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IEEE draft, namely Draft 3.0, and used that as the baseline for an interoperability certification of first-wave products in mid-2013. Later, and more in line with the ratification date of 802.11ac (that is, after December 2013), the Wi-Fi Alliance is expected to refresh its 802.11ac certification to include testing of the more advanced 802.11ac features.

This second-wave certification could include features such as channel bonding up to 160 MHz, four spatial streams, and MU-MIMO. Overall, this arrangement closely follows how 802.11n was rolled out. As of February 2014, the launch date for Wave 2 certification is yet to be determined.

Enterprise networks considering an investment in infrastructure Wi-Fi have two excellent choices: (1) buy 802.11n APs, since they deliver a remarkable level of performance, they are available today, and 802.11n is widely deployed in client products, or (2) wait for 802.11ac APs and their state-of-the-art performance. A third option avoids the wait: invest in a modular 802.11n AP such as the Cisco® Aironet®3600 Series Access Point, which is readily field-upgradable to 802.11ac, or the Cisco Aironet 3700 Series Access Point, which supports an integrated 802.11ac radio.

802.11ac will have a few effects on existing 802.11a/n deployments, even if the deployment is not upgraded to 802.11ac immediately: (1) the wider channel bandwidths of neighboring APs require updates to radio resource management, or RRM (and in particular the dynamic channel assignment algorithm), and (2) 802.11a/n wireless intrusion protection systems (WIPS) can continue to decode most management frames such as beacon and probe request/response frames (that are invariably sent in 802.11a format) but do not have visibility into data sent in the new 802.11ac packet format.

One thing not to worry about is compatibility. 802.11ac is designed in a deep way to coexist efficiently with existing 802.11a/n devices, with strong carrier sense, a single new preamble that appears to be a valid 802.11a preamble to 802.11a/n devices, and extensions to request-to-send/clear-to-send (RTS/CTS) to help avoid collisions with users operating on slightly different channels.

V. Differences between 802.11ac and 802.11n

 802.11ac has avoided the battles of 802.11n and instead has focused on extending the tremendous advances made in 802.11n to deliver the next generation of speed and robustness.

For instance, 802.11n pioneered aggregation through the selective use of A-MPDU, A-MSDU, and A-MPDU of A-MSDU 802.11ac actually requires every 802.11ac transmission to be sent as an A-MPDU aggregate. This is due in part to the intrinsic efficiency of A-MPDU, as well as to some other factors.

In a further example, 802.11ac extends the 802.11n channel access mechanism: virtual carrier sense and back-off occur on a single 20-MHz primary channel; CCA is then used for the remaining 20-MHz sub-channels immediately before transmitting on them.

Given the power of A-MPDU and the 802.11n channel access mechanism, 802.11ac actually didn’t need to innovate much in the MAC. Indeed, extensions to the RTS/CTS mechanism are the only new mandatory MAC feature.

802.11n does include many options with reduced value, and 802.11ac takes a very pragmatic approach to them. If a “useless” option is used and affects a third-party device, 802.11ac typically forbids an 802.11ac device (operating in 802.11ac mode) from using the option. If a “useless” option has not been used in 802.11n products or affects only the devices that activate the option, the feature is not updated for 802.11ac but is instead “left to die.”

For instance, there is no 802.11ac version of the “802.11n greenfield” preamble format. 802.11ac defines only one preamble format, which, to legacy 802.11a/n devices, will look safely like an 802.11a preamble followed by a payload with a bad CRC. This means that legacy devices won’t try to transmit over the top of the 802.11ac transmission, nor will they attempt to send a bad payload up the stack.

802.11n introduced reduced inter-frame spacing (RIFS), which reduces overheads between consecutive transmissions, but experience has shown that A-MDPU solves much the same problem even more efficiently.

802.11ac devices operating in 802.11ac mode are not permitted to transmit RIFS.

802.11n features that are not updated for 802.11ac (or are explicitly forbidden for 802.11ac devices operating in 802.11ac mode) include all the 802.11n sounding options, including extension LTFs, the calibration procedure, antenna selection, PCO, L-SIG TXOP protection, unequal modulation, 4×3 and 3×2 STBC modes, MCS32, and dual CTS protection.

VI. Analysis of the IEEE 802.11 Capacity

The Protocol capacity differentiates for the different MAC protocols, but this is also affected by a couple of other parameters, like in example, the number of active Stations and the route on which the active stations contribute for the load. max  is the capacity in case that there are M count active stations in the asymptotic conditions (in example, all the M workstations always have a ready for transfer packet); Single is the capacity in the extreme case of one active node. In a MAC protocol which is ideal from a utilization standpoint, both max and Single must be equal to 1.

