber node varies from 20 to We consider two modes of operations: FDD and TDD. With the FDD mode, separation between uplink and downlink transmission is achieved by TDD to FDD conversion at the by-pass amplifiers. Recall that the TDD to FDD solution doubles the bandwidth consumption. Thus, one can carry 3 Wi-Fi channels (i.e., 3 APs per fiber node) and a guard band on the frequency band between MHz. With the TDD mode, the separation is achieved by switches at the amplifiers. Thus, one can carry 7 Wi-Fi channels (i.e., 7 APs per fiber node) at the same frequency band. Accordingly, 1%-50% penetration ratio results in subscribed STAs per AP and subscribed STAs per AP with FDD and TDD respectively. Now, we assume that 15% of the STAs are active. Active STAs are polled each polling cycle. Inactive STAs are polled once in a while (once in 5 seconds) with no data sent or

7 received. Furthermore, we assume that at each polling cycle 20% of the active STAs send and receive data. The assumed Wi-Fi parameters are as follows: Link speed 54 Mbps. Downstream message length 1500 bytes. Upstream message length 192 bytes. Propagation delay 50 microseconds (10 Km- average distance). 50% of the STAs are CF-Pollable. Figure 3 presents the perceived bandwidth by a STA in kbps for various market penetration ratios. From the figure one can see that a fair bandwidth is available for each STA for a relatively high market penetration ratio, e.g., with TDD a downstream bandwidth of more than 1.5 Mbps and an upstream bandwidth of more than 200kbps is available for a high market penetration ratio of 40% Downstream FDD TDD Upstream FDD TDD Bandwidth per STA [kbps] Bandwidth per STA [kbps] Market penetration [%] Market penetration [%] Figure 3: STA bandwidth vs. market penetration ratio 5 Conclusions The CATV mounted Wi-Fi generates implementation of a broadband wireless access system. While the emerging WiMax provides wireless access through the air interface, our system provides Wi-Fi access by carrying the signals over the CATV infrastructure. We note that the IEEE

8 was primarily designed for short distances and cannot be immediately applied as a long-range broadband wireless access. Furthermore, the CATV infrastructure cannot immediately carry the Wi- Fi frequency band. Thus, the paper presents a new and unique technology that solves these obstacles. This technology is composed of: up/down frequency converters, by-pass amplifiers and a switching system that is centrally controlled. Moreover, we established that wireless broadband access should be based on a polling system rather than on a CSMA/CA MAC. Since the PCF polling mechanism is not widely implemented we proposed a new DCF based pseudo polling scheme. Finally, we showed, through a performance analysis of a typical scenario that the proposed Wi-Fi via cable access system delivers reasonable performance in term of bandwidth per station even for a high market penetration ratio. 6 References [1] IEEE Std , 1999 edition, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. [2] IEEE Std a, 1999, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, High-speed Physical Layer in the 5GHz Band. [3] IEEE Std b , Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extensions in the 2.4 GHz Band. [4] IEEE Std g , Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Further Higher Data Rate Extensions in the 2.4 GHz Band. [5] IEEE P802.11e/D6.0, November 2003, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Quality of Service (QoS) Enhancements. [6] IEEE Std , IEEE Standard for Local and metropolitan area networks, Part 16: Air Interface for Fixed Broadband Wireless Access Systems [7] K. K. Leung, B. McNair, L. J. Cimini, Jr., J. H. Winters, "Outdoor IEEE Cellular Networks: MAC Protocol Design and Performance", IEEE International Conference oncommunications, [8] Shiann-Tsong Sheu, Yun-Yen Shih, Yue-Ru Chuang, An -based polling strategy for the ieee wireless networks, IEEE Global Telecommunications Conference, GLOBECOM'03, Dec [9] Data-Over-Cable Interface Specifications Radio Frequency Interface Specification, MCNS Consortium, SP-RFIv , [10] Data-Over-Cable Service Interface Specifications - DOCSIS Radio Frequency Interface Specification, SP-RFIv2.0-I , 2003.