Wi-Fi, both Carrier grade and best effort, is becoming a more integral part of overall wireless experience from cellular networks. We consider what metrics and tools network planners can use to determine overall network performance, and ensure the best experience from the mix of assets and connection types available.
Taking a holistic view across Cellular and Wi-Fi
Many mobile networks are investing in their own Carrier Wi-Fi schemes and encourage use of Wi-Fi to offload traffic. It seems to me that often these schemes are seen as quite independent or just commercial arrangements with little combined network capacity planning and performance management.
During a conference presentation, Arieso (part of JDSU) report* that network operators are now asking for a more holistic view of the subscriber experience. They want to understand both cellular and Wi-Fi traffic activity from their customers. This combined mapping and reporting would require correlation between different radio technologies (macro, small cell, Wi-Fi) and allow planning of both Wi-Fi and Licenced spectrum assets, making best use of all available resources.
The focus is on Carrier Wi-Fi access points and open-access public small cells directly under the operator's own planning and management. There would be less visibility of residential and third party Wi-Fi access points or Femtocells.
A common set of statistics
Performance management systems digest a huge amount of data, crunch the numbers and produce consolidated reports. These high level KPIs provide a general indication of the health and average service quality delivered – good for reporting management targets. Anything up to 200 KPIs might be available, but often only 10-20 are focussed on. These systems also allow drill down into specific problem areas, such as identifying particular geographic trouble spots or traffic types. Call tracing can be enabled for individual users, tracking every interaction with the network when trouble shooting specific issues.
There are similar kinds of systems available for Carrier Wi-Fi and Cellular networks. What will be needed in future is a common platform that handles both and can track the end user experience as it switches between them.
Macrocells and Small Cell performance reporting doesn't always match
Each macrocell sector and small cell reports a wide range of usage statistics. There can be anything up to 500 different counters per cell reporting many different aspects such as:
- Data volumes (uplink and downlink)
- System load (mean/max users both passive and active, active call duration)
- Congestion (rejected connections)
- Handover analysis (failure causes, breakdown of handover types)
- RF signal quality (interference levels)
- Traffic modelling (traffic and service types)
Most RAN vendors provide their performance management data based on the 3GPP standard 32.401. In some cases, only 50% of the standard counters are implemented, supplemented by 100s of vendor specific extensions. Counters are typically aggregated hourly but the data refresh rate is moving to 15 minute updates.
Macrocells, remote radio heads and DAS systems are all based on the same underlying products. All are just seen as another cell on the network and provide the same set of statistics, including full RF trace capability.
The information available from small cell vendors varies, with some providing little or no 24x7 information about which IMSIs are using each cell. Others provide comprehensive data comparable with their larger cousins.
Wi-Fi Statistics report on a different basis
Carrier Wi-Fi access points generate usage records for each session, including:
- start/end times
- MAC address
- data transferred
- RF signal quality
The session end times aren't always accurate because they may be triggered by a timeout after you've left the area rather than through a specific disconnect. Wi-Fi wasn't originally designed to handle session handovers, and so each user appears as a nomadic session.
The main Wi-Fi device identifier is the MAC address, uniquely assigned to electronic equipment in the factory and from which the manufacturer (and sometimes model) can be determined.
Many Wi-Fi registrations are based on user ID and password, or they can be open access. This can make it difficult to correlate the user identity with an IMSI (cellular SIM identity). That's one reason why you are sometimes required to give your mobile number and enter a validation code texted to you.
The more recent use of Hotspot 2.0, which allows registration using the SIM card, implicitly reports the IMSI and facilitates this.
I would say there is a much greater gulf between Carrier/Enterprise and Private/residential Wi-Fi access points than between large and small cellular products. Low cost residential units are very much simpler devices and typically don't log sessions or transactions. Hotspot 2.0 registration using SIM authentication is rare for private/residential networks. Typically mobile operators won't have visibility of the Wi-Fi or fixed broadband access used by their mobile customers, although they may be relying on it to deliver better voice and data services.
Location – Where exactly are the cells and Access Points?
Often the exact geographic location of small cells and access points isn't well known. You might get a building Lat/Long and a room name, but it's rare for the network planning system to know the precise location. GPS, even where built-in to the small cell, may not always work indoors and/or is not always reported back to a central database. This is also an issue for DAS and Remote Radio Heads, where the antenna positions may not be recorded.
Height within a building is also often an unknown. There is no global standard for defining the floor number – is the first floor at ground level or one storey up? It depends which country you are in.
This is one area where cell asset management systems need more sophistication. The growing use of tablet apps during planning and commissioning should help here. The drive towards in-building location services, especially for emergency/first responders, should help encourage improvement.
Oversights and Gotchas
Arieso highlight these and other surprising issues which can catch out the unwary:
- Performance reporting that closely aligns between IMSI and MAC addresses for the same device is not yet really standardised
- Physical geo-location of cell/antenna locations (especially DAS)
- Large networks serving multiple timezones (e.g. USA, Australia)
- Data loss through FTP file batch transfer
- Rolling software upgrades, leading to simultaneous operation of different software per cell
The planning, operation and management of cellular and Wi-Fi systems differs. Fully managed Carrier Wi-Fi and cellular networks are more closely aligned, but the residential, amenity and Enterprise Wi-Fi may not provide the same full range of performance data.
Important steps to create a holistic picture of network operation include:
- Matching the different IDs used for Wi-Fi and cellular (MAC and IMSI)
- Recording the precise geo-location of every antenna
- Combining performance data from multiple networks into a single system
With the wider use of seamless Voice over Wi-Fi and use of Carrier Wi-Fi through Hotspot 2.0, better and more seamless reporting of the end-user experience is essential. This should allow planners to determine where/when/what new investments in technology are required to deliver the best service.
*Phil Claridge, Chief Innovation Architect as Arieso (now part of JDSU), gave an insightful talk on this topic to the Cambridge Wireless Small Cell SIG, from which we've drawn (and expanded with our own thoughts). [Presentation slides available here]