How To Solve A Network Problem In An Ipa (Ipa 2) (Ipam) (Networking) (For Acedo) (Ipta) (Powerline) (Network) (Nem

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1 Lund University ETSN01 Advanced Telecommunication Tutorial 7 : Link layer part I Author: Antonio Franco Course Teacher: Emma Fitzgerald February 12, 2015

2 Contents I Before you start 3 II Exercises 3 1 Link layer III Solutions 4 2 Link layer

3 Part I Before you start This tutorial is given to prepare you to the exam. Since time is limited, it is highly advised that you first try to solve the exercises (Part II) at home, then have a look at the solutions (Part III), and, finally, ask questions during the exercises sessions. Part II Exercises 1 Link layer 1.1 The IEEE standard has listed the following three functions since the first incarnation: 1. Point Coordination Function 2. Enhanced Distributed Coordination Function 3. HCF Controlled Channel Access Explain which problem each function addresses, how the function operates and compare/contrast the approaches. 1.2 Describe what is meant by the following two terms and draw an illustrative example of each: 1. The hidden node problem 2. The exposed node problem A mechanism called MACA has been designed to alleviate the problems associated with hidden nodes, how does this operate and how does this address the problem? 1.3 (a) Explain the steps of the Reservation ALOHA algorithm and draw a diagram of its operation. (b) What are the advantages and disadvantages of using this algorithm over slotted ALOHA? 3

4 (c) Consider a Reservation ALOHA system with N nodes. Assume a Poissonian packet generation process for each node with parameter λ. The round length is T s seconds that is, every T s seconds contains a reservation phase and a data transmission phase. Each data transmission phase contains k timeslots. What is the probability that a given node will have at least one packet to send in the second round? You can assume that all nodes begin the first round with no packets queued to send. (d) What is the expected number of nodes with at least one packet to send in the second round? (e) Each node that wishes to transmit selects a slot to reserve at random (with a uniform distribution), and each node may only send one packet per round, even if it has more packets queued to send. For N = 3, what is the probability of a collision in the reservation phase of the second round? Part III Solutions 2 Link layer The PCF is a centralised polling mechanism controlled by the AP whereby the AP hands out a transmission opportunity according to a polling list. This enables resource assignment but no traffic separation. 2. The EDCF is a distributed probabilistic function in which the STAs alter some parameters to achieve different probabilities in successfully accessing the channel at contention. The parameters are CWMIN/CWMAX, AIFSN and TXOPLimit. Traffic is divided into 4 access categories and given different treatment (priority) based on their classification. 3. HCCA is akin to PCF contention free but extended to provide traffic separation whereby the AP hands out a TxOP with different sizes to different flows using Traffic specifications. The scheme still suffers from the overheads associated with the PCF. Example illustrations of the hidden end exposed node problems are given in the lecture slides. MACA introduces the RTS and CTS frames for notifying all nodes within transmission range of the sender and receiver of the intent to transmit. If a node overhears either message it should refrain from transmitting for a set 4

5 period. Examples were given in the slides of situations where the RTS/CTS frames produced overhead but little or no benefit (both internal and external HN scenarios were given). The problems named were the probability of lost RTS/CTS frames due to collisions and the symmetry properties of traffic. 2.3 (a) In Reservation ALOHA, there are two phases: a reservation phase, followed by a transmission phase. In the reservation phase, stations reserve slots for the transmission phase using ALOHA. If the transmission phase has k slots, then the reservation phase is divided into k mini-slots (of much shorter duration than the transmission slots), so that a transmission in mini-slot i reserves transmission slot i. These two phases are then repeated. See the MAC slides, pp12-13, for a diagram. (b) Advantage: efficiency can be significantly increased (up to 80% from only 36% for slotted ALOHA), as no collisions are possible in the transmission phase. Disadvantage: delay increases due to waiting for the reserved timeslot, especially if a collision occurs during the reservation, and synchronisation is required between all stations to keep the reservation list consistent. (c) P(at least one packet to send) = 1 - P(no packets to send). (d) N (1 e λts ) (e) 1 P = 1 e λts (λt s ) 0 0! = 1 e λts [ (1 e λt s ) 3 k 1 k 2 k k + 3 ( 1 e λts) 2 e λt s k 1 k + 3 ( 1 e λts) ( e λts) 2 + ( ] e λts) 3 that is, the opposite event of the sum of the probability of all three transmitting and choosing different slots, plus the probability of two of them transmitting and choosing different slots, plus the probability of only one of them tranmsitting plus the probability of no one transmitting. 5