- 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 connectivity 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 background, 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?
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 connectionoriented 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 retransmitted 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
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 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."
Roaming Duration
Roaming duration is the time it takes for roaming to complete.
- 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 applications, such as VoIP, are extremely delay-sensitive and cannot tolerate large roaming durations.
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