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Wireless LAN Fundamentals: Mobility


  1. Characteristics of Roaming
  2. Layer 2 Roaming
  3. Layer 3 Roaming
  4. Summary

This chapter covers the following topics:

  • Characteristics of roaming

  • Layer 2 roaming

  • Layer 3 roaming and an introduction to Mobile IP

This book covers the major components of 802.11 wireless LANs (WLANs). Fundamental concepts such as medium access mechanisms, frame formats, security, and the physical interfaces build the foundation for understanding more advanced and practical concepts.

In keeping with this theme, this chapter covers mobility. Mobility is the quality of being capable of movement or moving readily from place to place. 802.11 WLAN devices provide this kind of untethered freedom. But there's more to mobility than the lack of a network cable. Understanding how mobility is implemented in 802.11 arms you with the knowledge you need to support or facilitate mobile applications. Many terms describe mobility, but this chapter uses the terms mobility and roaming to describe the act of moving between access points (APs).

Characteristics of Roaming

Defining or characterizing the behavior of roaming stations involves two forms:

  • Seamless roaming

  • Nomadic roaming

Seamless roaming is best analogized to a cellular phone call. For example, suppose you are using your cellular phone as you drive your car on the freeway. A typical global system for mobile (GSM) communications or time-division multiple access (TDMA) cell provides a few miles of coverage area, so it is safe to assume that you are roaming between cellular base stations as you drive. Yet as you roam, you do not hear any degradation to the voice call (that is what the cellular providers keep telling us). There is no noticeable period of network unavailability because of roaming. This type of roaming is deemed seamless because the network application requires constant network connectivity during the roaming process.

Nomadic roaming is different from seamless roaming. Nomadic roaming is best described as the use of an 802.11-enabled laptop in an office environment. As an example, suppose a user of this laptop has network connectivity while seated at his desk and maintains connec-tivity to a single AP. When the user decides to roam, he undocks his laptop and walks over to a conference room. Once in the conference room, he resumes his work. In the back-ground, the 802.11 client has roamed from the AP near the user's desk to an AP near the conference room. This type of roaming is deemed nomadic because the user is not using network services when he roams, but only when he reach his destination.

What happens to application sessions during roaming? Many factors influence the answer to this question. Consider the following:

  • The nature of roaming in 802.11.

  • The operation of the application. Is the application connection-oriented or connectionless?

  • The roaming domain. Does roaming occur with a single subnet or across multiple subnets?

  • Roaming duration. How long does the roaming process take?

The Nature of Roaming in 802.11

802.11 roaming is known as "break before make," referring to the requirement that a station serves its association with one AP before creating an association with a new one. This process might seem unintuitive because it introduces the possibility for data loss during roaming, but it facilitates a simpler MAC protocol and radio.

If 802.11 were "make before break," meaning a station could associate to a new AP before disassociating from the old AP, you would need safeguards in the MAC to ensure a loop-free topology. A station connected to the same Layer 2 broadcast domain via simultaneous network connections has the potential to trigger broadcast storms. A "make before break" architecture would necessitate an algorithm such as 802.1D spanning tree to resolve any potential loops, adding overhead to the MAC protocol. In addition, the client radio would have to be capable of listening and communicating on more than one channel at a time, increasing the complexity of the radio (and adding to the overall cost of the devices).

Operation of the Application

The way the application operates directly correlates to its resilience during the roaming process. Connection-oriented applications, such as those that are TCP-based, are more tolerant to packet loss incurred during roams because TCP is a reliable and connection-oriented protocol. TCP requires positive acknowledgments, just as the 802.11 MAC does. This requirement allows any 802.11 data lost during the roaming process to be retrans-mitted by TCP, as the upper-layer protocol.

Although TCP provides a tidy solution for applications running on 802.11 WLANs, some applications rely on User Datagram Protocol (UDP) as the Layer 4 transport protocol of choice. UDP is a low-overhead, connectionless protocol. Applications such as Voice over IP (VoIP) and video use UDP packets. The retransmission capability that TCP offers does little to enhance packet loss for VoIP applications. Retransmitting VoIP packets proves more annoying to the user than useful. As a result, the data-loss roaming might cause a noticeable impact to UDP-based applications.

Roaming Domain

Chapter 1, "Ethernet Technologies," defines a broadcast domain as a network that connects devices that are capable of sending and receiving broadcast frames to and from one another. This domain is also referred to as a Layer 2 network. The concept holds true for 802.11 as well. APs that are in the same broadcast domain and configured with the same service set identifier (SSID) are said to be in the same roaming domain. Recall from Chapter 2, "802.11 Wireless LANs," that extended service set (ESS) is similarly defined as multiple basic service sets (BSSs) that communicate via the distribution service (wired network). Therefore, a roaming domain can also be referred to as an ESS.Why are 802.11 devices limited to a Layer 2 network for roaming? What about roaming between Layer 3 subnets? Remember that 802.11 is a Layer 1 physical interface and Layer 2 data link layer technology. The 802.11 MAC protocol is Layer 3 unaware. That is not to say that Layer 3 roaming is impossible because it is not. It means that Layer 2 roaming is natively supported in 802.11 devices, and some upper-layer solution is required for Layer 3 roaming.

The distinction between whether a device roams within a roaming domain or between roaming domains has a large impact on application sessions. Figure 5-1 depicts a Layer 2 roaming domain. The roaming user can maintain application connectivity within the roaming domain and as long as its Layer 3 network address is maintained (does not change).

Figure 1Figure 5-1 Roaming in a Layer 2 Roaming Domain

Figure 5-2 illustrates roaming across roaming domains. The roaming user is roaming from an AP on Subnet A to an AP on Subnet B. As a result, the Layer 3 network address must change to maintain Layer 3 connectivity on Subnet B. As the Layer 3 address changes, the station drops all application sessions. This scenario is described later in this chapter in the section, "Mobile IP Overview."

Figure 2Figure 5-2 Roaming Across Roaming Domains

Roaming Duration

Roaming duration is the time it takes for roaming to complete. Roaming is essentially the association process that is described in Chapter 2; it depends on the duration of the following:

  • The probing process

  • The 802.11 authentication process

  • The 802.11 association process

  • The 802.1X authentication process

The cumulative duration of these processes equates to the roaming duration. Some appli-cations, such as VoIP, are extremely delay-sensitive and cannot tolerate large roaming durations.

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