IEEE 802.11.

Presentasi berjudul: "IEEE 802.11."— Transcript presentasi:

1 IEEE

2 Materi Overview 802.11 802.11 Architecture PHY Requirements
MAC Requirements

3 Overview 802.11 Wireless network distandarisasi oleh IEEE tahun 1997.
Disebut dengan standar IEEE Application Presentation Session Transport Network Data Link Physical ISO OSI 7-layer model Logical Link Control Medium Access (MAC) Physical (PHY) IEEE 802 standards 9/22/2018

4 Arsitektur 802.11 ESS DS AP AP STA STA STA STA BSS BSS
Infrastructure Network STA STA Ad Hoc Network Ad Hoc Network BSS BSS STA STA 9/22/2018

5 Komponen Arsitektur 802.11 Access Point (AP) Distribution System (DS)
Berfungsi sebagai relay dan bridge Distribution System (DS) AP berkomunikasi dengan AP yang lain, bisa menggunakan wired, wireless, atau switch Station Mobile node dengan standar IEEE Basic Service Set (BSS) Terhubung dengan backbone DS melalui AP Ad hoc  Independent BSS (IBSS) dengan tidak menggunakan AP Extended Service Set (ESS) Dua atau lebih BSS yang saling dihubungkan oleh DS 9/22/2018

6 Lapisan Arsitektur Standar IEEE mendefinisikan lapisan Medium Access Control (MAC) dan Physical (PHY) PHY  mengatur detail proses modulasi dan pengiriman penerimaan bit(Tx/Rx) MAC  mengatur sejumlah regulasi bagaimana cara mengakses lapisan media dan proses pengiriman data 9/22/2018

7 Phy Layer

8 Lapisan PHY Pada lapisan PHY ini dikenal dua jenis teknik modulasi spread spectrum, yaitu Frequency Hopping Spread Spectrum (FHSS) dan Direct Sequence Spread Spectrum (DSSS) Spread Spectrum digunakan untuk menghindari gangguan pengguna yang berlisensi dan non lisensi, serta menghindari noise. Efek modulasi ini untuk meningkatkan bandwidth dari sinyal yang akan ditransmisikan Sinyal termodulasi menggunakan urutan angka Spreading Code atau spreading sequence Digenerate oleh generator pseudonoise atau pseudo-random Pada penerimaan, urutan digit digunakan untuk demodulasi sinyal spread spectrum 9/22/2018

9 Frequency Hopping Sinyal dibroadcast melalui frekuensi radio yang acak
Sejumlah kanal dialokasikan untuk sinyal FH Lebar masing-masing kanal sesuai dengan bandwidth sinyal input Sinyal melompat dari frekuensi ke frekuensi pada interval tetap Transmitter beroperasi di satu kanal pada satu waktu Bit ditransmisikan menggunakan beberapa skema enkoding Pada setiap interval, frekuensi carrier yang baru dipilih Spreading code menggunakan daftar frekuensi yang digunakan untuk sinyal carrier (hopping sequence) Pada receiver, melakukan lompatan antara frekuensi dalam sinkronisasi dengan transmitter Bandwidth 2.4 GHz, data rate 1 – 2 Mbps (USA/Europe) Keuntungan: Penyadap hanya mendengar unintelligible blips Dapat menghilangkan interferensi sinyal Tahan terhadap multipath fading 9/22/2018

10 Frequency Hopping 9 8 7 6 Time 5 4 3 2 1 2.400 GHz 2.483 GHz Frequency Digunakan untuk 79 channel, dengan selisih 1 MHz (USA/Europe) Perubahan frekuensi (hop) setiap 0.4 detik FH menggunakan MFSK 9/22/2018

11 Direct Sequence Setiap bit dalam sinyal asli direpresentasikan oleh beberapa bit dalam sinyal yang ditransmisikan Spreading code menyebar sinyal melalui band frekuensi yang lebih luas Teknik mengkombinasikan aliran informasi digital dengan aliran bit spreading code menggunakan exclusive-OR (XOR) Bandwidth 2.4 GHz, data rate 1 – 2 Mbps, DS pada b data rate menjadi 5 – 11 Mbps DS menggunakan BPSK Keuntungan: Tahan terhadap jamming yang disengaja atau tidak Sharing pada single kanal antara beberapa pengguna 9/22/2018

