Whispers & Screams
And Other Things

How Wi-Fi works

If you want to know how to fix your Wi-Fi, first you need to understand how it works

Before you set about fixing your Wi-Fi, it helps to know how the technology works.

That way, you can make an informed decision about the equipment you need to solve your issues, or whether a change of settings might help.

It’s a complicated subject, and we won’t attempt to cover everything (such as packet data, TCP/IP, or the ins and outs of wireless security), but by the end of this section, you should have a firm grasp of Wi-Fi’s fundamentals.

Signals and spectrum

Wi-Fi’s core premise is pretty simple – routers and adapters send and receive data using radio waves. It’s the same basic technology that’s used by radio and TV to receive terrestrial signals, mobile phones to make and receive calls, as well as video senders, baby monitors, and all sorts of other wireless devices.

In effect, all a wireless router or adapter does is translate the data it receives into a radio signal, which is decoded back into data at the other end.

Specifically, wireless routers use frequencies of 2.4GHz (or the range 2.412GHz-2.484GHz to be more precise) and, in the case of more expensive dual-band routers, 5GHz (4.195GHz-5.825GHz) to send and receive information.

But there’s far more to it than simply slinging streams of data to and fro. Each of these bands is further divided into channels, of which your router can use one or two simultaneously (when two are used simultaneously, it’s called channel bonding – see below for more details). In the 2.4GHz band there are up to 14 channels available, and up to 42 in the 5GHz band.

The idea is that by using different channels, neighbouring networks avoid stepping on each other’s toes. In an ideal world, for maximum performance and stable operation, your router should be running on a channel that no other network in range is using.

In reality, the true number of available channels is lower than these theoretical maximums, depending on where you live and which router you’re using.

In the UK and Europe, you’re legally allowed to use only channels 1 to 13 in the 2.4GHz space, and you’re restricted to 18 of the 42 in the 5GHz space. A Netgear router we use in our office, meanwhile, makes only four channels in the 5GHz space available for use.

This is compounded by the fact that when your router transmits on each channel, the effective width of its signal is about 20MHz, which, in the 2.4GHz space, means it can overlap up to eight neighbouring channels.

It doesn’t take a genius to work out that when more than three wireless networks are in close proximity to one another, co-channel and adjacent channel interference can become a problem.



Channel bonding (the ability some routers have to group two channels together, doubling the potential throughput) makes the congestion even worse – with several 40MHz wide channels hogging such a narrow spectrum, it’s like trying to squeeze several 21-stone men into a small lift.

Why 5GHz?

There is a solution to hand, however – 5GHz wireless. The advantages it holds over 2.4GHz are threefold. First, it’s far less congested. Fewer people own dual-band 5GHz routers and devices, so the chances are you’ll be able to set up your network on a completely congestion-free channel, which you perhaps wouldn’t over 2.4GHz.

Second, since the channels are further apart than in the 2.4GHz band (with 20MHz between each, compared with 4MHz or 5MHz) there’s much less opportunity for adjacent channel overlap. Even in the unlikely event that many 5GHz routers and devices are in close proximity to each other, maintaining a steady signal should be much easier.

Finally, and potentially the biggest bonus of all, there are relatively few non-networking devices currently using the 5GHz space.

Where users of 2.4GHz must contend with all manner of domestic interlopers, from microwaves to cordless phones, 5GHz networks are comparatively clutter-free.



Physical barriers

It isn’t all rosy in the 5GHz garden, though. Since the signal is of a higher frequency than 2.4GHz, it deals less well with walls, windows and floors, and this hits its ability to transmit and receive speedily at long range.

In Rustyice tests, we’ve routinely seen routers perform well over 2.4GHz, flawlessly transferring files wirelessly at a distance of about 40m, with two walls in the way.

When tested in the same location over 5GHz, most suffer a significant drop in transfer speed and weaker signal reception. Some fail to maintain a solid connection entirely. That means the more objects blocking your signal path, the worse the reception in the 5GHz band gets. It isn’t only building materials that get in the way – everything from humans to heavy rain can attenuate a wireless signal.

Choosing a 5GHz router

Restricted range isn’t the only problem afflicting 5GHz routers. Many devices, such as smartphones, internet radios and games consoles, don’t send or receive signals in that band.

It’s really only laptops and PCs with premium wireless cards that will take advantage of the 5GHz band.

That’s why high-end routers typically offer the choice of 2.4GHz and 5GHz bands, but you should take care when choosing a dual-band router.

Some routers can transmit on both bands simultaneously, while others require you to manually flick between the two. Needless to say, the former is the better choice.
Continue reading
1092 Hits
1 Comment

Wi-Fi security luddite? The ICO is coming for you!