The capacity of the IEEE 802.11 MAC protocol is estimated by increase in the asymptotic conditions, the coefficient between the average length of a message and the average time tv, which is needed on the channel for the message transfer; tv is also addressed to the average virtual time for transfer.

For this analysis another value S is need to show the time needed for a successful transfer, in example, the interval between the beginning of the transfer, where there are no collisions and the and the receiving of its’ ACK plus DIFS.

Lemma 1.  When m is representing the time needed for the packet transfer and is representing the maximum propagation delay in the transfer between two WLAN stations, then

S ≤ m + 2+ SIFS + ACK + DIFS

Verification:

We are considering that the successful transfer is performed by station A, which at the time t0, transfers a packet to station B. AB is the delay in the transfer between the two stations, without any loss of generality we assume AB < m

Fig.6: Events in a successful transfer

As shown in Fig.6 the order of events in a successful transfer is:

1- A, begins to transmit at time t0 ,

2- B, begins reception at time t0+ AB ,

3- A completes transmitting at time t0+m ,

4- B completes reception at time t0+m+ AB,

5- B begins the ACK transmission at time t0+m+ AB +SIFS,

6- A begins the ACK reception at time t0+m+ AB +SIFS+t,

7- B completes the ACK transmission at time t0+m+ AB +SIFS+ACK,

8- A completes the ACK reception at time t0+m+ AB +SIFS+ACK+AB,

9- A can start the next transmission at time t0+m+ AB +SIFS+ACK+AB+DIFS,

Hence S = m + 2AB+ SIFS + ACK + DIFS from which Lemma 1 immediately follows.

Single can be calculated, noting that when only one station is active its’ average delay time is E[CW] and then tv = E[S] + E[CW].

From Lemma 1 where m  is the average time for transmission and E [CW1] is equal to the half of the minimal value of CW.

The only unknown value in (1) is m . It is considered, that the lengths of the packets are integers multiple of the sloth length, tsloth. Moreover the lengths of the packets are geometrically distributed with the value q. Therefore m = tsloth ∫(1-q).

When more of one station is active the virtual time for transmission includes the successful transmission and the intervals of collisions (Fig. 7)

Fig.7 Structure of virtual transmission time

Figure 7 shows that, before successful transmission, collisions may appear together with the periods, in which the area of transmission is in awaiting of effect from the algorithm (Idle periods). We have to note that additional overheads are associated with collisions: because of the carrier sensing mechanism colliding messages prevent the network stations from observing that the channel is idle for a further time interval less or equal to the maximum propagation time  . Furthermore, according to the MAC protocol, after each collision the medium must remain idle for an interval equal to a DIFS. From these observations it follows that

Where Idle_pi  and Colli are the lengths of the i-th idle period and collision, respectively; Nc is the number of collisions in a virtual time. In the IEEE 802.11 protocol the length of a collision is equal to the maximum length of the colliding packets, and hence it depends on the packet size distribution and on the backoff algorithm which determines the number of colliding stations. The length of the idle periods and the number of collisions depends on the backoff algorithm. To compute the unknown quantities in (2) by exactly taking into consideration the backoff algorithm used in the standard is very difficult, if not impossible, due to the temporal dependencies which it introduces. According to the standard, by denoting with I the number of attempts to successfully transmit a packet, a station for each packet will experience I backoff times {B1, B2,…. BI} which are sampled in a uniform way in intervals of length {CW1, CW2,…. CWI}. In this paper to simplify the protocol analysis we assume that the backoff times have a different distribution. Specifically, we assume that the tagged station for each transmission attempt uses a backoff interval sampled from a geometric distribution with parameter p, where p=1/(E[B]+1) and E[B] is the average value of {B1, B2,…. BI}, expressed in number of slots. Lemma 2 provides an expression for E[B].

Lemma 2. E[B]=(E[CW]-1)/2, where E[CW] is the average contention window.

The assumption on the backoff algorithm implies that the future behavior of the station does not depend on the past and hence, in a virtual transmission, i) the idle periods time {Idle_pi } are i.i.d sampled from a geometric distribution with average E [Idle_p]; ii) the collision lengths { Colli } are i.i.d with average E[Coll]. Thus equation (2) can be rewritten as

tv =E[Nc]{E[Coll]++ DIFS}+E[Idle_p].( E[Nc]+1)+E[S]

In the following we assume that E[CW] is known and we derive exact expressions for the unknowns in the above equation: E[Idle_p], E[Nc] and E[Coll].

