Local Metropolitan Area Networks ACOE 322 Lecture 3

Local & Metropolitan Area Networks ACOE 322 Lecture 3 LAN types Dr. L. Christofi 1

0. LAN types • The types of LANs we will examine in this section are as follows: 1. Ethernet (IEEE 802. 3) 2. Token Ring (IEEE 802. 5) 3. Fiber Distributed Data Interface (FDDI) 4. Wireless LANs (IEEE 802. 11) Dr. L. Christofi 2

1. Ethernet LANs 1. 1 Traditional Ethernet (CSMA/CD) 1. 2 10 Mbps Ethernet 1. 3 100 Mbps Ethernet (Fast Ethernet - FE) 1. 4 1000 Mbps Ethernet (Gigabit Ethernet - GE) Dr. L. Christofi 3

1. 1 Ethernet (CSMA/CD) • The most widely used LANs today are based on Ethernet and have been standardized by the IEEE 802. 3 standards committee • The Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as the access method • Recall: — Layers specified by 802. 3: • Ethernet Physical Layer • Ethernet Medium Access (MAC) Sublayer • Possible Topologies: — Bus — Branching non-rooted tree for large Ethernets Dr. L. Christofi 4

Network Interface Card (NIC) • Each station on an Ethernet network (such as a PC, workstation or printer) has its own NIC. • The NIC fits inside the station and it is the interface between the station and the network. • In most desktop computers, the NIC is an Ethernet card (10, 100 or 1000 Mbps) that is plugged into a slot on the computer motherboard. Dr. L. Christofi 5

How does Ethernet work? • The NIC provides the station with a 6 -byte physical address, normally written in hex notation. This is the MAC address. • Using MAC addresses to distinguish between machines, Ethernet transmits frames of data across baseband cables using CSMA/CD (IEEE 802. 3) Dr. L. Christofi 6

What is a MAC Address? • Media Access Control (MAC) Address is the physical address of any device, e. g. a NIC in a computer on the network. • The MAC address is 6 -bytes long and has two parts of 3 bytes long. Example: 06 7 A D 3 The first 3 bytes specify the company that made the NIC Dr. L. Christofi 01 BF 2 C the second 3 bytes are the serial number of the NIC 7

Minimal Bus Configuration Coaxial Cable Transceiver Terminator Transceiver Cable Host Dr. L. Christofi 8

Typical Large-Scale Configuration Repeater Host Ethernet segment Dr. L. Christofi 9

Ethernet Configuration Dr. L. Christofi 10

Ethernet Physical Layer • Transceiver Cable — 4 Twisted Pairs — 15 Pin Connectors • Channel Logic — Manchester Phase Encoding — 64 -bit preamble for synchronization Dr. L. Christofi 11

Ethernet Physical Configuration (for thick coaxial cable) • Segments of 500 meters maximum • Maximum total cable length of 1500 meters between any two transceivers • Maximum of 2 repeaters in any path • Maximum of 100 transceivers per segment • Transceivers placed only at 2. 5 meter marks on cable Dr. L. Christofi 12

Manchester Encoding Ethernet uses Manchester Encoding scheme Data stream 1 0 1 1 0 0 Encoded bit pattern Recall: • 1 bit = high->low voltage signal • 0 bit = low->high voltage signal Dr. L. Christofi 13

Ethernet Synchronization • 64 -bit frame preamble used to synchronize reception • 7 bytes of 1010 followed by a byte containing 10101011 • Manchester encoded, the preamble appears like a sine wave Dr. L. Christofi 14

Ethernet Cabling Options • • • 10 Base 5: Thick Coax 10 Base 2: Thin Coax (“cheapernet”) 10 Base-T: Twisted Pair 10 Base-F: Fiber optic Each cabling option carries with it a different set of physical layer constraints (e. g. , max. segment size, nodes/segment, etc. ) Dr. L. Christofi 15

Ethernet: MAC Layer • Data encapsulation —Frame Format —Addressing —Error Detection • Link Management —CSMA/CD —Backoff Algorithm Dr. L. Christofi 16

Ethernet Frame Format Multicast bit Destination (6 bytes) Source (6 bytes) Length (2 bytes) Data (46 -1500 bytes) Pad FCS (4 bytes) 64 Bytes < Ethernet frame size < 1518 Bytes Dr. L. Christofi 17

