Frequency hopping pattern is used to separate different Pico nets. The Pico net channel is represented by a pseudo-random hopping sequence. The hopping sequence is unique for the Pico net and is determined by the device address of the master of the Pico net. A Pico net is a collection of Bluetooth devices, which are synchronized to the same hopping sequence. In order for devices to be able to communicate between each other, the master determines the hopping sequence or pattern in the Pico net and the slaves have to synchronize to this pattern.
Each Pico net has a unique hopping pattern, which is determined by the device ID, a 48-bit unique identifier. A collision occurs if two or more Pico nets use the same carrier frequency at the same time. This might happen as the hopping sequences are not coordinated.
The phase in the hopping pattern is determined by the master’s clock. After adjusting the internal clock according to the master a device may participate in the Pico net. Frequency-hopping/time -division duplex scheme is used for transmission with a fast hopping rate of 1,600 hops per second. The time between two hops is called a slot, which is an interval of 0.625 ms, each slot uses different frequency. Bluetooth uses 79 hop carriers equally spaced within 1MHz.
Each device participating in a certain Pico net hops at the same time to the same carrier frequency. Within each slot the master or one out of the seven slaves may transmit data in an alternating fashion. Master and slaves alternately transmit and listen. 7. Explain how tunneling works in general and especially for mobile IP using IP-in-IP, minimal, and generic routing encapsulation, respectively. Discuss the advantages and disadvantages of these three methods.
Tunneling is a mechanism used for forwarding packets between home agent and care-off address. A tunnel establishes a virtual pipe for packets between an entry-to-end point. The packet itself is unchanged during the process of entry-exit the tunnel. Sending the packets through the tunnel is achieved by using encapsulation. With mobile IP the HA takes the original packet with the MN as destination, puts it into the data part of a new packet and sets the new IP header in such a way that the packet is routed to the COA. The new header is called “outer header”. IP-in-IP encapsulation is mandatory for IP.
The entire IP datagram becomes payload in new IP datagram. The original, inner IP header is unchanged except for the time to live number, which is decremented by 1. The outer header is a full IP header, while the inner header is identical to the original header. The big advantage of this technique is that the whole tunnel is considered a single hop from the original packet’s point of view. This feature allows the MN to behave as if it were attached to the home network. No matter how many number of hops the packet has to take in the tunnel, it is just a one logical hop away from the MN.
Minimal encapsulation is an optional encapsulation technique. With this method the tunnel entry point and the endpoint are specified. A new header is inserted between the original IP header and original IP payload. The original IP header is modified to form new outer IP header. This technique avoids repetition of identical fields, as TTL, IHL, version and TOS. It is only applicable for the unfragmented packets, because no space is left for the fragment identification. Generic routing is a method that allows the encapsulation of packet of one protocol suite into the payload portion of a packet of another protocol suite.
The outer header is the standard IP header with HA as source address and COA as destination address. The big advantage of this technique is that it supports other network layer protocols in addition to IP. 8. Explain the principle of the routing protocol DSDV (Destination Sequenced Distance Vector). DSDV is an enhancement to distance-vector routing for ad-hoc networks. It is of a proactive type, where each node maintains routing information for all known destinations. Distance vector routing is needs periodic updates and therefore it results in traffic overhead even if there is no change in network topology.
DSDV keeps the simplicity of distance vector, it guarantees loop freeness. DSDV features new table entry for destination along with a feature as a sequence number, which allow fast reaction to topology changes. It also makes immediate route advertisement on significant changes in routing table. DSDV adds two main functions. * Sequence numbers: Each routing advertisement comes with a sequence number which helps to apply the advertisement in correct order and therefore it avoids loop that are likely to be a part of distance vector algorithm.
Damping: Advertisement containing changes in the topology currently stored and therefore not spread further between the routers. This technique includes a timer, which relates to the waiting time between the first and the best announcement of a path to a specific destination. DSDV routing tables contain flags and a setting time, that helps with stable status of paths or links, and are used to delete stale entries from the table. Update information is compared to own routing table, the route is selected with the higher destination sequence number, ensuring that the newest information from destination is used.
It also supports immediate advertisements which means that change in the network such as information on new routes, broken links, and metric are immediately propagated to neighbours. 9. What is the reaction of standard TCP in case of packet loss? In what situation does this reaction make sense and why is it quite often problematic in the case of wireless networks and mobility? TCP assumes a network congestion if acknowledgment does not arrive in time. Standard TCP reacts with slow start if acknowledgments are missing.
It is practical in fixed networks but this process doesn’t facilitate in the case of transmission errors over wireless link and during handover. This behavior results in severe performance degradation and decreases the efficiency of TCP used in mobile network. TCP acknowledgment mechanism cannot distinguish between the different causes of packet loss, which means that missing acknowledgment due to a transmission error is misinterpreted for congestion or network overload.