Advances in Electrical and Computer Engineering Volume 9, Number 3, 2009 Considerations on VoIP Throughput in 802.11 Networks Alin D. POTORAC Stefan cel Mare University of Suceava str.Universitatii nr.13, RO-720229 Suceava [email protected] Abstract—Voice data packets have to arrive at the Overlap between the channels cause unacceptable destination in time, with a defined cadence and with low and degradation of signal quality and throughput. Basically the constant delay in order to allow the real time voice radio channel overlapping is accepted in 802.11 standards reconstruction. From this point of view, transmitting voice over [2]. IP networks is the most sensitive category of applications, especially when wireless medium is involved. The paper In infrastructure wireless LANs with one access point, the discusses the possibilities of transmitting the maximum data frames do not travel directly among clients. A wireless number of simultaneous voice streams over 802.11 wireless client sends the data frame to the access point and then the networks considering the main factors which impact with VoIP access point resends the payload content of the original data throughput, in a basic scenario. Starting from a proposed frame, packed in a new data frame, to the receiving client. communication model, the number of simultaneous possible The AP bandwidth (and the radio space) is shared between VoIP sessions is calculated, taking into consideration the contribution of the protocol overheads, the security overheads, the AP radio clients and the user available bandwidth is thus the PHY level timings and the CODEC proprieties. Numerical split among those clients [6]. results are generated and compared. RTS/CTS (Request to Send, Clear to Send) mechanism is the basic solution for managing 802.11b/g mixed wireless Index Terms—throughput, quality of service (QoS), IEEE networks. One client is asking the permission for 802.22 wireless LAN (WLAN), voice over IP (VoIP), voice transmission by sending a RTS message to the access point. codec At its turn, the access point is answering with a CTS message. The clients who receive the CTS will stop the send I. INTRODUCTION initiatives and avoid the collisions. The throughput is thus Sending the voice in real world IP networks is not a trivial reduced due to the RTS/CTS exchange times. problem. This is even more complicated on wireless Considering one radio client in a clean radio environment, networks. The voice applications are the most sensitive ones the above limitations could be neglected as a first approach. as they are time sensitive. In 802.11 communications, In such conditions, there are no overlapping, no bandwidth supplementary overheads and timing intervals are necessary sharing and no RTS/CTS mechanism. However, the protocol for every carried data packet. Additionally, the radio overheads and radio communication timing intervals cannot transmission technology has some limitations due to channel be avoided and they are extensively discussed further in this overlapping, radio bandwidth sharing, legacy support, paper, being the most throughput resources consuming overheads and inter-packet times. elements. APP Application Layer (RTP) RTP Voice Packet 12B (Codec Dependent) UDP Transport Layer (UDP) UDP Data Message 8B INT Internet Layer (IP) IP Datagram 20B LLC LLC Sub- Layer MAC Security LLC IP Data Unit 30B Overheads 4B CRC MAC MAC Sub- Layer 0-20B MAC Body 4B MAC Data PLCP MAC Protocol Data Unit (MPDU) Preamble Header PHY 18B 6B PLCP Protocol Data Unit (PPDU) Figure 1. VoIP packet encapsulation. Digital Object Identifier 10.4316/AECE.2009.03009 45 Advances in Electrical and Computer Engineering Volume 9, Number 3, 2009 II. PROTOCOL OVERHEADS Finally, when no security method is involved, the PLCP For each voic e packet whic h has to be sent, different Prot ocol Data Unit (PPDU) which is launched into the overheads are added when the data unit passes to each physical transmission medium contains a total number of protocol layer. We have to notice that the package length is overhead bytes equal with 102 (78+24), but transmitted at increased each time when de data unit is transferred to the two different rates. next down layer. In Figure 1, we show how these things happen. At the application level, the voice data stream is III. SECURITY OVERHEADS compressed by a specific codec, resulting voice packets with When security issues are used, as we can see in Table 1, a certain length. The compressed packets have different 8-16-20 bytes are supplementary carried for each voice dimensions depending on the codec. They are carried by frame. Accordingly, the security overheads will have an using RTP protocol (Real Time Transport Protocol) which impact on the voice channel extending the data frame. With adds its header of 12 bytes length. Next, at the transport WEP (Wired Equivalent Privacy) we arrive at 110 bytes, layer, a new UDP 8 bytes header is necessary (UDP is User with WPA (Wi-Fi Protected Access) using TKIP (Temporal Datagram Protocol). The IP frame creation needs another 20 Key Integrity Protocol) algorithm, 122 bytes are added, and bytes header. With no security involved, the MAC layer with WPA CCMP (Wi-Fi Protected Access based on adds a total overhead of 38 bytes (with LLC contribution Counter Mode with Cipher Block Chaining Message included) and PHY has another 24 bytes as PLCP (Physical Authentication Code Protocol) the security overheads are Layer Convergence Protocol) preamble and header. 