For large values of M the number of stations ready for transmission is less dependent on the virtual time evolution, hence assumptions i) and ii) become more and more realistic as M increases. The results presented in this paper also indicate that for M=10 the above assumptions do not introduce significant errors in the capacity analysis.

LEMMA 3. Assuming that, for each station, the backoff interval is sampled from a geometric distribution with parameter p:

The average virtual transmission time in asymptotic conditions is completely defined by the relationships defined in Lemma 3. However, before being able to compute the virtual transmission time we need to estimate the parameter p.

VII. Standards for WLAN security

All technologies for wireless connection use one or another type of data encryption for security reasons. Networks using the IEEE 802.11 standard, use WEP (Wired Equivalent Privacy) functions for Data encryption. By the type of the setup the encryption can be 64, 128 or 256 –bit. WEP was included as the privacy component of the original IEEE 802.11 standard ratified in 1997. WEP uses the stream cipher RC4 for confidentiality, and the CRC-32 checksum for integrity. It was deprecated in 2004 and is documented in the current standard. Standard 64-bit WEP uses a 40 bit key (also known as WEP-40), which is concatenated with a 24-bit initialization vector (IV) to form the RC4 key. At the time that the original WEP standard was drafted, the U.S. Government\'s export restrictions on cryptographic technology limited the key size. Once the restrictions were lifted, manufacturers of access points implemented an extended 128-bit WEP protocol using a 104-bit key size (WEP-104). A 64-bit WEP key is usually entered as a string of 10 hexadecimal (base 16) characters (0–9 and A–F). Each character represents 4 bits, 10 digits of 4 bits each gives 40 bits; adding the 24-bit IV produces the complete 64-bit WEP key (4 bits × 10 + 24 bits IV = 64 bits of WEP key). Most devices also allow the user to enter the key as 5 ASCII characters (0–9, a–z, A–Z), each of which is turned into 8 bits using the character\'s byte value in ASCII (8 bits × 5 + 24 bits IV = 64 bits of WEP key); however, this restricts each byte to be a printable ASCII character, which is only a small fraction of possible byte values, greatly reducing the space of possible keys. A 128-bit WEP key is usually entered as a string of 26 hexadecimal characters. 26 digits of 4 bits each gives 104 bits; adding the 24-bit IV produces the complete 128-bit WEP key (4 bits × 26 + 24 bits IV = 128 bits of WEP key). Most devices also allow the user to enter it as 13 ASCII characters (8 bits × 13 + 24 bits IV = 128 bits of WEP key).A 152-bit and a 256-bit WEP systems are available from some vendors. As with the other WEP variants, 24 bits of that is for the IV, leaving 128 or 232 bits for actual protection. These 128 or 232 bits are typically entered as 32 or 58 hexadecimal characters (4 bits × 32 + 24 bits IV = 152 bits of WEP key, 4 bits × 58 + 24 bits IV = 256 bits of WEP key). Most devices also allow the user to enter it as 16 or 29 ASCII characters (8 bits × 16 + 24 bits IV = 152 bits of WEP key, 8 bits × 29 + 24 bits IV = 256 bits of WEP key). Usually more than one passwords can be used on the devices in the network, but at one stage only one is used, and the change is made manually by the user. This is the main disadvantage of WEP. In every Access point ESSID is used for access control. Without this information a workstation cannot connect to the Access Point. Also there can be configured a list of MAC addresses of devices that can access the network. The encryption of the signal is made by the RC4 algorithm with a 40-bit key, but there are simpler methods of data and signal encryption.

In some publications, it is said that the WEP key can be decrypted in 24 hours, by spoofing the packet traffic, so the best solution is to change the WEP keys, but this also brings discomfort.

Newer methods to the WEP keys are present: WPA (Wi-Fi Protected Access) and WPA2. They are more secure that their predecessor WEP. Also based on the RC4 encryption algorithm, but with more dynamical change of the Keys. The PSK (PreShared Keys) mechanism for access to the network and additional TKIP (Temporal Key Integrity Protocol) are used. TKIP changes the keys of every sent packet, which makes the packet spoofing and decryption more difficult.

WPA2 uses AES (Advanced Encryption Standard), which makes it the most secure of the three.

VIII. Specifications of WLAN

Number of Mobile Users-20

Location of the Users- Small Business Office – 20X20

Services Provided by WLAN:

-access to unique resources (Disk array, printer, scanner, and other), which are only in the network;

 -optimization of the volume of information, stored in the system by maintaining a minimal count of copies of programs, Documents, data and archives;

-Possibility of working “In group” on Shared files and data bases in real time (online);

-E-mails, exchange of short messages in real time (chat), audio and video conferences, exchange of document, meetings and briefings;

-Constant connection of the type 24/7/365;

-Flexible resources for security of data, by User profiles (permissions), authentication and encryption of transfers.