12 Direct Sequence 9/22/2018

13 Distribusi Kanal (Global)
Perkembangan pada IEEE b dengan metode DSSS 5 – 11 Mbps pada range bandwidth 2.4 GHz Lebar kanal 22 MHz dengan 3 kanal non-overlapping (kanal 1, 6, 11) 9/22/2018

14 Distribusi Kanal (Global) Lanj.
Channel 14 hanya diijinkan di Jepang, Channel 12 dan 13 diijinkan hampir di seluruh negara kecuali USA, dimana hanya channel 1 – 13 yang legal digunakan. 9/22/2018

15 MAC Layer

16 Lapisan MAC Lapisan MAC mencakup tiga bidang fungsional:
Pengiriman data yang reliable Kontrol akses Keamanan Lapisan MAC pada terdiri dari 2 sub-layer Distributed coordination function (DCF) Point coordination function (PCF) 9/22/2018

17 IEEE 802.11 DCF DCF sublayer menggunakan p-persistent CSMA
Lakukan listening ke medium Jika medium idle, kirim frame dengan probabilitas p Jika medium sibuk, tunggu sampai idle Tidak ada mekanisme CD (Collision Detection) seperti pada Ethernet Media wireless tidak dapat melakukan transmisi dan sensing secara paralel Sinyal wireless mengalami atenuasi/pelemahan sehingga sulit untuk dipakai mendeteksi collision The DCF sublayer makes use of a simple CSMA (carrier sense multiple access) algorithm. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise the station must wait until the current transmission is complete before transmitting. The DCF does not include a collision detection function (i.e., CSMA/CD) because collision detection is not practical on a wireless network. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a single delay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail.

18 IEEE DCF (2) menggunakan mekanisme CSMA/CA (Collision Avoidance) Dasarnya dari p-persistent CSMA Menggunakan random back-off counter sebelum memulai transmisi Menggunakan mekanisme Request to Send (RTS)/Clear to Send (CTS) untuk menghindari hidden terminal problem Menggunakan mekanisme Acknowledgement (ACK) untuk meningkatkan reliabilitas Untuk setiap pengiriman frame, ada jeda waktu tertentu yang disebut Inter Frame Space (IFS)

19 Backoff Interval When channel is busy, choose a backoff interval in the range [0, cw]. Count down the backoff interval when medium becomes idle. 1 slot time count down = 20 µs Count down is suspended if medium becomes busy again. When backoff interval reaches 0, transmit RTS. Binary exponential backoff in DCF: When a node fails to receive ACK, cw is doubled up (up to an upper bound). 9/22/2018

20 IEEE 802.11 Medium Access Control Logic
Using an IFS, the rules for CSMA access are as follows (Stallings DCC8e Figure 17.6): 1. A station with a frame to transmit senses the medium. If the medium is idle, it waits to see if the medium remains idle for a time equal to IFS. If so, the station may transmit immediately. 2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over. 3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off a random amount of time and again senses the medium. If the medium is still idle, the station may transmit. During the backoff time, if the medium becomes busy, the backoff timer is halted and resumes when the medium becomes idle. 4.If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred. To ensure that backoff maintains stability, binary exponential backoff, described in Chapter 16, is used. Binary exponential backoff provides a means of handling a heavy load. Repeated failed attempts to transmit result in longer and longer backoff times, which helps to smooth out the load. Without such a backoff, the following situation could occur: Two or more stations attempt to transmit at the same time, causing a collision. These stations then immediately attempt to retransmit, causing a new collision.

21 Basic CSMA/CA operations

22 Transmission without RTS/CTS

23 Inter Frame Space (IFS)
SIFS (Short IFS) DIFS (distributed coordination function IFS) PIFS (point coordination function IFS) The preceding scheme is refined for DCF to provide priority-based access by the simple expedient of using three values for IFS: • SIFS (short IFS): The shortest IFS, used for all immediate response actions, as explained in the following discussion • PIFS (point coordination function IFS): A midlength IFS, used by the centralized controller in the PCF scheme when issuing polls • DIFS (distributed coordination function IFS): The longest IFS, used as a minimum delay for asynchronous frames contending for access