The Information Commissioner's Office today published new guidance for home Wi-Fi security after a YouGov report found that 40% of home users did not understand how to manage the security settings on their networks.

The survey also found that in spite of most ISPs now setting up and installing security on Wi-Fi equipment, 16% of the people surveyed were unsure whether or not they were using a secured network, or were aware they weren't, but didn't give a toss either way.

The new guidance includes information on managing encryption settings and how to think of a secure password. Top tip? Don't use pa55w0rd.

Giving people unsolicited access to your network could reduce connection speed, cause you to exceed data caps, or allow hordes of criminals to use your network for nefarious purposes, said the ICO.

Welcoming the move, D-Link's Chris Davies pointed out that there was no excuse for being caught out.

"There is no doubt that in the past setting up security on wireless networks could be tricky," said Chris. "But this is no longer the case with most wireless products.

"Security can be set up wiin a couple of minutes with no prior technical knowledge required. We've also been working with ISPs to help them ship products to consumers with security pre-configured."

Let's just hope the ICO doesn't start fining home users for data breaches. Or maybe that would be the kick in the butt some of them need?
Continue reading
893 Hits
0 Comments

An examination of DHCP Snooping with option 82 on Cisco.

DHCP snooping is a DHCP feature that provides security by filtering untrusted DHCP messages from hosts or other devices on the network. DHCP snooping accomplishes this level of security by building and maintaining a DHCP snooping binding table.

An untrusted DHCP message is a DHCP message that the switch receives from outside the network or firewall or from an unauthorised DHCP server that can cause security attacks within a network. DHCP snooping is used along with the interface tracking feature, which inserts option 82 in the DHCP messages by the switch. Option 82 is the Relay Agent Information Option as described in RFC 3046.

The use of DHCP snooping extends existing security capabilities, including the capability to trust a port as a DHCP server and prevent unauthorised DHCP server responses from untrusted access ports. Another DHCP snooping supported feature is per-port DHCP message rate limiting, which is configurable in packets per second (pps) and is used to prevent DoS attacks. The DHCP snooping feature is useful in ISP networks, university campuses and Long Range Ethernet (LRE) network scenarios to prevent misconfigured or malicious DHCP servers from causing user-connectivity problems (such as giving out bogus DHCP addresses).

DHCP snooping builds a DHCP binding table that contains client IP addresses, MAC addresses, ports, VLAN numbers, leases and binding types. Switches support the enabling of the DHCP snooping feature on a per VLAN basis. With this feature the switch intercepts all DHCP messages within the layer 2 VLAN domain. With option 82 enabled, the Supervisor Engine adds the ingress module, port, VLAN and switch MAC address to the packet before forwarding the DHCP request to the DHCP server. The DHCP server can track the IP address that it assigns from the DHCP pool.

With this feature the switch restricts end-user ports (untrusted ports) to sending only DHCP requests, while all other types of DHCP traffic, such as DHCP offer responses, are dropped by the switch. DHCP snooping trusted ports are the ones connected to the known DHCP servers or uplink ports to the distribution switch that provide the path to the DHCP server. Trusted ports can send and receive any DHCP message . In this manner the switch allows only trusted DHCP serves to give out DHCP addresses via DHCP responses. Therefore this feature prevents users from setting up their own DHCP servers and providing unauthorised addresses.

In summary, DHCP snooping with option 82 provides an excellent mechanism to prevent DHCP DoS attacks or misconfigured clients from causing anomalous behaviour in the network.

Continue reading
582 Hits
0 Comments

Femtocells & Relays in Advanced Wireless Networks

With the huge growth of mobile phones complementing a revolution wireless network technologies there has been a huge change in the consumer’s lifestyle and dependence on mobile phones. With the emergence of smart phones (mobile web) consumers are replacing not only their fixed lines but have started downsizing the number of PC's in the home. Fundamentally, consumers want great voice quality, reliable service, and low prices. But today’s mobile phone networks often provide poor indoor coverage and expensive per-minute pricing. In fact, with the continued progress in broadband VoIP offerings such as Vonage and Skype, wireless operators are at a serious disadvantage in the home.

Hence the wireless operators are looking to enhance their macro-cell coverage with the help of micro-cell coverages(indoor) by deploying small base stations such as Femtocells or with the help of Relay technology. These miniature base stations are the size of a DSL router or cable modem and provide indoor wireless coverage to mobile phones using existing broadband Internet connections.

