Networking in the home is significantly different than networking in the enterprise or inside the service provider infrastructure. Likewise, ensuring quality of service (QoS) on a home network requires different treatment than ensuring QoS in the enterprise or broadband access (e.g. DSL, cable modem) infrastructure.
Enterprise infrastructure or broadband access infrastructure networks typically utilize professionally managed equipment and engineered cable plants using technologies that provide a constant underlying bandwidth. In contrast, home networking uses a wide variety of existing cabling or non-engineered placement of wireless access points using technologies that have varying underlying bandwidth.
Users without much experience or knowledge of networking management often install the home networking equipment and those users typically do not desire to actively manage their networks. Finally, the applications in the home tend to be multimedia intensive with a wide variety of bandwidth and packetization characteristics.
Once these types of characteristics of the home network environment have been identified, an evaluation of the techniques to provide an appropriate QoS scheme can begin. The resulting QoS system should answer the following design considerations (Figure 1):
- Media access: How do individual devices on the network access the shared media?
- Packet forwarding: How are packets forwarded within in-home gateways, routers, or bridges?
- QoS Characteristics: How are the desired QoS characteristics managed and communicated by applications, users, and operators for the devices on the home network?
Figure 1: Three QoS design considerations: media access, packet forwarding, and QoS characteristics.
To provide QoS in a home network, the network interfaces on the devices must perform two basic functions: media access and packet forwarding. QoS media access is the process of determining when a packet may be transmitted from one device into a network so as not to preclude the timely transmission of a packet on the media by another device. Specifically, it is the rules to indicate the acceptable time the network interface on each of the devices may transmit data under the assumption that multiple devices will want to transmit at nearly the same time.
QoS packet forwarding is the process of transmitting packets that have arrived from one or more network interfaces within a single device according to set of rules that provide QoS. Typically, the residential gateway is the single device on a home network that has multiple network interfaces and some rules are necessary to forward between the multiple network technologies.
The two basic QoS functions can be realized in one of two different paradigms: prioritized (differentiated) QoS or parameterized (scheduled) QoS. The prioritized paradigm is for shared-media technologies and differentiated media access. Under this approach, marking is done as packets enter the network and these markings are honored by differentiated forwarding/queuing engines at devices like the residential gateway.
The parameterized paradigm is for shared-media technologies and planned media access opportunities. In this approach, all devices and applications must request specific amounts of time to transmit their data and planned media forwarding is performaned on per-flow state and signaling at every hop.
In the discussion below, we will show how priority-based media access schemes are superior for home networks. We'll start the discussion by showing how to apply packet forwarding to ensure QoS between the various in-home networking technologies. We'll then discuss a system to assign and manage the priorities for all of the devices on the in-home network. Finally, we'll discuss the CableHome QoS11 and UPnP QoS14 architectures designed around the priority-based QoS scheme that implement the in-home media access, packet-forwarding, and characteristics management as a complete system.
Note: This article assumes that the network segments within the home are IP based. Data and content are typically delivered to the home on an IP based access network and IP-based in-home network allows the underlying network segments to be transparent to the applications.
Practical Media Access Considerations
There are three basic technologies that are used for home networking today: 10/100baseT Ethernet, IEEE 802.11g/b/a (Wi-Fi), and HomePNA. In the future, other technologies such as HomePlug may become popular.
While many people tend to divide those technologies along the lines of wired and wireless, for QoS considerations, a better categorization would be shared-media versus point-to-point. Switched 10/100BaseT Ethernet is a point-to-point technology. Essentially, each piece of media only has a single device on it. All of the other technologies are shared-media technologies. In these other approaches, multiple devices share a single media segment and some mechanism is needed to control how devices transmit on the media.
Most existing hubs in the home do not support either parameterized or IEEE 802.1p/q priority schemes and, more than likely, cannot be upgraded. However, when a service such as CableHome is added to a home network, existing hubs are likely to be replaced with switches that are integrated in the residential gateway device.
For switched Ethernet, differentiated media access is not of much value since traffic is point-to-point and there is essentially no contention on such a link. Many residential gateways have a four- or five-port Ethernet switch built-in, so QoS is likely not much of an issue for each of the Ethernet ports. Finally, 100BaseT bandwidth is sufficient to address most home networking bandwidth and latency requirements, especially when it is dedicated usage on a point-to-point link. Thus, for the purposes of QoS media access, Ethernet within the home can be considered a point-to-point technology and treated separately from shared media technologies.