24 SIFS Jeda waktu singkat untuk kegiatan yang butuh respon segera e.x. ACK, CTS SIFS memberikan prirotas tertinggi untuk pengirim SIFS used in following circumstances: Acknowledgment (ACK) station responds with ACK after waiting SIFS gap for efficient collision detect & multi-frame transmission Clear to Send (CTS) station ensures data frame gets through by issuing RTS and waits for CTS response from destination Poll response see Point coordination Function (PCF) discussion next Consider first the SIFS. Any station using SIFS to determine transmission opportunity has, in effect, the highest priority, because it will always gain access in preference to a station waiting an amount of time equal to PIFS or DIFS. The SIFS is used in the following circumstances: • Acknowledgment (ACK): When a station receives a frame addressed only to itself (not multicast or broadcast), it responds with an ACK frame after waiting only for an SIFS gap. This has two desirable effects. First, because collision detection is not used, the likelihood of collisions is greater than with CSMA/CD, and the MAC-level ACK provides for efficient collision recovery. Second, the SIFS can be used to provide efficient delivery of an LLC protocol data unit (PDU) that requires multiple MAC frames. In this case, the following scenario occurs. A station with a multiframe LLC PDU to transmit sends out the MAC frames one at a time. Each frame is acknowledged by the recipient after SIFS. When the source receives an ACK, it immediately (after SIFS) sends the next frame in the sequence. The result is that once a station has contended for the channel, it will maintain control of the channel until it has sent all of the fragments of an LLC PDU. • Clear to Send (CTS): A station can ensure that its data frame will get through by first issuing a small Request to Send (RTS) frame. The station to which this frame is addressed should immediately respond with a CTS frame if it is ready to receive. All other stations receive the RTS and defer using the medium. • Poll response: This is explained in the following discussion of PCF.

25 Besaran SIFS

26 PIFS dan DIFS PIFS digunakan oleh controller terpusat
for issuing polls has precedence over normal contention traffic but not SIFS DIFS digunakan untuk jeda pengiriman frame biasa DIFS = SIFS + 2 x slot time The next longest IFS interval is the PIFS. This is used by the centralized controller in issuing polls and takes precedence over normal contention traffic. However, those frames transmitted using SIFS have precedence over a PCF poll. Finally, the DIFS interval is used for all ordinary asynchronous traffic.

27 IEEE 802.11 MAC Timing Basic Access Method
Stallings DCC8e Figure 17.7a illustrates the use of these time values.

28 Problem Problems Sensing range  Transmission range
Hidden terminal problem Exposed terminal problem Sensing range  Transmission range Contention matters only at the receiver’s end 9/22/2018

29 Hidden Terminal Problem
X No carrier  OK to transmit 9/22/2018

30 Exposed Terminal Problem
Y X Presence of carrier  holds off transmission 9/22/2018

31 Existing Work MACA [Karn 1990] MACAW [Bharghanvan 1994] IEEE 802.11
Proposes to solve the hidden terminal problem by RTS/CTS dialog MACAW [Bharghanvan 1994] Increasing reliability by RTS/CTS/DATA/ACK dialog IEEE Distributed Coordination Function (DCF) Also use RTS/CTS/DATA/ACK dialog 9/22/2018

32 RTS/CTS dialog (1) Any node hearing this RTS will defer medium access
9/22/2018

33 RTS/CTS dialog (2) Any node hearing this CTS will defer medium access
9/22/2018

34 RTS/CTS/DATA/ACK dialog
Defer Defer Data ACK 9/22/2018

35 Transmission with RTS/CTS

36 IEEE MAC Frame Format Stallings DCC8e Figure 17.8 shows the frame format. This general format is used for all data and control frames, but not all fields are used in all contexts. The fields are: • Frame Control: Indicates the type of frame (control, management, or data) and provides control information. Control information includes whether the frame is to or from a DS, fragmentation information, and privacy information. • Duration/Connection ID: If used as a duration field, indicates the time (in microseconds) the channel will be allocated for successful transmission of a MAC frame. In some control frames, this field contains an association, or connection, identifier. • Addresses: The number and meaning of the 48-bit address fields depend on context. The transmitter address and receiver address are the MAC addresses of stations joined to the BSS that are transmitting and receiving frames over the wireless LAN. The service set ID (SSID) identifies the wireless LAN over which a frame is transmitted. • Sequence Control: Contains a 4-bit fragment number subfield, used for fragmentation and reassembly, and a 12-bit sequence number used to number frames sent between a given transmitter and receiver. • Frame Body: Contains an MSDU or a fragment of an MSDU. The MSDU is a LLC protocol data unit or MAC control information. • Frame Check Sequence: A 32-bit cyclic redundancy check.

37 Disadvantages of IEEE 802.11 DCF
High power consumption Hidden terminal problem not totally solved (e.g., collision of RTS) Exposed terminal problem not solved Fairness problem among different transmitting nodes Only providing best-effort service 9/22/2018