Ethernet MAC Frame Address Field • Destination and Source Addresses: — 6 bytes each • Two types of destination addresses — Physical address: Unique for each user — Multicast address: Group of users — First bit of address determines which type of address is being used 0 = physical address 1 = multicast address Dr. L. Christofi 18

Communicating Within the LAN Unicast Broadcast Multicast Dr. L. Christofi 19

IEEE 802. 3 frame format (1) 64 Bytes* < Ethernet frame size < 1518 Bytes* * Excluding Preamble and SFD Dr. L. Christofi 20

IEEE 802. 3 frame format (2) • Preamble: a 7 -byte pattern of alternating 1 s and 0 s to establish bit synchronization • Start frame delimiter (SFD): The sequence 10101011 indicates the actual start of the frame • Destination address (DA): specifies the station(s) for which the frame is intended • Source address (SA): specifies the station that sent the frame • Length/type: length of LLC data field in bytes or Ethernet type field • LLC data: data unit supplied by LLC • Pad: bytes added to ensure that the frame is long enough for proper CD operation. Filled when Length < 46. • Frame check sequence (FCS): a 4 -byte CRC check, based on all fields except preamble, SFD and FCS. Dr. L. Christofi 21

CSMA/CD revisited • Recall: — CSMA/CD is a “carrier sense” protocol. • If channel is idle, transmit immediately • If busy, wait until the channel becomes idle — CSMA/CD can detect collections. • Abort transmission immediately if there is a collision • Try again later according to a backoff algorithm • Carrier sense reduces the number of collisions • Collision detection reduces the impact of collisions Dr. L. Christofi 22

CSMA/CD inefficiency • Inefficiency of CSMA/CD — When two frames collide, the medium remains unusable for the duration of transmission of both damaged frames — For long frames, compared to propagation time, the amount of wasted capacity can be considerable. — This waste can be reduced if a station continues to listen to the medium while transmitting Dr. L. Christofi 23

CSMA/CD and Ethernet • Ethernet: — Short end-to-end propagation delay — Broadcast channel • Ethernet access protocol: — CSMA/CD with Binary Exponential Backoff Algorithm Dr. L. Christofi 24

Ethernet Backoff Algorithm: Binary Exponential Backoff • If collision, — Choose one slot randomly from 2 k slots, where k is the number of collisions the frame has suffered. — One contention slot length = 2 x end-to-end propagation delay This algorithm can adapt to changes in network load. Dr. L. Christofi 25

Binary Exponential Backoff (cont’d) slot length = 2 x end-to-end delay = 15 ms A Dr. L. Christofi B @ t=0 ms: Assume A and B collide (k. A = k. B = 1) A, B choose randomly from 21 slots: [0, 1] Assume A chooses 1, B chooses 1 @ t=30 ms: A and B collide (k. A = k. B = 2) A, B choose randomly from 22 slots: [0, 3] Assume A chooses 2, B chooses 0 @ t=45 ms: B transmits successfully @ t=75 ms: A transmits successfully 26

Binary Exponential Backoff (cont’d) • In Ethernet, — Binary exponential backoff will allow a maximum of 15 retransmission attempts — If 16 backoffs occur, the transmission of the frame is considered a failure. Dr. L. Christofi 27

Ethernet Performance Dr. L. Christofi 28

Ethernet Features and Advantages • Passive interface: — No active element • Broadcast: — • All users can listen Distributed control: — Each user makes own decision Simple Reliable Easy to reconfigure Dr. L. Christofi 29

Ethernet Disadvantages • Lack of priority levels • Cannot perform real-time communication • Security issues Dr. L. Christofi 30

Ethernet Switching • Recent development: Connect many Ethernet segments or subnets through an “Ethernet switch” Dr. L. Christofi to segment 4 to segment 1 to segment 3 to segment 2 31

Why Ethernet switching? • LANs may grow very large — The switch has a very fast backplane — It can forward frames very quickly from one segment to another • Cheaper than upgrading all host interfaces to use a faster network Dr. L. Christofi 32