118 bytes [10]. The PLCP Preamble and Header (PCLP overheads) are It is important to note that the voice packets length transmitted at the basic channel data rate. The basic rate is 1 usually extend from 10 to 160 bytes (Table 2). For example, Mbps for 802.11b, 24 Mbps for pure 802.11g and 6 Mbps when a G729 codec is used (with a 10 bytes packet length), for 802.11a. The rest of the frame is transmitted at the employing WPA-TKIP security method, 20 bytes will be channel data rate. At this moment and in these conditions, it added for each 10 bytes payload. This is the worst case is realistic to consider the overhead for 802.11 MAC layer scenario, with the minimum packet length and maximum only, which has a length of 78 bytes, and to include the security overhead. In other words, for one packet carrying PLCP transmission time together with the other timing voice, another double length sequence has to be added. intervals of 802.11 communications. These timing intervals However, the security overheads impact on the number of are DIFS (Distributed Interframe Space), SIFS (Short possible simultaneously VoIP sessions is reduced because of Interframe Space) and CW (Contention Window, backoff the fact that VoIP goodput is less sensible in relation with time) [5], [9] and they will be later explained as 802.11 overheads variations [1] while timing intervals are dominant radio environment characteristics. in the transmission budget, as we will conclude in this We will consider now the overheads for the Application paper. Layer (RTP), the Transport Layer (UDP), the Internet Layer (IP) and the MAC Sub-Layer (MAC) as suggested in figure TABLE 1. SECURITY OVERHEADS IN BYTES/PACKET WEP WPA WPA2 1. The MAC sub-layer contributes with the MAC header (30 Security Protocol WEP TKIP CCMP bytes), with the MAC CRC trailer (4 bytes) but also with Data integrity CRC-32 MIC CCM LLC overheads (3 or 4 bytes) and with optional security Security level Poor Medium High overheads if WEP or WPA is used. Overheads 8 20 16 Accordingly, at the PHY level, excluding the PHY IV. VOICE CODECS overheads, we arrive to an already included overheads amount H, having a length of 78 bytes, as it is calculated in Since the audio information is a continuous one, even in equation (1). quiet time intervals some background noise is to be transmitted. If uncompressed voice is carried, the necessary H = H + H + H + H = RTP UDP IP MAC speed for the basic data flow is 8 x 8 = 64 kbps (PCM). If = 12 + 8 + 20 + 38 = 78 bytes (1) we are adding 102 bytes as overheads to each 1 byte sample, The PHY level contribution was not included in the we get an unacceptable channel usage efficiency of general overheads because it is always transmitted at the 1/(1+102) = 0.97%. Therefore, packing the elementary basic channel data rate and not at the channel rate [1], [9]. samples into larger packets is a necessity. When, on a radio channel, a certain data rate is established, The packets have to be long enough to assure good the bits flow for data and overheads are transmitted at this channel efficiency, but also short enough to allow time speed. The PLCP preamble and header (24 bytes in total) are multiplexing with other packets and also they have to be however transmitted at the basic rate. Based on that, we can tailored for specific carrying frames. A short packet is also a transform this PHY overhead stream in a transmission time good solution for shorter retransmissions time when errors interval, with a constant time value, not related with the occur. Each codec is defining a packet length and a packet channel data rate and addable with other time intervals, inter-arrival time, as shown in Table 2. The protocols similar with the inter-frame intervals (which will be overheads for usual codecs are also summarized. The codecs explained as 802.11 radio channel propriety). parameter values are included into ITU Recommendations 46 Advances in Electrical and Computer Engineering Volume 9, Number 3, 2009 TABLE 2. THE PARAMETERS OF THE MAIN CODECS TCP/IP Layer Voice codec G.726-32 G711 G729 G723.1 Application Layer Packet inter-arrival time [ms] 20 20 10 30 Voice packet length [bytes] 80 160 10 24 RTP layer overhead [bytes] 12 12 12 12 Transport Layer UDP layer overhead [bytes] 8 8 8 8 Internet Layer IP layer overhead [bytes] 20 20 20 20 Data link Sublayer MAC layer overhead [bytes] 36 36 36 36 Physical Sublayer PHY layer overhead [bytes] 24 24 24 24 [3], [4] and they are based on acceptable coding/decoding If the medium is sensed to be available for the duration of delays compared with the human sound perception.