8.1 WLAN Requirements:

- Little possibility for transfer errors;

- Possibility for multichannel transmission with separation in frequency and time, with the method of jumping frequency and effective use of the specter;

- Flexible schematics for transmission- Multicasting and broadcasting;

- Possibility of p2p Transmission (Point-to-Point);

- Easy configuration and management;

- Stable Parameters in moments of high usage;

- Low cost- the cost of the WLAN does not exceed 30% of the cost of the linked devices.

8.2 Necessary resources for Designing WLAN:

-Access points (AP hubs) - 3 pcs

-LAN junction board- 1 pcs

-Mobile Stations- 20 pcs

-Cable- STP cable cat 5- 100m

Structure of the Network

For constricting the WLAN with the parameters given it is enough to install 3 Access Point, performing the function of wireless Hubs with radio interface to the (Mobile) Workstations and Ethernet interfaces for connection with the LAN junction boards.

Around every access point a coverage zone is formulated, called micro-cell or just cell. With the installation of the wireless points, we will create a cell network, in the design of which we will keep in mind a few rules:

Firstly, all APs will be configured with the same SSID – Network Name.

Secondly, two neighbor cells will work on different channel, so they do not disturb each other.

Thirdly, the cells will overlap spatially in the range of 15-20% in the end zones of coverage, so there is a fluent transition between the cells.

Choosing The Technology for realizing the WLAN:

By the given requirements and resources for the realization of the WLAN, the standards that are chosen are IEEE802.11ac for the wireless segment and FastEthernet IEEE 802.3u 100BaseTX for the Local Area Network segment.

The dynamical assignment of IP addresses will be provided by a DHCP server, which will be configured in the APs.

For Data security in the wireless network, the WPA2 method will be used. WPA2 is based on the RC4 encryption algorithm, but with dynamic change of the keys. PSK mechanism (Reshaped Keys) and additional TKIP (Temporal Key Integrity Protocol) will be used for access to the Network.

The chosen technologies will most fully cover the requirements for the build of the Network- provision of services, coverage of the provided space, number of users and secure traffic in the wireless network.

Realization of the WLAN and assessment of needed resources

There is a great number of manufacturers of network devices like:

Trend Net; 2 Wire; 3Com; Acer NeWeb Corporation; Acrowave Systems Co., Ltd; Agere; Aironet; Alcatel; Apple Computer, Inc.; Breezecom; Bromax; Cisco Systems; CyLink; Delta Networks, Inc.; D-Link; Fujitsu; Hewlett-Packard; Intel; Linksys Group, Inc.; Lucent Technologies; Melco Inc.; Nokia Networks; Nortel Networks Corporation; Proxim; Samsung; SMC Networks; Toshiba and many others.

IX. Needed resources for the creation of the WLAN:

1. Access Points LINKSYS LAPAC2600 – 3 pcs

LINKSYS LAPAC2600 - the newest business access point from Linksys will be used in this WLAN. It is a next generation MU-MIMO access point for high-density wireless environments. The access point keeps the small to medium business running smoothly with combined speeds of up to 2.53 Gbps. It also greatly simplifies administration with its Clustering feature that lets you manage multiple access points from a single point of control. The LAPAC2600 Access Point uses MU-MIMO (Multi-User, Multiple-Input, Multiple-Output) technology to ensure multiple simultaneous connections. MU-MIMO provides a dedicated data stream for each connected client, so the employees won\'t have to compete with one another for bandwidth. Multiple users will be able to simultaneously engage in video conferencing, download large files, and perform other data-intensive tasks with minimal or no latency on the devices.

1.1 TECHNICAL SPECS

Model Name:

-Linksys Business Pro Series Wireless-AC Dual-Band MU-MIMO Access Point

Network Standards:

-IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac  

-MU-MIMO

-802.11r and 802.11k

-Power over Ethernet Plus (PoE+) IEEE 802.3at

Radio Frequency Bands:

-2.4GHz and 5GHz (Concurrent)  

Ports:

-1 x Primary Gigabit LAN Port with IEEE 802.3at PoE+  

-1 x Secondary Gigabit LAN Port for Link Aggregation  

-1 x 12V/2.5A power port

LEDs:

-1 x System LED  

Buttons:

-Reset Button  

MIMO:

-4x4

Antenna Type:

-Internal PCBA Antenna  

Antenna Gain:

-4.4 dBi @2.4GHz; 5.2 dBi @5GHz  

RF Transmit Power:

-High Power PA  

Radio Frequency Channels:

-2.412 to 2.462 GHz: 11 channels

-5.180 to 5.240 GHz: 4 channels

-5.745 to 5.825 GHz: 5 channels

Receive Sensitivity:

-802.11b @ 11Mbps: -85dBm  

-802.11a/g @ 54Mbps: -73dBm  

-802.11n @ HT20 MCS7: -67dBm  

-802.11n @ HT40 MCS7: -65dBm  

-802.11n @ HT40 MCS7: -65dBm

Maximum Power Consumption:

-24W  

Dimensions (LxWxH):

-9.57 x 9.33 x 1.72 inches  

Weight:

-1.87 lb

Operating Temperature:

-0° to 40°C (32° to 104°F)  

Storage Temperature:

-20° to 70°C (-4° to 158°F)  

Operating Humidity:

-10% to 85% non-condensing  

Storage Humidity:

-10% to 90% non-condensing  

Mounting:

-Ceiling

-Wall

Security Features:

-Multiple SSIDs (16)  

-SSID Broadcast  

-SSID to VLAN Mapping  

-Wi-Fi Protected Access (WPA/WPA Mixed, WPA2) Personal and Enterprise  

-RADIUS  

-IP and Mac-Based Access Control  

-Rogue AP Detection  

-802.1X Supplicant

Security Lock:

-Kensington Security Lock  

Quality Of Service:

-WMM, Bandwidth Management, Client QoS, DiffServ Policy (IPv4 and IPv6)

Management Interface:

-HTTP

-HTTPS

-SNMP

-Regulatory Compliance:

-FCC, CE, IC  

Advanced Features:

-Wireless Distribution System (WDS), Workgroup Bridge, IGMP/MLD Snooping, Scheduler, Band Steering, IPv6 Interface, Link Aggregation, Dual Image Support, Captive Portal, Clustering

Event Notification:

Local Log, Remote Syslog, and Email Alerts  

Network Diagnostics:

Log, Ping and Packet Capture  

Warranty and Support:

Limited Lifetime Warranty  

Centralized Management System:

Single Point Control: Clustering

2. LAN Commutator D-LINK DES-3018 – 1 pcs

Technical Specifications:

• Dimensions (WxDxH) 44.1 cm x 20.7 cm x 4.4 cm• Weight 2.1 kg

• Ports 16x Ethernet 10Base-T, Ethernet 100Base-TX - RJ-45 (half/full duplex mode), RS-232 - DB9

• Network Protocol Ethernet, Fast Ethernet• Standards IEEE 802.3, IEEE 802.3u, IEEE 802.1D, IEEE 802.1Q, IEEE 802.1p, IEEE 802.3x, IEEE 802.3ad (LACP), IEEE 802.1w, IEEE 802.1x, IEEE 802.1s•

Power supply 120/230 V (50/60 Hz)

LAN Cable STP cat 5 – 100 m

LAN cable FTP, 4 twisted pairs, with PVC insulation

3. Mobile Workstations Dell Inspiron 5758- 20 pcs

Technical Specifications:

PRODUCT INFORMATION

SKU

115675

Mfr Part#

I5758-2857BLK

UPC

884116204046

GENERAL INFORMATION

Series

Inspiron

Model Number

i5758-2857BLK

Lifestyle-Desktop Replacement

Color – Lid - Black Gloss

Color – Base - Black

Color – Keyboard - Black

Platform - Notebook PC

OPERATING SYSTEM

Windows 10 Home 64-bit

PROCESSOR

CPU Brand - Intel

CPU Core - Dual-Core

CPU Type - Core i3

CPU Speed - 4030U (1.9GHz)

Smart Cache - 3MB

FSB - DMI2 5GT/s

CPU Main Features

- Hyper-Threading Technology, Virtualization Technology, Intel 64, Enhanced Intel SpeedStep Technology, Idle States, Thermal Monitoring Technologies, Execute Disable Bit, Virtualization Technology for Directed I/O (VT-d), Intel Anti-Theft Technology, Intel VT-x with Extended Page Tables (EPT), Intel Identity Protection Technology, Intel Secure Key, Intel HD Audio Technology, Intel Rapid Storage Technology, Intel Smart Connect Technology, Intel AES New Instructions