1. 2 10 Mbps Ethernet • Physical configurations — 10 BASE 5 — 10 BASE 2 — 10 BASE-T — 10 BASE-F Dr. L. Christofi Note: 10 refers to data rate in Mbps, BASE=Baseband 5= 50Ω coaxial cable 2= thinner 50Ω coaxial cable T= Unshielded twisted pair cable F = Fiber Optic cable 33

10 BASE-T medium specification • Unshielded twisted pair (UTP) cables are found prewired in office buildings as excess telephone cable and can be used for LANs • 10 BASE-T defines a star topology • Stations attach to a multiport repeater via a point-to-point link • The link consists of two UTPs. • The data rate is 10 Mbps using Manchester encoding • Length of link is limited to 100 m • If an optical cable is used, the maximum length is 500 m. • A 10 BASE-T system can be mixed with 10 BASE 5 and 10 BASE 2 systems via repeaters • Maximum transmission path between any two stations is 5 segments and 4 repeater sets. — A segment is a point-to-point link or a coaxial cable — The maximum number of coaxial cable segments in a path is 3. Dr. L. Christofi 34

10 BASE-F medium specification • Added to IEEE 802. 3 in 1993 • Advantages of the distance and transmission on optical fiber • 10 BASE-FP (passive): passive-star topology for stations/repeaters with up to 1 km per segment; makes synchronous retransmission • 10 BASE-FL (link): a point-to-point link for connecting stations/repeaters at up to 2 km; asynchronous signaling; any timing distortions are propagated through a series of repeaters • 10 BASE-FB (backbone): a point-to-point link for connecting repeaters at up to 2 km; a cascade up to 15 repeaters in sequence to activate greater length. Dr. L. Christofi 35

High speed LANs Standard Ethernet LANs and MANs (up to 10 Mbps) are based on one copper wire. This is OK for low speed and short distances but not suitable for high speed and longer distances -> use fiber instead. § Fast and Gigabit Ethernet § FDDI (Fiber Distributed Data Interface) § HIPPI (High Performance Parallel Interface) Dr. L. Christofi 36

1. 3 Fast Ethernet • Operates at 100 Mbps • Standardized in IEEE 802. 3 as 100 BASE-T and 100 BASE-F • Basic idea behind Fast Ethernet (FE) was simple: keep all the old packet format, interfaces, procedural rules and cables, but just reduce the bit time from 100 ns to 10 ns. Dr. L. Christofi 37

IEEE 802. 3 100 BASE-T Physical Layer Medium Alternatives Dr. L. Christofi 38

Configuration and operation • In its simplest form, a 100 BASE-T network is configured in a star-wire topology, with all stations connected directly to a central point (multiport repeater). • The repeater is responsible for detecting collisions, not the attached devices. Its functions are: — A valid signal appearing on any input is repeated on all output links — If two inputs occur at the same time, a jam signal is transmitted on all links Dr. L. Christofi 39

Collision domain • Used to define a single CSMA/CD network • This means that if two stations transmit at the same time, a collision will occur. — Stations separated by a simple multiport repeater are within the same collision domain — Stations separated by a bridge are in different collision domains Bridged Ethernet Dr. L. Christofi Separates collision domains 40

Example: 100 -Mbps Ethernet Backbone Strategy Dr. L. Christofi 41

1. 4 Gigabit Ethernet (GE) • Operates at 1 Gbps • Same strategy as FE • CSMA/CD protocol and frame format, same as in 10 Mbps and 100 Mbps Ethernet • Compatible with 100 BASE-T and 10 BASE-T Dr. L. Christofi 42

Protocol architecture • The MAC layer is an enhanced version of the basic 802. 3 MAC algorithm • A separate Gigabit Medium-Independent Interface (GMII) is optional except for the UTP Dr. L. Christofi 43

GE Physical layer • 1000 BASE-LX — Long-wavelength option — Supports duplex links up up to 550 m for multimode fiber or up to 5 km for single mode fiber • 1000 BASE-SX — Short-wavelength — Supports duplex links up to 550 m for multimode fiber • 1000 BASE-CX — Supports 1 Gbps among devices located within a single room using copper jumpers (STP cable up to 25 m long) • 1000 BASE-T — Makes use of 4 pairs CAT 5 UTP cables to support devices over a range of up to 100 m Dr. L. Christofi 44

GE Medium Access Layer (1) • Same CSMA/CD frame format and MAC protocol as in 10 Mbps Ethernet and FE. Dr. L. Christofi 45