Pointing out some key advantages of Femtocells and Relays we will then focus on their adoption in advanced wireless networks(WiMAX and LTE)

fnr-femtocells1

 

 

 

 

 

FEMTOCELLS

Technical Advantages:

Low Cost: The Business Model would be initially by offering Femtos as a consumer purchase through mobile operators

Low Power: around 8mW- 120 mW lower than Wi-Fi APs.

Easy to Use: Plug-and-Play easily installed by consumers themselves

Compatibility & Interoperability: Compatibility with UMTS,EVDO standards and WiMAX,UMB & LTE standards

Deployment: In Wireless Operator owned licensed spectrum unlike WiFi

Broadband connected: Femtocells utilize Internet protocol (IP) and flat base station architectures, and will connect to mobile operator networks via a wired broadband Internet service such as DSL, cable, or fibre optics.

Customer’s point of view:

Increased Indoor Coverage: Coverage radius is 40m – 600m in most homes providing full signal throughout the household

Load sharing: Unlike in macro cells which supports hundreds of users, Femtos will support 5-7 users simultaneously  enabling lesser contention in accessing medium delivering higher data rates/user.

Better Voice Quality: As the users will be in the coverage envelope and closer to Femtos, they will definitely be supported with a better voice and sound quality with fewer dropped calls

Better Data/Multimedia Experience: It will deliver better and higher data performance with streaming musics, downloads and web browsing with lesser interruptions and loss of connections compared to a macro-cell  environment.

Wireless Operator’s point of view:

Lower CAPEX: Increased usage of femtocells will cut down huge capital costs on macro cell equipments & deployments. This includes costs savings in site acquisitions, site equipments, site connections with the switching centers.

Increased network capacity: Increased usage of femtocells will reduce stress on macro cells increasing overall capacity of mobile operators

Lower OPEX: With lesser macro cell sites it reduces the overall site maintenance, equipment maintenance and backhaul costs.

Newer Revenue Opportunities: With provision of excellent indoor coverage and superior user experience with voice and multimedia data services operators has an opportunity of raising its ARPU with more additions to family plans

Reduced Churn: Due to improved coverage, user multimedia experience and fewer dropped calls, will lead to a significant reduction in customer churn

Technical hurdles:

Spectrum: Femtocells works on licensed spectrum and as the spectrum is the most expensive resource it will be a major technical hurdle for the wireless operator for frequency planning.

RF Coverage Optimization: Radio tuning and optimization for RF coverage in macro cells is manually done by technicians which is now not possible at each femtocell level, henceforth self optimization and tuning over time according to the indoor coverage map has to be done either automatically or remotely which is a technical challenge.

RF Interference: Femtocells might be prone to femto-macro interference and also femto-femto interference in highly dense macro or micro environments which might affect the user experience.

Automatic System Selection: When an authorized user of a femto cell moves in or out of the coverage of the femto cell – and is not on an active call – the handset must correctly select the system to operate on. In particular, when a user moves from the macro cell into femto cell coverage, the handset must automatically select the femto cell, and visa versa.

Handoffs: When an authorized user of a femto cell moves in or out of coverage of the femto cell – and is on an active call – the handset must correctly hand off between the macro cell and femto cell networks. Such handoffs are especially critical when a user loses the coverage of a network that is currently serving it, as in the case of a user leaving the house where a femto cell is located.

Security & Scalability: A femto cell must identify and authenticate itself to the operator’s network as being valid. With millions of femto cells deployed in a network, operators will require large scale security gateways at the edge of their core networks to handle millions of femto cell-originated IPsec tunnels.

Femto Management: Activation on purchase and plug and play by end user is an important step and with a proper access control management allowing end-user to add/delete active device connections in the household. In addition, operators must have management systems that give first-level support technicians full visibility into the operation of the femto cell and its surrounding RF environment.

RELAYS:

Relay transmission can be seen as a kind of collaborative communications, in which a relay station (RS) helps to forward user information from neighboring user equipment (UE)/mobile station (MS) to a local eNode-B (eNB)/base station (BS). In doing this, an RS can effectively extend the signal and service coverage of an eNB and enhance the overall throughput performance of a wireless communication system. The performance of relay transmissions is greatly affected by the collaborative strategy, which includes the selection of relay types and relay partners (i.e., to decide when, how, and with whom to collaborate).

Relays that receive and retransmit the signals between base stations and mobiles can be used to effectively  increase throughput extend coverage of cellular networks. Infrastucture relays do not need wired connection to network thereby offering savings in operators’ backhaul costs. Mobile relays can be used to build local area networks between mobile users under the umbrella of the wide area cellular networks



Advantages:

Increased Coverage: With multi-hop relays the macro cell coverage can be expanded to the places where the base station cannot reach.

Increased Capacity: It creates hotspot solutions with reduced interference to increase the overall capacity of the system.