All of the other standards-based shared media technologies such as 802.11 b/g/a1, HomePNA2, and HomePlug3 have or will shortly have some support for a priority-based QoS scheme. In general, these technologies' priority-based media access is accomplished by controlling which device gets to access the media first based upon priorities. The devices with highest priority traffic are allowed first opportunity to transmit and then, depending upon the bandwidth availability, devices with lower priority traffic will get the opportunity to transmit their data on the media. The amount of bandwidth consumed by the highest priority traffic typically cannot be strictly controlled with the prioritized scheme.
Parameterized QoS relies on the assumption that the underlying physical layer/media access control (PHY/MAC) technology is able to deliver very constant bandwidth and minimal jitter. This assumption is acceptable for networking technologies such as Ethernet or Docsis, but is inappropriate for home networking technologies such as wireless LAN, phoneline, and powerline, where the underlying throughput/jitter can be strongly influenced by rapidly changing and sporadic interference.
To have a high confidence for parameterized guarantees, very conservative estimates must be made on the underlying parameters. Conservative estimates lead to worst-case operation that may be significantly different from typical operation. Further, aside from audio or video (AV), most applications are not QoS-aware. They need—and would tend to over subscribe—the very bandwidth-limited media. Thus, priority-based media access is preferred for home networks.
Feasibility Considerations
If new services were being added to the home network that require QoS, the new device would need to include the necessary QoS signaling functionality. Prior to the installation of such device, a service provider-managed residential gateway that includes QoS functionality is assumed to be in place. That residential gateway is likely to support existing legacy PCs and Internet appliances that do not have manageable QoS functionality.
Often, changes to those existing hosts, whether software or hardware, may not be feasible. Thus, the in-home QoS scheme must function in the presence of legacy devices.
Prioritized media access can be overlaid on existing shared-media home networks. Even though the various home networking technologies mentioned above have a means for setting priority, in general there is no entity setting the priorities. Traffic using those technologies tends to only be transmitted as best effort. Thus, when QoS is added to a residential gateway, traffic transmitted from the residential gateway can be prioritized.
Likewise, network client devices that conform to a QoS specification can use prioritized media access. Non-compliant devices will continue using best effort priority. Thus, QoS for a mix of devices that do and do not support QoS on a home network can be engineered by setting priorities on the shared interfaces and only sending appropriate traffic on the switched Ethernet interfaces of the Residential Gateway.
An example of a prioritized QoS system overlaid on a legacy 802.11 Wi-Fi13 network is WiFi's Wireless Multimedia Enhancements (WME) based on the EDCA portion of draft IEEE 802.11e. For higher priority traffic, WME shortens the access parameters so that the packets are transmitted before traffic from legacy devices can be transmitted. WME also can lengthen the access parameters to allow a background class of traffic with lower priority than legacy traffic.
Unfortunately, parameterized media access cannot be overlaid on legacy networks since legacy devices can not be prevented from transmitting during times assigned by the central controller to (new) conforming devices. Parameters, reservations, and bandwidth managers cannot operate with legacy devices on shared networks.
Queuing and Packet forwarding
In any device with multiple network interfaces, such as a residential gateway, a mechanism is necessary to control packets that are received from multiple interfaces that are to be retransmitted through a single interface. This control process is referred to as forwarding priorities or queuing. Several potential methodologies exist depending on the QoS capabilities of the interfaces. Note: See IEEE802.1p/q4 or RFC24745for a discussion on differentiated services queuing.
There are multiple ways to implement a priority-based packet forwarding mechanism. If the interfaces on which the packets are received contain a Layer 2 (L2) or Layer 3 (L3) priority mechanism, and if the priorities on different interfaces have global relevance, the packets may be queued for transmission simply by their relative priorities.
L3 mechanisms tend to require packets to be marked/labeled and the label must be mapped to L2 mechanisms in order to enforce priorities on shared media. If the differentiation markings on L2 and L3 are mapped, queuing decisions can be made either on L2 or L3 indicators.
The regeneration feature of 802.1p offers the ability to examine packet contents and to label the packets for priority treatment at the next packet forwarder. However, even if the contents of packet received at a residential gateway were examined to determine if it was high priority or not, there is little merit in tagging it for transmission on the WAN (DSL or cable modem) or back onto the LAN since the residential gateway simply must make an internal decision on which queue to place it in.
