Wireless LANs

Advantages very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... Disadvantages typically very low bandwidth compared to wired networks (1-10 Mbit/s) due to shared medium many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11) products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000 Design goals for wireless LANs global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) transparency concerning applications and higher layer protocols, but also location awareness if necessary Comparison: infrared vs. radio transmission Infrared uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices Example Many different products 802.11 - Physical layer (classical) 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mW (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchronization

Comments

Post a Comment

Popular posts from this blog

Handling of Skew

Handling of Skew 􀂄 The distribution of tuples to disks may be skewed — that is, some disks have many tuples, while others may have fewer tuples. 􀂄 Types of skew: 􀃌 Attribute-value skew. 􀂾 Some values appear in the partitioning attributes of many tuples; all the tuples with the same value for the partitioning attribute end up in the same partition. 􀂾 Can occur with range-partitioning and hash-partitioning. 􀃌 Partition skew. 􀂾 With range-partitioning, badly chosen partition vector may assign too many tuples to some partitions and too few to others. 􀂾 Less likely with hash-partitioning if a good hash-function is chosen. Handling Skew in Range-Partitioning 􀂄 To create a balanced partitioning vector (assuming partitioning attribute forms a key of the relation): 􀃌 Sort the relation on the partitioning attribute. 􀃌 Construct the partition vector by scanning the relation in sorted order as follows. 􀂾 After every 1/nth of the relation has been read, the value...
Read more

Fragment-and-Replicate Join

Fragment-and-Replicate Join 􀂄 Partitioning not possible for some join conditions 􀃌 e.g., non-equijoin conditions, such as r.A > s.B. 􀂄 For joins were partitioning is not applicable, parallelization can be accomplished by fragment and replicate technique 􀃌 Depicted on next slide 􀂄 Special case – asymmetric fragment-and-replicate: 􀃌 One of the relations, say r, is partitioned; any partitioning technique can be used. 􀃌 The other relation, s, is replicated across all the processors. 􀃌 Processor Pi then locally computes the join of ri with all of s using any join technique. 􀂄 General case: reduces the sizes of the relations at each processor. 􀃌 r is partitioned into n partitions,r0, r1, ..., r n-1;s is partitioned into m partitions, s0, s1, ..., sm-1. 􀃌 Any partitioning technique may be used. 􀃌 There must be at least m * n processors. 􀃌 Label the processors as 􀃌 P0,0, P0,1, ..., P0,m-1, P1,0, ..., Pn-1m-1. 􀃌 Pi,j computes the join of ri with sj. In ...
Read more

Interquery Parallelism

Interquery Parallelism 􀂄 Queries/transactions execute in parallel with one another. 􀂄 Increases transaction throughput; used primarily to scale up a transaction processing system to support a larger number of transactions per second. 􀂄 Easiest form of parallelism to support, particularly in a sharedmemory parallel database, because even sequential database systems support concurrent processing. 􀂄 More complicated to implement on shared-disk or shared-nothing architectures 􀃌 Locking and logging must be coordinated by passing messages between processors. 􀃌 Data in a local buffer may have been updated at another processor. 􀃌 Cache-coherency has to be maintained — reads and writes of data in buffer must find latest version of data. Cache Coherency Protocol 􀂄 Example of a cache coherency protocol for shared disk systems: 􀃌 Before reading/writing to a page, the page must be locked in shared/exclusive mode. 􀃌 On locking a page, the page must be read from disk ...
Read more