MEMORY

Total Memory - 8GB

Memory Speed - DDR3L-1600

Form Factor - 204-pin SO-DIMM

Memory Slots (Total) - 2

Memory Slots (Available) - 0

Maximum Memory Supported - 16GB

Memory Configuration - 2 x 4GB

HARD DRIVE

HD Interface - SATA 6Gb/s

HD RPM - 5,400 RPM

HD Capacity - 1TB

HD Configuration - 1 x 1TB

MULTIMEDIA DRIVES

Optical Drive Interface - SATA

Optical Drive - DVDRW

Optical Drive Specifications - Tray Load DVD Drive (Reads and Writes to DVD/CD)

DISPLAY

Screen Size - 17.3\"

Display Type - HD+ TruLife LED-backlit LCD

Resolution - 1600x900

Widescreen - Yes

Front Camera - Yes

Front Camera Resolution - 720p

Front Camera Features - HD

VIDEO

GPU Type - Intel HD

Video Memory - Shared

Video Connectors - 1 x HDMI

AUDIO

Speakers - Stereo Speakers

Built-in Microphone - Dual Microphones

Audio System - Waves MaxxAudio

COMMUNICATIONS

LAN - Ethernet LAN

LAN Data Transfer Rate - 10/100Mbps

WLAN - 1x1 Dual Band 802.11ac

Wireless Technology - 802.11ac

Bluetooth - Bluetooth 4.0

CARD READER

Media Supported - Secure Digital, SDHC, SDXC

Memory Card Reader - 3-in-1 Digital Media Reader

EXPANSION

Expansion Slots - 1 x M.2

PORTS & CONNECTORS

USB 2.0 - 2 x USB 2.0 (Type-A)

USB 3.1 - 1 x USB 3.1 (Gen 1 Type-A)

HDMI - 1

LAN RJ-45 - 1

Audio - 1

SECURITY

Physical Device Security - Security Lock Slot

KEYBOARD & MOUSE

Keyboard Features - Backlit Keyboard

Keyboard - Standard

Mouse Type - Touchpad

POWER

Battery - 4-Cell Lithium-Ion

Battery Life

Up to 5 Hours, 45 Minutes (Stated battery Life is estimated based on manufacturers engineering testing for a new battery. Actual performance will vary based on notebook settings, environmental conditions, and usage. Battery capacity decreases over time and use.)

PHYSICAL SPECIFICATIONS

Width - 16.41\"

Depth - 11.15\"

Height - 1.06\"

Weight - 6.22 lbs.

Box Size - 21.90\" x 3.30\" x 13.90\"

MANUFACTURER WARRANTY

Parts - 1 Year Limited

Labor - 1 Year Limited

X. Structural Schematics of WLAN

XI. Building an 802.11ac Network

Building a network may begin with detailed information gathering to make a good prediction of the number and location of APs required, or it may be more iterative, where a few APs are used to “test the waters” with a deployment in a key gathering spot for users. In iterative deployments, using the management capabilities of the wireless LAN system you are evaluating is a good way to obtain feedback on your assumptions. Is the client mix what was expected? Are the supposed key applications the most com‐ monly used applications? Channel Selection At first glance, 802.11ac’s addition of yet another channel width would seem to com‐ plicate the configuration process because it means network designers must manage yet another parameter with backward compatibility implications. However, the design of 802.11ac’s channel coexistence mechanisms provides a rough guideline to channel al‐ location. Because 802.11ac clients can measure the available bandwidth, an 802.11ac network can take up as much capacity as is available, and two 802.11ac networks sharing the same frequency space can share the wide channels. Figure 5-4 shows how a network can be built with minimum channel overlap. For the purpose of the figure, each AP’s frequency space is represented by a “stack” of bars, where the shortest bar is the primary 20 MHz channel, the next-longest bar is the pri‐ mary 40 MHz channel, and the longest bar is the primary 80 MHz channel. When two APs share a channel, the relevant bar is blended between two colors.

The figure shows a network being brought up in the following steps:

1. When the first AP is powered up, it is straightforward. There is no existing network, and therefore the AP can choose any channel. In the figure, the AP represented by blue bars chooses channel 40. It will therefore take channel 40 for its 20 MHz transmissions, channels 36 and 40 for its 40 MHz transmissions, and channels 36 through 48 for its 80 MHz transmissions.

2. The second AP poses no problems, either. There is a free 80 MHz channel from channels 52 through 60, so the AP represented by green bars chooses, say, channel 60. (All four channels will choose the non-overlapping 80 MHz channel, so they are all equivalent.)

3. When the third AP, represented by orange bars, is added, it has no free 80 MHz channel. Therefore, it needs to choose a minimum-interference channel. Stepping down from the desired 80 MHz channel width, the orange AP can choose the 40 MHz channel of channels 44 and 48. The overlap between the orange and blue APs is shown by the way that the 80 MHz channel is blended between orange and blue.

