ay in 2. Explain why the vulnerable time in ALOHA depends on the frame transmission time (Tfr), but in CSMA it depends on frame propagation time (Tp)?
The vulnerable times are in which there is a possibility of collision. We assume that the stations send fixed length frames with each frame taking Tfr S to send. Suppose Consider Station A sends a frame at time t. Now imagine station B already sent frame between t-Tfr and t.
That means it leads the collision between the frames of station A and B.The other end of B's frame collides with beginning of A's frame. Pure ALOHA has vulnerable time of 2 *Tfr. A station send one frame to another station has started.Establisation of Slotted ALOHA was invented to improve the efficiency of pure ALOAH. In the slotted ALOAH we divide the time into slots of Tfrs and force the station to send only beginning of the time slot.
3. In the CSMA protocol, if a station successfully receives an acknowledgement, it knows that its frame has been correctly received at the destination station. Now, if the station has another frame to send, the protocol forces the station to re-enter the back-off phase in which the station chooses a random back-off value and counts down this value when the channel is sensed idle. What rationale might the designers of CSMA/CA have had in mind by having such a station not transmit the second frame immediately –after the DIFS period (if the channel is sensed idle)?
Diagram of CSMA/CA:
Description of the diagram:
CSMA/CA: It is defined as a combination of physical carrier sensing and virtual carrier sensing.
Frame: It is a collection of a sequence of bits.
Working of CSMA/CA:
1. The source station should sense the medium before sending the frame by checking the energy level of the carrier sensing.
2. Persistence strategy should be with back-off until the channel is idle.
3. When the destination is found to be idle, then it waits for a period called Distributed
inter-free space (DIFS).
4. Then the station that sends the control frame is called as a request to send(RTS).
5. The waiting period after receiving the RTS is called as Short Inter-frame space (SIFS).
6. Therefore, the destination sends the control frame called the clear to send(CTS) to
the source station. Then the destination is ready to receive the data from the
frames.
7. The destination station waits for an amount of time which is equal to the waiting
time of SIFS.
8. An ACK acknowledgment is shown after receiving the data.
9. ACK is needed for the protocol to check the arrival of the data to the destination
station.
6. An organization has deployed a campus network using cellular IP. The cellular IP network extends the wired infrastructure of the campus as shown in the figure below. The gateways in cellular IP connect their respective wireless segments to the wired segment. Each Base station in the wireless segment provides layer 3 functionality (in other words, they act as IP forwarding engines). The campus network supports multicast routing. All the components in the wireless part strictly adhere to cellular IP standard. Assume that this campus network deploys a source based multicast routing scheme – a protocol that builds source based multicast distribution trees (Refer to Appendix for a brief explanation on Source based multicast routing). A visiting mobile node MN enters the wireless network and initially gets registered to its home network via BS1-G1 and acquires a Care of Address (CoA – an address from the campus network address space). The CoA assigned to the MN remains unchanged irrespective of MN’s movements within the campus network. In other words, MN does not need to acquire a new CoA upon moving to a wireless segment under the same or under a different gateway. While initially at BS1, the MN joins a multicast group G whose scope is local to this campus network (all sources,
and members reside in the campus network). Assume that hosts H1, H3, H5, and H7 are members of this group with H1 and H7 also acting as Multicast sources. Furthermore, assume that MN itself is a source for multicast traffic. In this scenario, explain what problems can arise in multicast communication due to MN’s mobility. (300 Words)
1. The most common issue in a multicast network is packets transmitted by the source not reaching receivers.
One of the common causes of packets not reaching receivers is Reverse Path Forwarding (RPF) check failure. Once a multicast data distribution tree is formed using a multicast routing protocol, routers use RPF check to forward multicast packets from one interface to
another. The RPF check verifies whether the packet arrived on the correct interface pointing toward the source to avoid loops. RPF failures may occur when there are multiple paths between devices that forward multicast traffic from a source to receivers, and the unicast routing topology is not congruent or the same as the data distribution tree of multicast topology.
2. Another multicast issue could be related to the building of the data distribution tree itself.
Each multicast routing protocol has its own mechanism to build and maintain the tree. Dense mode protocols rely on flood and prune behavior, which should not cause problems. Sparse mode protocols rely on a device functioning as Rendezvous Point (RP) to build the tree for a multicast group, and every router in the network needs to know the RP-to-group
mapping. This is done through manual configuration or automatic information distribution, which also involves some basic configuration. There are also mechanisms like filters available to control the distribution of RP information. Improper configuration of these features could lead to failure in building the distribution tree and affect the forwarding of multicast traffic.