GE Medium Access Layer (2) • For hub operation (half duplex) there are two enhancements to the basic CSMA/CD scheme — Carrier extension • Appends a set of special symbols to the end of short MAC frames so that the resulting block is at least 4096 bit times in duration, up from the minimum 512 bit times imposed at standard Ethernet and FE. — Frame bursting • Allows for multiple short frames to be transmitted consequentially without releasing control for CSMA/CD between frames • Avoid the overhead of carrier extension when a single station has a number of small frames ready to send Dr. L. Christofi 46

2. Token Ring • Widely used although not as popular as Ethernet • Based on IEEE 802. 5 standard • We will now examine the Medium Access Layer and then the Physical layer aspects of this specification Dr. L. Christofi 47

IEEE 802. 5 Token Ring Medium Access Control Dr. L. Christofi 48

Token ring operation • Based on the use of a small frame, called a token, that circulates when all stations are idle • Whenever a station wishes to send a frame, it first waits for the token • On receipt of the token, it initiates transmission of the frame, which includes the address of the intended recipient at its header • The frame is repeated by all stations in the ring, until it circulates back to the initiating station, where it is removed • The intended recipient also retains a copy of the frame Dr. L. Christofi 49

Token Passing (1) Token is traveling along the ring Dr. L. Christofi Station A captures the token and sends its data to D 50

Token Passing (2) Station D copies the frame and sends the data back to the ring Dr. L. Christofi Station A receives the frame and releases the token 51

MAC frame (1) Dr. L. Christofi 52

MAC frame (2) • The MAC frame consists of the following fields: — Starting Delimiter (SD): Indicates start of frame — Access Control (AC): Contains info for priority, reservation variables and monitor bit — Frame Control (FC): indicates if this is an LLC data frame — Destination address (DA): As with 802. 3 — Source address (SA): As with 802. 3 — Data Unit (DU): Contains LLC data unit — Frame Check Sequence (FCS): As with 802. 3 — End Delimiter (ED): Contains the error detection bit (E), which is set if any repeater detects an error and the Intermediate (I) bit, which is used to indicate that this is a frame other than the final one of a multiple frame transmission — Frame Status (FS): Contains the address recognized (A) and frame Copied (C) bits. Dr. L. Christofi 53

Token maintenance • To overcome error conditions, one station is designated as the active monitor — Periodically issues an Active-Monitor-Present control frame to assure other stations that there is an active monitor on the ring — To detect a lost-token condition, the monitor uses a valid-frame timer that is greater than the time required to traverse the ring. — The timer is reset every valid token or data frame — If the time expires, the monitor issues a priority 0 token — If it sees a persistently circulating data frame with the monitor bit set, it absorbs it and transmits a priority 0 token — No token should circulate completely around the ring at a constant nonzero priority level — If the active monitor detects another active monitor, it immediately goes into standby status. Dr. L. Christofi 54

Dedicated Token Ring (DTR) • A Ring can be configured in a star topology by use of a hub, or concentrator. • The token-passing algorithm can be used so that the ring capacity is still shared and access control is determined by the token. • It is also possible to have the central hub function as a switch with the connection between each station as a fullduplex point-to-point link. • The DTR specification defines the use of stations and concentrators in the switched mode. • The DTR concentrator acts as a frame-level relay rather than a bit-level repeater: each link from concentrator to station is a dedicated full-duplex link with immediate access possible (token passing is not used). Dr. L. Christofi 55

Example DTR configuration (1) Dr. L. Christofi 56

Example DTR configuration (2) • A DTR switch consists of C-ports and a data transfer unit (DTU). A C-port may operate in 2 modes: — Transmit immediate protocol (TXI) • Full duplex mode of transmission • C-port or device does not need the capture of a token to transmit frames and can transmit frames at any time — Token-passing protocol (TPP) • Classic IEEE 802. 5 protocol, that allows integration of classic tokenpassing stations and ring-concentrators into a DTR configuration • C-port behaves as if it is part of a ring and passes the token accordingly • The DTU is the switching mechanism that connects the Cports within a DTR switch Dr. L. Christofi 57