If a receiver interface implements a QoS mechanism based on parameters, queuing decisions probably can be based on the latency or jitter parameters for flow planning. However, it can be problematic to determine if a packet should be transmitted or not based on its length if the length would mean that a potential low latency packet were to be received and must be transmitted at a specific transmission opportunity.
Feasibility Considerations
Since a QoS priority packet forwarding methodology resides entirely within the residential gateway and can be thought of as a minor enhancement to the existing packet handling capability, this new QoS functionality is relatively easily accomplished through a software upgrade. Fortunately this methodology also handles all legacy devices connected to the interfaces since QoS packet forwarding is implemented solely within the residential gateway.
QoS Characteristic Management
Above, we discussed how prioritized data is moved throughout the home network and within the residential gateway. But how do hosts and applications decide what data should have priority? The QoS characteristics management functionality answers that question by performing two primary functions:
- Setting the basic network wide policy determining which applications receive which priority.
- Communicating the application information and assigned QoS priorities between the devices on the home network.
The users of the home network hold the decision power on which applications should be given priority over other applications. For example, a user may prefer that streaming video be given precedence over networked gaming.
If the residential gateway is supplied by a service provider, the provider may suggest default priorities or offer to manage the priorities through the residential gateway. The default priorities may be uniform over the operator service area or may be customized on an individual home basis to account for the differences in the mix and number of services and applications for a given household.
The policy of priorities may use standard information such as the service or application name. In addition to application or service-based definition of priorities, finer-grain control based on individual network devices or destination devices may also be desirable.
Once the policy on the priority assignments has been determined, that policy must be shared with all of the compliant devices on the home network. Thus, the second function of QoS characteristic management is providing the means for the management headend or QoS policyholder to communicate the desired QoS Characteristics to the residential gateway and then further to the hosts.
In addition to the existing management interfaces on the residential gateway, there must be a management interface base (MIB) allowing the operator to manage and modify the default QoS behavior. The residential gateway must also be informed of the existence of the applications on the various hosts. The media access characteristics of QoS characteristic management are then conveyed by the residential gateway to the hosts via a specified messages and protocol. This informs the host which transmission priorities the application(s) on the host shall use.
Feasibility Considerations
On legacy PCs, prioritized media access may be accomplished by a software addition that takes advantage of existing APIs. Some operating systems and network stacks allow applications to signal priorities or parameters. For example, recent versions of Windows include the GQoS API where 802.1p priorities or RSVP parameters can be requested by applications through the Winsock2 network stack.7. Other platforms, such as Linux kernel 2.4.14 and above with skb-priority using the vconfig8 utility, have similar QoS-enabled network stacks.9 Two QoS characteristic management systems that take advantage of those QoS-enabled interfaces are CableHome and UPnP QoS, described below.
Application of a Priorities-Based QoS System
With the above considerations on media access and packet queuing on in-home networks, the CableHome QoS architecture specified a priority-based paradigm.11 This is in contrast to the parameter-based paradigm that is typically used for QoS on the cable broadband access network. Thus, the domain of this CableHome QoS architecture focuses solely on providing QoS between the Residential Gateway and networked devices within the home. The overall goal becomes enabling home networking applications to establish prioritized data transmission between the CableHome hosts and the CableHome residential gateway using conformant messaging.
To achieve that project goal, three primary design guidelines were established to answer the three design consideration questions introduced at the beginning of this article:
- QoS media access: CableHome QoS defines a mechanism that controls transmission access using priorities on shared media for the CableHome residential gateway and CableHome hosts. The mechanism provides prioritized media access to various devices and applications on the home network.
- QoS forwarding: The CableHome residential gateway supports a queuing mechanism that prioritizes packets that are received from multiple interfaces and are to be retransmitted (forwarded) through each of its interfaces.
- QoS characteristic management: CableHome QoS specifies a signaling and management mechanism for communication of QoS characteristics (priority settings) between the CableHome residential gateway and CableHome hosts desiring QoS within a home network. This mechanism is aggregated and managed in the CableHome residential gateway.