5. Consider a novel Mobile IP scheme in which a mobile station announces its permanent
(home) IP address to agents in foreign networks. These agents, in turn, announce this
information to other routers using their regular routing protocol update messages. What
are some benefits and drawbacks of this scheme when compared to the IETF Mobile IP
scheme? (300 Words)
A lthough the Internet offers access to information sources worldwide, typically we do not expect to benefit from that access until we arrive at some familiar point–whether home, office, or school. However, the increasing variety of wireless devices offering IP connectivity, such as PDAs, handhelds, and digital cellular phones, is beginning to change our perceptions of the Internet.
To understand the contrast between the current realities of IP connectivity and future possibilities, consider the transition toward mobility that has occurred in telephony over the past 20 years. An analogous transition in the domain of networking, from dependence on fixed points of attachment to the flexibility afforded by mobility, has just begun.
Mobile computing and networking should not be confused with the portable computing and
networking we have today. In mobile networking, computing activities are not disrupted when the user changes the computer's point of attachment to the Internet. Instead, all the needed reconnection occurs automatically and noninteractively.
Truly mobile computing offers many advantages. Confident access to the Internet anytime, anywhere will help free us from the ties that bind us to our desktops. Consider how cellular phones have given people new freedom in carrying out their work. Taking along an entire computing environment has the potential not just to extend that flexibility but to fundamentally change the existing work ethic. Having the Internet available to us as we move will give us the tools to build new computing environments wherever we go. Those
who have little interest in mobility per se will still benefit from the ability to resume previous applications when they reconnect. This is especially convenient in a wireless LAN office environment, where the boundaries between attachment points are not sharp and are often invisible.
The evolution of mobile networking will differ from that of telephony in some important respects. The endpoints of a telephone connection are typically human; computer applications are likely to involve interactions between machines without human intervention. Obvious examples of this are mobile computing devices on airplanes, ships, and automobiles. Mobile networking may well also come to depend on position-finding devices, such as a satellite global positioning system, to work in tandem with wireless access to the Internet.
Another difference may well be rate of adoption. It took many years for mobile phones to become cheap and light-weight enough to be perceived as convenient. Because wireless mobile computing devices such as PDAs and pocket organizers have already found user acceptance, mobile computing may become popular much more quickly.
However, there are still some technical obstacles that must be overcome before mobile networking can become widespread. The most fundamental is the way the Internet Protocol, the protocol that connects the networks of today's Internet, routes packets to their destinations according to IP addresses. These addresses are associated with a fixed network location much as a nonmobile phone number is associated with a physical jack in a wall. When the packet's destination is a mobile node, this means that each new point of attachment made by the node is associated with a new network number and, hence, a new IP address, making transparent mobility impossible.
Mobile IP (RFC 2002),1 a standard proposed by a working group within the Internet Engineering Task Force, was designed to solve this problem by allowing the mobile node to use two IP addresses: a fixed home address and a care-of address that changes at each new point of attachment. This article will present the Mobile IP standard in moderate technical detail and point the reader toward a wealth of further information.2 ,3 In addition, readers can go to the sidebar Mobile IP Web Resources in this issue's IC Online at http://computer.org/internet/ for a convenient set of hyperlinked resources.
I also describe how Mobile IP will change with IP version 6,4 , 5 the product of a major effort within the IETF to engineer an eventual replacement for the current version of IP.6 Although IPv6 will support mobility to a greater degree than IPv4, it will still need Mobile IP to make mobility transparent to applications and higher level protocols such as TCP.
There is a great deal of interest in mobile computing and apparently in Mobile IP as a way to provide for it. A quick Web search for items related to Mobile IP returned over 60,000 hits– impressive even given the notorious lack of selectivity for such procedures. Mobile IP forms the basis either directly or indirectly of many current research efforts and products. The Cellular Digital Packet Data (CDPD),7 for example, has created a widely deployed communications infrastructure based on a previous draft specification of the protocol. In addition, most major router vendors have developed implementations for Mobile IP.
