This depends upon the manufacturer. Some hardware access points have a recommended limit of 10, with other more expensive access points supporting up to 100 wireless connections. Using more computers than recommended will cause performance and reliability to suffer.
Software access points may also impose user limitations, but this depends upon the specific software, and the host computer's ability to process the required information.
multiple access points can be connected to a wired LAN, or sometimes even to a second wireless LAN if the access point supports this.
In most cases, separate access points are interconnected via a wired LAN, providing wireless connectivity in specific areas such as offices or classrooms, but connected to a main wired LAN for access to network resources, such as file servers.
A wireless computer can "roam" from one access point to another, with the software and hardware maintaining a steady network connection by monitoring the signal strength from in-range access points and locking on to the one with the best quality. Usually this is completely transparent to the user; they are not aware that a different access point is being used from area to area. Some access point configurations require security authentication when swapping access points, usually in the form of a password dialog box.
Access points are required to have overlapping wireless areas to achieve this
*** NOT ALL ACCESS POINTS SUPPORT ROAMING
LAN to LAN Wireless Communication Each Access Point acts as a Router or Bridge to connect its own LAN to the wireless network
Mobile Communication Technology according to IEEE Local wireless networks WLAN 802.11 802.11a 802.11b 802.11i/e/…/w 802.11g WiFi 802.11h 802.15.4 802.15.1 802.15.2 Bluetooth 802.15.4a/b ZigBee 802.15.3 802.20 (Mobile Broadband Wireless Access) + Mobility WiMAX 802.15.3a/b 802.15.5
Station (STA): terminal with access mechanisms to the wireless medium
Independent Basic Service Set (IBSS): group of stations using the same radio frequency
802.11 LAN IBSS 2 802.11 LAN IBSS 1 STA 1 STA 4 STA 5 STA 2 STA 3
IEEE standard 802.11 mobile terminal access point fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC
Medium Access Medium Busy Contention next frame Time DIFS Direct Access if “Medium is Free” >= DIFS DIFS PIFS SIFS Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses PCF Inter-frame spacing (PIFS): medium priority, time bounded service DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority
Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses
PCF Inter-frame spacing (PIFS): medium priority, time bounded service
DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority
802.11 - competing stations - simple version(for broadcast) t busy bo e station 1 station 2 station 3 station 4 station 5 packet arrival at MAC DIFS bo e bo e bo e busy elapsed backoff time bo r residual backoff time busy medium not idle (frame, ack etc.) bo r bo r DIFS bo e bo e bo e bo r DIFS busy busy DIFS bo e busy bo e bo e bo r bo r
St-3 has the first request and sends the packet. St-3 senses the medium, waits for DIFS and accesses the medium.
Stns 1,2 & 5 have to wait for at least DIFS after Stn-3 stops sending the data.
All three stations now start off a back off timer and start counting down their back off timers.
Back off time = Elapsed back off time
+ residual back off time.
@@ It is to be noted that if the residual time of device-1 is more than that of device-2, it means that device-1 had waited for a lesser time as compared to device-2 and so, device-2 gets a priority to access the medium.
If the contention window is small, too many stns will contend and so, collisions will be substantial.
If the contention window is larger, there will be noticeable delays.
802.11 - CSMA/CA access method II (for unicast) t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention
Sending unicast packets
station has to wait for DIFS before sending data
receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)
automatic retransmission of data packets in case of transmission errors (No ACK is sent)
But sender has to wait for the medium access. No special privileges for retransmitted data.
No. of retransmissions are limited.
802.11 – DFWMAC Hidden Terminal Avoidance using RTS & CTS) t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS SIFS NAV (RTS) NAV (CTS)
Sending unicast packets
station can send RTS with reservation parameter after waiting for DIFS (RTS specifies receiver’s Id, amount of time needed for transmission of data and also time for ACK )
acknowledgement via CTS after SIFS by receiver (if ready to receive) (all stns receive this)
sender can now send data at once, acknowledgement via ACK
other stations store medium reservations distributed via RTS and CTS. Other stns have to set ‘Net allocation Vector(NAV)’ (contained in CTS) that specifies how long they need to wait before trying again for transmission.
While transmitting frag1, one more duration is also transmitted corresponding to the duration of the following fragment and the acknowledgement.
Thus the medium is reserved for the following fragment(frag-2)
Other nodes which receive this will adjust their NAV
If there is no network change (static network), the set of nodes receiving this duration is the same as that indicated in the original RTS control packet.
Because of mobility, this is not the case in most situations.
The receiver will receive frag1 and send an ACK1 that contains duration of the net fragment transmission.
