TCP Connectivity Analysis in Mobile
7.5 Simulation Environment, Test-Bed, and Results
In this section, a test bed is set up using a DSR-based MANET scenario. Figure 7.6 shows a typical ad hoc mobile-IP architecture with essential entities such as Home Agent (HA), Correspondence Node (CN), hosts, and ad hoc elements, and optional entities such as Access Points and ad hoc manager.
HA is a server located at the base location, the same location as the mobile host. When the mobile host is at its base location, only HA serves it, and no additional roaming, handoff, and handover procedures are involved. Whenever an entity requires to communicate with the mobile host, it sends its queries to the HA, and whenever the mobile host needs to communicate, it communicates through the HA as well. As soon as the mobile host leaves its home location, the HA is not able to serve the mobile host directly. In this situation, ad hoc managers and Access Points come to play their roles.
When the mobile host is away from the base location, HA will send the related queries first to the ad hoc manager, which will be in touch with the current Access Point. An ad hoc manager manages a wireless domain of ad hoc entities, and an Access Point directly registers a mobile host as it is roaming inside a local region.
FIGURE 7.5 DSR fixed and option headers.
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7.5.1 Simulation Parameters
NS-II simulation software is used for the simulation purposes. Within NS-II, one is able to create close-to-real-life channel conditions. A realistic modeling is considered for the following physical layer-related factors:
• Free space and ground reflection propagation
• Transmission power
• Antenna gain
• Receiver sensitivity
• Propagation delay
• Carrier sense
• Discarding packets below carrier-sense threshold
• CSMA/CD (CA) collision leftovers (packets not discarded due to collisions)
An efficient and complete distributed coordination function (DCF) MAC layer model is used, which applies to IEEE 802.11-based wireless LAN protocol standards.
The following parameters specify the traffic conditions:
• Traffic rate of the channel is set to 2 Mbps.
• Ad hoc elements use DSR protocol for routing on top of IPv6.
• For traffic generation, file transfer protocol (FTP) is used over TCP for all the flows in the network.
• 802.11b is used for MAC layer protocol.
The simulation shows the retransmission pattern in the course of mobile host movements from one Access Point to the next (during a handoff/handover procedure). Figure 7.7 shows this pattern, in which a handoff/handover takes place between the 3rd and the 4th seconds. This results in a slight degradation FIGURE 7.6 Typical ad hoc mobile-IP architecture.
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of the connection due to the physical occurrence of handoff. The number of retransmissions rises to three in the worst case. The same happens between the 8th and 9th seconds.
7.6 Conclusion
This chapter has discussed the importance of TCP as a reliable transport layer in communication between mechatronic devices. TCP was shown to play an important role in error detection, forward error correction (retransmission), and congestion control. Different TCP flavors were introduced with descriptions on how each of them deals with the congestion issue. Different fields of TCP’s frame header were studied, and special attention was given to the retransmission procedure. Furthermore, the definition of ad hoc networks was introduced. It was discussed how TCP mechanism could be affected due to longer delays and physical limitations incurred within ad hoc domains. The end of the chapter included a simulation environment and test bed to study the effect of handoff/handover procedure on the number of retransmissions of TCP. For most mechatronic applications, TCP seamless connectivity has to maintain connectivity for high-performance connection-oriented handshaking with peers. During handoff/handover, which is the worst case for TCP retransmissions, the maximum number of retrans-missions rises to three, which, depending on the specific application, could be nontolerable.
References
1. Wilson, D.J. and Dragnea, R., IPv6 in Fixed and Mobile Networks, Technology White Paper, Alcatel Corp. Press, Alcatel Headquarters, Paris, France, November 2004.
2. Kurose, J., Connection-Oriented Transport: TCP, Lecture Notes, University of Massachusetts, Amherst, MA, May 2004.
3. Haden, R., TCP, The Data Network Resource, (http://www.rhyshaden.com/tcp.htm),1996–2006.
4. Transmission Control Protocol, RFC 793, IETF, Information Sciences Institute, University of Southern California, Marina del Rey, California, 1981.
5. Port Numbers and Services Database (www.sockets.com/services.htm).
6. List of TCP and UDP port numbers, Wikipedia Web site (http://en.wikipedia.org/wiki/
List_of_TCP_and_UDP_port_numbers).
FIGURE 7.7 TCP retransmission pattern during mobile-IP activity.
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7. Xu, S. and Saadawi, T., Performance evaluation of TCP algorithms in multi-hop wireless packet networks, Journal of Wireless Communications and Mobile Computing, Vol. 2, No. 1, 85–100, 2002.
8. Stoica, I., A Comparative Analysis of TCP Tahoe, Reno, New-Reno, SACK and Vegas, Communication Networks, Student Project, University of California, Berkeley, CA, 2005.
9. Adibi, S., Different flavors of mobile ad-hoc networks (MANETs), Photonic Networking, ECE 710 Final Course Project, Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, Canada, July 2006.
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