The outlook for Mobile IP in the complex Internet marketplace is far from clear, and some technical problems remain, security being the most important. However, once the security
solutions are solid, nomadic users may finally begin to enjoy the convenience of seamless untethered roaming and effective application transparency that is the promise of Mobile IP.
HOW MOBILE IP WORKS
IP routes packets from a source endpoint to a destination by allowing routers to forward packets from incoming network interfaces to outbound interfaces according to routing tables. The routing tables typically maintain the next-hop (outbound interface) information for each destination IP address, according to the number of networks to which that IP address is connected. The network number is derived from the IP address by masking off some of the low-order bits. Thus, the IP address typically carries with it information that specifies the IP node's point of attachment.
To maintain existing transport-layer connections (see the sidebar "Nomadicity: How Mobility Will Affect the Protocol Stack" on the next pages) as the mobile node moves from place to place, it must keep its IP address the same. In TCP (which accounts for the overwhelming majority of Internet connections), connections are indexed by a quadruplet that contains the IP addresses and port numbers of both connection endpoints. Changing any of these four numbers will cause the connection to be disrupted and lost. On the other hand, correct delivery of packets to the mobile node's current point of attachment depends on the network number contained within the mobile node's IP address, which changes at new points of attachment. To change the routing requires a new IP address associated with the new point of attachment.
Mobile IP has been designed to solve this problem by allowing the mobile node to use two IP addresses (see the sidebar "Mobile Networking Terminology" for definitions of italicized terms). In Mobile IP, the home address is static and is used, for instance, to identify TCP connections. The care-of address changes at each new point of attachment and can be thought of as the mobile node's topologically significant address; it indicates the network number and thus identifies the mobile node's point of attachment with respect to the network topology. The home address makes it appear that the mobile node is continually able to receive data on its home network, where Mobile IP requires the existence of a network node known as the home agent. Whenever the mobile node is not attached to its home network (and is therefore attached to what is termed a foreign network), the home agent gets all the packets destined for the mobile node and arranges to deliver them to the mobile node's current point of attachment.
Whenever the mobile node moves, it registers its new care-of address with its home agent. To get a packet to a mobile node from its home network, the home agent delivers the
packet from the home network to the care-of address. The further delivery requires that the packet be modified so that the care-of address appears as the destination IP address. This modification can be understood as a packet transformation or, more specifically,
a redirection. When the packet arrives at the care-of address, the reverse transformation is applied so that the packet once again appears to have the mobile node's home address as the destination IP address. When the packet arrives at the mobile node, addressed to the home address, it will be processed properly by TCP or whatever higher level protocol logically receives it from the mobile node's IP (that is, layer 3) processing layer. More information on the abstract modeling as a way to perform layer 3 redirection on packets can be found in Bhagwat, Perkins, and Tripathi.8
In Mobile IP the home agent redirects packets from the home network to the care-of address by constructing a new IP header that contains the mobile node's care-of address as the destination IP address. This new header then shields or encapsulates the original packet, causing the mobile node's home address to have no effect on the encapsulated packet's routing until it arrives at the care-of address. Such encapsulationis also called tunneling, which suggests that the packet burrows through the Internet, bypassing the usual effects of IP routing.
Mobile IP, then, is best understood as the cooperation of three separable mechanisms:
• Discovering the care-of address;
• Registering the care-of address;
• Tunneling to the care-of address.
4 In MACAW, the scheme of having the congestion information disseminated explicitly by media access protocol produced a fairer allocation of resources. What problems can surface when using such a scheme?
• Considering the two-cell configuration Stream X1-Y1 -> Channel utilization -> 0 Stream Y2-X2 -> Channel utilization -> 20.35
The above shows the throughput as a result from the version the media access protocol incorporating the scheme of having the congestion information disseminated explicitly by media access protocol. The stream X1- Y1 is not having any access( access denied), whereas stream Y2-X2 is having the complete utilization of channel. It is because whenever X1 try and intiate a data transfer, Y1 is not able to hear it due to the transmission of Y2. B1 will be able to initiate a transfer when its RTS arrives in the short gap between completion of Y2's net RTS and complete data transmission.
The main problem is lack of synchronization information since X1 does not have a way to know about the contention periods.
7. Briefly explain the architectural difference between IETF Mobile-IP and Columbia Mobile-IP. Highlight the architectural difference along the following dimensions:
a. HA
b. FA
c. Location Directory d. Location Update
here…