The other set of nodes will adjust their NAV
Thus, the current fragments would contain info about the following ones.
DFWMAC-PCF I PIFS stations‘ NAV wireless stations point coordinator D 1 U 1 SIFS NAV SIFS D 2 U 2 SIFS SIFS SuperFrame t 0 medium busy t 1
DFWMAC-PCF I (Point Coordination Function -Polling)
To achieve a time bounded service, the PCF is used on top of the DCF. It requires an access point. The access point splits time into super frame periods.
Super Frame : contention-free period
+ contention period.
In the scheme above, contention-free period should ideally start at to. But the medium is busy till t1.
Actually, PCF has to wait for PIFS before accessing the medium. As PIFS is less than DIFS, no other stn can send the data earlier than PCF.
PCF sends data to stn-1(polling starts)
Stn-1 responds after SIFS.
After waiting for SIFS, the PCF sends data to Stn-2. Stn-2 answers with U2.
Polling continues for D3 but D3 has no data.
Finally, after polling all stns, the PCF can send a ‘End Marker (CFend) indicating that the contention period can start all over.
Using PCF automatically sets NAV and prevents other stns from sending data.
DFWMAC-PCF I (Point Coordination Function -Polling)
The Process of polling with PCF is exactly like TDMA where all users get a fair and equal chance to send data.
If a node does not send data, it is an overhead.
DFWMAC-PCF II t stations‘ NAV wireless stations point coordinator D 3 NAV PIFS D 4 U 4 SIFS SIFS CF end contention period contention free period t 2 t 3 t 4
802.11 - Frame format Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC 2 2 6 6 6 6 2 4 0-2312 bytes Protocol version Type Subtype To DS More Frag Retry Power Mgmt More Data WEP 2 2 4 1 From DS 1 Order bits 1 1 1 1 1 1
control frames, management frames, data frames
important against duplicated frames due to lost ACKs
- DA Transmitter Address(TA) Receiver Address(RA) 1 1 - DA SA BSSID 0 1 - SA BSSID DA 1 0 - Basic Service Set Id (BSSID) Source Address (SA) Distribution address (DA) 0 0 Address 4 Address 3 Address 2 Address1 From DS To DS
For sender, it is not an issue as the transmitter knows when it is ready for sending frames.
Transmitter has to buffer the frame to make sure that it will transmit when the receiver is ready to receive.
stations wake up at the same time periodically and listen to the transmitter.
Waking up at the right time needs the TSF.
Along with beacon, a Traffic Indication Map(TIM- containing the list of stns
for which buffering has been done in the AP.) is also sent.
Traffic Indication Map (TIM)
list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)
list of broadcast/multicast receivers transmitted by AP
Ad-hoc Traffic Indication Map (ATIM)
announcement of receivers by stations buffering frames
more complicated - no central AP
collision of ATIMs possible (scalability?)
Power saving with wake-up patterns (infrastructure) .........Only One Station Shown........... TIM interval t medium access point busy D busy busy busy T T D DTIM interval B B station p d d T TIM D DTIM B broadcast/multicast awake p PS poll d data transmission to/from the station
Power saving with wake-up patterns (infrastructure)
All stations wake up prior to TIM/DTIM.
CASE WITH TIM :
Access Point buffers frames when the receiver is in the sleep mode.
With every beacon, a Traffic Information Map (TIM-containing the list of stations for which uni-cast buffers are stored in AP) is sent.
AP sends a broadcast frame and the receiver stays awake to receive it.
Receiver then sleeps and wakes up just before the next TIM.
TIM is delayed since the medium is busy. So, the receiver stays awake.
AP has nothing to send and so, the receiver goes to sleep.
In the next TIM interval, the AP indicates that the stn is the destination for a buffered frame.
Stn answers with a PS poll and stays awake to receive data.
In the next DTIM interval, the AP has more broadcast data to send.
This is deferred since medium is busy.
Power saving with wake-up patterns (ad-hoc) awake D transmit data t station 1 B 1 B 1 B beacon frame station 2 B 2 B 2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data A transmit ATIM
Each participating station has to buffer the data since access points do not exist.
In the period that all stations are awake, all the participating station send a list of buffered frames and the stations that are targeted to receive these. These are sent through “Adhoc Traffic Information Map(ATIM)”
All stations stay awake during this ATIM period and listen to the ATIM.
In the example, ATIM of station-1 contains the address of station-2.
Stn-2 acknowledges the ATIM, waits for the data and later acknowledges the data.
With more stations wanting to send their frames, collisions can be substantial.
Access delay is not easy to predict and so, QoS can’t be guaranteed.