A logical architecture, instead of a device-oriented architecture, is used to specify requirements for conforming equipment. The CableHome QoS architecture is illustrated in Figure 2 and is composed of the following entities:
- Q-Domain encompassing the CableHome QoS sphere of influence. The headend equipment manages the CableHome QoS functions but is not within the Q-Domain.
- CableHome residential gateway (CRG) containing the portal services (PS) logical element.
- CableHome host (CH) containing the boundary point (BP) logical element.
- CableHome QoS portal sub-element (CQP)
- CableHome QoS boundary point sub-element (QBP)
Figure 2: Example of cable QoS elements (Adapted from ITU-T J.190).
The CQP acts as an aggregator for conformant applications. Its primary function is to enable priorities-based QoS for the devices within the home network. It performs priorities-based queuing/forwarding and media access for the traffic originating from the PS. It is also responsible for communication of QoS characteristics to various devices within the home. In the UPnP QoS architecture, this functionality is known as the QoS manager service.
The QBP sub-element performs priorities-based media access for the traffic originating from the BP. It is also responsible for the reception of QoS characteristics information from the PS. In the UPnP QoS architecture, this functionality is known as QoS device service.
Those elements provide QoS using the following functions in the ITU-T J.190 and J.192 IPCable2Home10 or CableLab's CableHome architectures:
- PS forwarding: Prioritized queuing and packet forwarding functionality in the CQP.
- PS media access: Prioritized shared media access functionality in the CQP.
- QoS management server: This CQP functionality is responsible for maintaining a repository of QoS priority characteristics for various devices and applications within the home network and also for communication of these characteristics to these devices and applications.
- BP media access: Prioritized shared media access functionality in the QBP.
- QoS management client: This QBP functionality, with the aid of QoS Management Server, determines QoS characteristics that a particular application/device needs to use.
In the CableHome QoS environment, applications or end points associated with a BP are identified and communicated by the QBP to the CQP during a discovery phase using protocols in UPnP.12 The prioritized treatment for those applications is determined by consulting a database of operator and user assigned priorities. In the UPnP QoS architecture this database is known as the QoS policy holder service. Those per application priorities are consistently applied to IP packets generated by the BPs and passing through the PS throughout the entire in-home network.
Standardized messages infrequently communicate the priority assignment information between the QBPs and CQP. BPs use the native L2 prioritized media access mechanisms such as WME to transmit packets on shared media networks. The CQP implements a prioritized queuing mechanism for packets traversing its interfaces. Those standardized message can be enhanced by taking advantage of the UPnP QoS architecture. UPnP QoS messages allow QoS capable consumer electronics devices to discover each other and then uniformly assign priorities across the applications and home network using the QoS policy holder service that typically would reside in the residential gateway.
Wrap Up
A priorities-based media access and packet-forwarding approach is recommended for in home networks desiring QoS. This recommendation is based on the feasibility of managing bandwidth over varying home networking technologies in the presence of legacy network devices. The complementary in home QoS and access system QoS would provide a cost-effective way to deliver high quality, revenue-generating services without the need to replace existing legacy in-home networking devices.
Author's Note: The author wishes to thank Steve Saunders, Thomas Kuehnel, Matthew Fischer, Marc Abrams, and anonymous reviewers for their assistance, guidance, and valuable input to this article.
References
- http://grouper.ieee.org/groups/802/11/
- http://homepna.org
- http://homeplug.org
- http://grouper.ieee.org/groups/802/1/
- http://www.ietf.org/rfc/rfc2474.txt
- http://www.ietf.org/html.charters/diffserv-charter.html
- http://msdn.microsoft.com/msdnmag/issues/01/04/QOS/
- http://www.candelatech.com/~greear/vlan.html
- http://diffserv.sourceforge.net/
- http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent=T-REC-J.190
- http://www.cablelabs.com/projects/cablehome/specifications/
- http://www.upnp.org/
- http://www.wi-fi.org/
- http://www.upnp.org/download/summitslides2003/01-08 QoS for Europe Summitt1.ppt
About the Author
Stephen Palm is a principal engineer in the Broadband Communications Group of Broadcom Corporation. He chairs and participates in committees of ITU-T SG15, ITU-T SG9, Wi-Fi, UPnP, and CableLab's CableHome. Stephen holds a BSEE from the University of California, Irvine, an MSECE from Carnegie Mellon University, and a Ph.D. from the University of Tokyo. He can be reached at qos@kiwin.com.