Lower CAPEX & OPEX: Relays extending the coverage eliminates the need of additional base stations and corresponding backhaul lines saving wireless operators deployment costs and corresponding maintenance costs. The relays can be user owned relays provided by operators and can be mounted on roof tops or indoors.

Better Broadband Experience: Higher data rates are therefore now available as users are close to the mini RF access point.

Reduced Transmission power: With Relays deployed there is a considerable reduction in transmission power reducing co-channel interference and increased capacity.

Faster Network rollout: The deployment of relays is simple and quickens the network rollout process with a higher level of outdoor to indoor service and leading to use of macrodiversity increasing coverage quality with lesser fading and stronger signal levels.

As a hot research topic with great application potential, relay technologies have been actively studied and considered in the standardization process of next-generation mobile communication systems, such as 3GPP LTE-Advancedand IEEE 802.16j (multihop relays for WiMAX standards).
Relay Types




Two types of RSs have been defined in 3GPP LTE-Advanced and 802.16j standards, Type-I and Type-II in  3GPP LTE-Advanced, and non-transparency and transparency in IEEE 802.16j.



Specifically, a Type-I (or non-transparency) RS can help a remote UE unit, which is located far away from an eNB (or a BS), to access the eNB. So a Type-I RS needs to transmit the common reference signal and the control information for the eNB, and its main objective is to extend signal and service coverage.Type-I RSs mainly perform IP packet forwarding in the network layer (layer 3) and can make some contributions to the overall system capacity by enabling communication services and data transmissions for remote UE units.



On the other hand, a Type-II (or transparency) RS can help a local UE unit, which is located within the coverage of an eNB (or a BS) and has a direct communication link with the eNB, to improve its service quality and link capacity. So a Type-II RS does not transmit the common reference signal or the control information, and its main objective is to increase the overall system capacity by achieving multipath diversity and transmission gains for local UE units.

Pairing Schemes for Relay Selection

One of the key challenges is to select and pair nearby RSs and UE units to achieve the relay/cooperative gain. The selection of relay partners (i.e., with whom to collaborate) is a key element for the success of the overall collaborative strategy. Practically, it is very important to develop effective pairing schemes to select appropriate RSs and UE units to collaborate in relay transmissions, thus improving throughput and coverage performance for future relay-enabled mobile communication networks.

This pairing procedure can be executed in either a centralized or distributed manner. In a centralized pairing scheme, an eNB will serve as a control node to collect the required channel and location information from all the RSs and UE units in its vicinity, and then make pairing decisions for all of them. On the contrary, in a distributed pairing scheme, each RS selects an appropriate UE unit in its neighborhood by using local channel information and a contention-based medium access control (MAC) mechanism. Generally speaking, centralized schemes require more signaling overhead, but can achieve better performance

Relay Transmission Schemes

 

Many relay transmission schemes have been proposed to establish two-hop communication between an eNB and a UE unit through an RS

Amplify and Forward — An RS receives the signal from the eNB (or UE) at the first phase. It amplifies this received signal and forwards it to the UE (or eNB) at the second phase. This Amplify and Forward (AF) scheme is very simple and has very short delay, but it also amplifies noise.

Selective Decode and Forward — An RS decodes (channel decoding) the received signal from the eNB (UE) at the first phase. If the decoded data is correct using cyclic redundancy check (CRC), the RS will perform channel coding and forward the new signal to the UE (eNB) at the second phase. This DCF scheme can effectively avoid error propagation through the RS, but the processing delay is quite long.

Demodulation and Forward — An RS demodulates the received signal from the eNB (UE) and makes a hard decision at the first phase (without decoding the received signal). It modulates and forwards the new signal to the UE (eNB) at the second phase. This Demodulation and Forward (DMF) scheme has the advantages of simple operation and low processing delay, but it cannot avoid error propagation due to the hard decisions made at the symbol level in phase one.

Comparison between 3GPP LTE Advanced and IEEE 802.16j RSs

Below shows comparison between Type I(3GPP- LTE Advanced) and Non-Transparency(IEEE -802.16j) RSs



Technical Issues

Practical issues of cooperative schemes like signaling between relays and different propagation delays due to different locations of relays are  often overlooked.  If  the difference in time of arrival between the direct path from source to destination and the paths source-relay-destination is constrained then relays must locate inside the ellipsoid as depicted below. Thus,  in practice, such a cooperative system shoiuld be a narrow band one, or guard interval between transmitted symbols should be used to avoid intersymbol interference due to relays.

In band relays consume radio resources and Out of band relays need multiple transceivers.
Continue reading
1029 Hits
1 Comment