程序代写 FIT3165/FIT4165 Tutorial #8 TCP/IP Layered Architecture Week 9

FIT3165/FIT4165 Tutorial #8 TCP/IP Layered Architecture Week 9 – Semester 1 – 2022
25 April 2022
Revision Status
Updated by Dr. , Rosanna F Alam and , April 2022.

Instructions
1. Students work individually to solve this week’s exercise.
2. Each student must answer the following review Q’s and complete the practical tasks
given in part 2.
PART 1 – Review Questions
1. Explain why the MAC protocol is more important in wireless LANs than wired LANs?
In the case of wired LANs, we have moved from a shared medium to a dedicated medium (point-to-point communication). In this situation a MAC protocol, such as CSMA/CD, is not needed anymore. In the case of a wireless LAN, the medium (air) is still shared between the users. We need MAC protocols such as CSMA/CA or channelization protocols to control sharing the medium.
2. Why is SNR in a wireless LAN normally lower than SNR in a wired LAN?
SNR is the ratio of the signal power to the noise power. If the signal power is decreased or the noise power is increased, SNR will decrease. In a wireless LAN, the signal power is less (using batteries). The noise power is higher in a wireless LAN because the noise is not controlled. The noise from any source can affect the signal exchanged between the sender and the receiver.
3. What is multipath propagation? What is its effect on wireless networks?
In a wireless environment, a receiving station may receive more than one signal from the same sender related to the same message. One of these signals can be the one received directly; the others are signals reflected back from some barrier. Since the signals have travelled different distances, they can be out of phase. The combination of these signals creates a signal which is the distorted version of the original signal sent by the sender. It is sometimes difficult to detect the original message.
4. There is no acknowledgment mechanism in CSMA/CD, but we need this mechanism in CSMA/CA. Explain the reason for this.
The lack of collision in CSMA/CD serves as an indication that data arrived safely. In CSMA/CA, the sending station does not check the possibility of collision; an acknowledgment is needed to ensure that a collision did not occur and the frame arrived at its destination.
©2022, Faculty of IT, Monash University

5. An AP may connect a wireless network to a wired network. Does the AP need to have two MAC addresses in this case?
No! The answer is negative. The MAC addresses are coming from the same address space. Only one MAC address can serve in this case.
6. An AP in a wireless network plays the same role as a link-layer switch in a wired network. However, a link-layer switch may or may not have a MAC address, but an AP normally needs a MAC address. Explain the reason.
In a wired network, a link-layer switch is connected to the hosts via point-to-point dedicated connections; there is no need for addresses for communication between hosts and the switch.
In a wireless network, an AP is connected to the hosts via a multicast network (air); the MAC addresses of the host and the AP make the communication more efficient; when a host sends a frame to the AP, all other hosts drop the received copy of the frame at the MAC sublayer when they find that the frame does not belong to them.
7. Explain why we have only one frame type in a wired LAN, but four frame types in a wireless LAN?
In a wired LAN, the medium access process is achieved using the collision detection mechanism. In a wireless LAN, the same task is done using the RTS, CTS, and ACK frames. We need all of these as well as data frames.
8. Do the MAC addresses used in an 802.3 (Wired Ethernet) and the MAC addresses used in 802.11 (Wireless Ethernet) belong to two different address spaces?
The answer is negative. The addresses are selected from the same address space. For example, if locally there is one wireless and one wired device, they cannot have the same MAC address; addresses should be unique.
©2022, Faculty of IT, Monash University

PART 2 – Practical Tasks
Activity: Design IP subnets for the network shown in Figure 1 using the network address block 10.100.192.0/18. Build the network with routers connecting multiple LANs together. Use dynamic routing to implement connectivity between the LANs.
(i) Two company routers A and B are connected to the router called “Internet” playing the role of the Internet. The company network is divided into client and server subnets. Design IP subnets for the following specifications using the network address 10.100.192.0/18 and build the network using CORE as shown in Figure 1.
Network address
Address Range
Company A Clients
254 IP addresses needed
Company A Servers
254 IP addresses needed
©2022, Faculty of IT, Monash University

Company B Clients
254 IP addresses needed
Company B Servers
254 IP addresses needed
Company B – Internet
2 IP addresses needed
Company A – Internet
2 IP addresses needed
Use DHCP to assign IP addresses to all the clients and servers. Therefore, Company-A and Company-B routers will be configured to work as DHCP servers.
(ii) Configure DHCP servers in the company-A and company-B routers using the IP addressing scheme designed in the part (i). You can use the following example configuration given as a template.
subnet 10.0.4.0 netmask 255.255.255.0
range 10.0.4.127 10.0.4.254; default-lease-time 600; option routers 10.0.4.1;
Enable DHCP client service on servers and clients. Run the simulation and check the clients’ and servers’ IP configuration using “ifconfig” command at the command prompt. You may check network connectivity by using the ping command to reach any host from any other.
(iii) Use a dynamic routing protocol, RIP for the packets to reach hosts from one subnet to another. This routing configuration should allow any host from any network to reach any other host in the entire network in Figure 1. Please follow the steps given below to configure RIP in all the routers.
● Step 1: Find “Quagga” in the services option of each router.
● Step 2: Enable “Zebra”, “RIP” and “vtysh“
● Step 3: Configure “Zebra” with the following details
Example configuration for Company – B router
interface eth0
ip address 10.100.196.1/30 !
interface eth1
ip address 10.100.194.1/24 !
interface eth2
ip address 10.100.195.1/24 !
©2022, Faculty of IT, Monash University

router rip
redistribute static redistribute connected redistribute ospf network 10.100.194.0/24 network 10.100.195.0/24 network 10.100.196.0/30
You can configure all the routers with appropriate interface IP addresses and the connected network addresses following the example given above.
(iv) Run the simulation and check network connectivity by using the ping command to reach any host from any other. If you have any problem in reaching full connectivity, you may use the “ip route show” command in the routers to check the routing information that the router holds.
©2022, Faculty of IT, Monash University

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