4. The addition of the fourth AP, represented by purple, takes a similar path as the addition of the orange AP in the previous step. It has no free 80 MHz channel, so it must choose the least-overlapping 40 MHz channel. The only unoccupied 40 MHz channel is channels 52 and 56, so it chooses either of those two primary 20 MHz channels as its operating channel. The figure shows it choosing channel 56.

5. Finally, when the fifth AP (represented by the color red) comes up, it cannot choose an unoccupied 80 MHz channel or an unoccupied 40 MHz channel. Therefore, it must choose a free 20 MHz channel. In the figure, it is shown occupying channel 48. The 40 MHz channel composed of channels 40 and 48 is blended between orange and red to show that it is being shared between those two APs, and the 80 MHz channel is blended between blue, orange, and red to show that all three APs share the 80 MHz channel.

This process illustrates one important advantage of 802.11ac: supporting multiple channel widths at the same time enables 802.11ac clients to “burst” capacity when it’s available. Network administrators should design their networks for minimum channel overlap for wide channels, and let the narrower transmissions fall where they must to accomplish that goal. Keeping the wide 80 MHz channels as free as possible will enable as many fast transmissions as possible from 80–MHz-capable clients and is a worthy goal.

Network Tuning and Optimization

Part of monitoring the network is watching for conditions that will lead to substandard service, and, if possible, applying new configurations to network devices to improve performance and functionality. Fundamentally, the 802.11 MAC manages airtime. APs turn available airtime into bits sent to and from the network. Performance tuning in 802.11ac uses similar techniques to performance tuning in previous physical layers: reduce airtime contention whenever possible, and work to pack as many bits as possible into each available microsecond.

With its emphasis on technologies that assist in improving dense networks, 802.11ac APs will be packed together quite tightly. Reducing the coverage area of each AP is an important way of providing more radio capacity, but it is by no means the end of the story. Even though the 2.4 GHz band is not capable of supporting 802.11ac, it still has an important role to play as a source of capacity in busy networks. When serving areas with maximum density, enable load-balancing features in your wireless network equip‐ ment. Many products support multiple forms of load sharing to optimize network performance. Identifying 802.11ac clients, especially those capable of wide channel op‐ 120 | Chapter 5: 802.11ac Planning erations, and moving them to 802.11ac radios will be an important component of boosting network capacity. In high-capacity areas, multiple adjacent APs on nearby channels will need to share capacity.

Many manufacturers select default settings that are generally good for data networking and will deliver acceptable performance for web-based applications and email. In fact, many APs include a feature that gives priority to high-speed 802.11ac frames because they move data much more quickly than the older 802.11a/b/g/n frames. When trans‐ mitting a 1,500-byte Ethernet frame, 802.11ac is lightning-fast compared to its prede‐ cessors, especially if a wider channel is available for the transmission. Preferential treat‐ ment for fast 802.11ac frames has the apparent effect of speeding up the network for 802.11ac users with only minimal impact to users of older devices. The ability of a network to treat traffic differently to serve the overall user population is often called “airtime fairness” because when the throughput is optimized for the entire client pop‐ ulation, the result is “fair.”

One important performance tuning technique that is no longer available to 802.11ac network administrators is control of data rates. In 802.11a/b/g/n, it was possible for network administrators to control which data rates were supported. To avoid devices falling back to airtime-hungry low data rates, network administrators often disable low data rates. Deactivating low rates often has another second desirable side effect in that it encourages devices to move off APs with marginal connections toward better APs. However, the 802.11ac protocol does not offer control of individual data rates. Devices must support all non-256-QAM data rates, and the only control offered by the protocol in the MAC capability information element (see “The VHT Capabilities Information element” on page 40) is over the 256-QAM rates.

Unlike 802.11n, the 802.11ac protocol does not provide the capabil‐ ity to control individual data rates. In 802.11n, all 76 MCS rates could be individually selected. 802.11ac uses a different method of deter‐ mining data rates and has a much smaller MCS set. In 802.11ac, only three choices are available: MCS 0–7, MCS 0–8, or MCS 0–9.

Voice

In contrast to data-oriented networks, some special configuration may be helpful for networks that support extensive amounts of voice traffic. Voice traffic is demanding because it cannot be buffered, so many of the efficiency enhancements in 802.11ac are not used by voice handsets. The core of voice tuning is reducing latency for as much traffic as possible. Here are some of the techniques that can be used:

QoS configuration: enable Wi-Fi Multi-Media (WMM) and priority queuing WMM is a quality-of-service specification that can dramatically improve the quality of voice at the receiver. Not all vendors turn on WMM by default, or even make Building an 802.11ac Network | 121 9. For more information on the operation of the DTIM, see Chapter 8 in 802.11 Wireless Networks: The Definitive Guide. voice the highest-priority traffic type. The single most important configuration change you can make to support higher-quality voice calls is to ensure that WMM is enabled. Some vendors also have an option for strict priority scheduling, which delivers frames in order to the receiver.

Enable admission control (WMM-AC)

Admission control requires voice client devices to request capacity for a call before enabling the call to be established. For example, a voice handset using G.711 could request that the AP allocate 80 kbps of capacity. The AP is then free to accept the request and reserve capacity, or reject the request due to a lack of capacity.

Enable fast roaming

Multiple techniques for fast roaming may be used, but the most common are op‐ portunistic key caching (OKC) and 802.11r. Check with your voice client vendor to figure out which of them are supported.

 Increase data rate used for Beacon frame transmission

Voice handsets are often very aggressive in roaming between APs, so tuning efforts will focus on decreasing the effective coverage area of APs and reducing large areas of coverage overlap. One of the most effective ways of limiting the effective range of an AP is to make its Beacon transmissions travel a shorter distance. While it is not possible to design a radio wave that stops at a certain distance, increasing the data rate of Beacon frames can be used to limit the effective range of the network. Typically, the Beacon rate will be set at a minimum of 24 Mbps, and sometimes even higher. (802.11a/g rates should be used because many voice handsets do not use 802.11n.)

Shorten DTIM interval

Many voice products use multicast frames for control features or push-to-talk (PTT) features. Multicast frames are held for transmission until the DTIM is trans‐ mitted.9 Many APs will ship with a DTIM of 3, so multicast transmissions are de‐ livered after every third Beacon. Setting the DTIM to 1 makes multicast delivery more frequent, at the cost of some battery life on handsets that need to power on after every Beacon to receive multicasts.

Reduce retry counters

Voice applications are highly sensitive to latency. 802.11 will automatically retry failed transmissions, but retransmissions take additional time. In voice transmis‐ sion, frames should arrive on time or not at all. Using network capacity to retransmit frames after the target delivery time does not improve call quality, but it can delay other voice frames in the transmit queue. Somewhat counterintuitively, reducing the frame retry count can improve overall latency, and therefore voice quality.

Multicast

Multicast applications are often similar to voice applications in terms of the demands placed on the network. Multicast traffic streams are often video, and may not be easily buffered if they are real-time streams. Furthermore, multicast traffic has a lower effective quality of service than unicast traffic on a wireless LAN because multicast frames are not positively acknowledged. In a stream of unicast frames, each frame will be ac‐ knowledged and retransmitted if necessary. Multicast transmission has no such relia‐ bility mechanism within 802.11, so a stream of multicast frames may not be received and there is no protocol-level feedback mechanism to report packet loss. Here are some steps you can take to optimize multicast transmissions:

Shorten the DTIM interval

Just as with voice, many multicast applications depend on receiving data promptly. Setting the DTIM interval as low as possible improves the latency of multicast delivery.

 Increase the data rate for multicast frames

By default, many products will select a low data rate, often 2 Mbps, for multicast transmissions in an effort to be backward compatible. While this is a laudable goal, and the choice of 2 Mbps was reasonable during the 802.11b-to-802.11g transition in 2004, low data rates for multicast no longer serve that goal. Unless there are critical applications running on 2 Mbps devices, or there are a large number of such old devices on the network without any upgrade path, you should increase the multicast data rate to reduce airtime contention. Many APs can automatically set the multicast data rate to the minimum data rate used for unicast frames to asso‐ ciated clients, or even the minimum unicast rate for clients in the multicast group. With 802.11ac, it is no longer possible to disable the low MCS rates, so the best that can be done is to disable the low data rates for previous physical layers.

Enable multicast-to-unicast conversion

Some APs implement a feature that converts a single multicast frame into a series of unicast frames. Multicast frames must be transmitted at a rate that can be decoded by all receivers and therefore is often relatively slow. Unicast frames can be trans‐ mitted much faster if the receivers are close to the AP. A series of positively ac‐ knowledged unicast frames may take approximately the same amount of airtime, but have significantly greater reliability. Group Management Protocol (IGMP) snooping One of the best ways to limit the load imposed by multicast traffic is to ensure that it is not forwarded on to the radio link if no clients are listening. Many APs imple‐ ment IGMP snooping, and even if your APs do not, IGMP snooping can be con‐ figured on the switched network connecting the APs. IGMP snooping monitors membership in multicast groups and only forwards multicast traffic if there are listeners to the stream.

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