Token ring trade-offs • Advantage — Flexible control over access that it provides • Disadvantage — The requirement for token maintenance • Loss of token prevents further utilization of the ring • Duplication of the token cal also disrupt ring operation • One station must be selected as monitor to ensure that exactly one token is on the ring and to reinsert a free token if necessary Dr. L. Christofi 58

IEEE 802. 5 Token Ring Physical layer • The standard imposes a maximum frame size of — 4. 550 bytes at 4 Mbps, and — 18. 200 bytes for 16 Mbps and 100 Mbps — This compares with a maximum frame size of 1518 bytes for IEEE 802. 3 LANs • At 4 Mbps and 16 Mbps, token-passing access control or switched DTR technique can be used • At 100 Mbps, the DTR technique is mandatory. Dr. L. Christofi 59

IEEE 802. 5 physical layer medium alternatives Dr. L. Christofi 60

3. FDDI (Fiber Distributed Data Interface) • • • This is a 100 Mbps token Ring LAN/MAN specification Uses Optical Fiber cabling High reliability (dual rings) l Distances of up to 200 km Immune to interference l Up to 1000 hosts attached Standardized by ANSI Dr. L. Christofi 61

Fiber Distributed Data Interface (FDDI) Standards • ANSI ASC X 3 T 9. 5 Standard Committee • ISO 9314 Standard Series • FDDI serves both LAN and MAN • FDDI – a token ring scheme, similar to the IEEE 802. 5 specification • FDDI was designed to accommodate better the rate of 100 Mbps • Some differences are at the MAC layer and at the physical layer. Dr. L. Christofi 62

FDDI operation • FDDI protocols are closely modelled to the 802. 5 protocols. – To transmit data, a station must first capture the token. – Then it transmits a frame and removes it when it comes around again. • The difference between FDDI and 802. 5 is that in 802. 5 a station may not generate a new token until its frame has gone all the way around and come back. – With FDDI, the amount of time wasted waiting for the frame is substantial due to the large distances and large number of station involved. For this reason, a station is allowed to put a new token back onto the ring as soon as it has finished transmitting its frames. Dr. L. Christofi 63

FDDI MAC frame format Similar to Token Ring Dr. L. Christofi 64

FDDI MAC Frame • Preamble: synchronizes the frame with each station’s clock • Starting Delimiter (SD): Indicates the start of frame • Frame Control (FC): Indicates if this is a synchronous or asynchronous frame • Destination address (DA): specifies the station(s) for which the frame is intended • Source address (SA): specifies the station that sent the frame • Information: Contains LLC data unit or control info • Frame Check Sequence (FCS): 4 -byte CRC • End Delimiter (ED): marks the end of the frame • Frame Status (FS): Contains the error detected (E), address recognized (A) and frame copied (F) indicators Dr. L. Christofi 65

FDDI token frame • Preamble: synchronizes the frame with each station’s clock • Starting Delimiter (SD): Indicates the start of frame • Frame Control (FC): has the bit pattern 1000 0000 or 1100 0000 to indicate that this is a token • End Delimiter (ED): marks the end of the token frame Dr. L. Christofi 66

Comparison of FDDI and IEEE 802. 5 Token Ring (1) Dr. L. Christofi 67

Comparison of FDDI and IEEE 802. 5 Token Ring (2) • Quite similar • The FDDI frame includes a preamble to aid in clocking, which is more demanding at the higher data rate • Both 2 -byte and 4 -byte addresses are allowed in the same network with FDDI — More flexible than the scheme used on all 802 standards • Some differences in control bits — FDDI does not include priority and reservation bits — Capacity allocation is handled in a different way Dr. L. Christofi 68

FDDI MAC Protocol • Fundamentally, the same as IEEE 802. 5 Token Ring MAC Protocol but with two differences: • First Difference — In FDDI, a station waiting for a token seizes the token by aborting (failing to repeat) the token transmission as soon as the token frame is recognized. The station begins transmitting one or more data frames. No flipping a bit to convert a token [high data rate]. • Second Difference — A station that has been transmitting data frames releases a new token as soon as it completes data frame transmission, even if it has not begun to receive its own transmission. Dr. L. Christofi 69

FDDI Token Ring Operation (1) Dr. L. Christofi 70

FDDI Token Ring Operation (2) • With reference to the previous slide: — After station A has seized the token, it transmits frame F 1 and immediately transmits a new token. — F 1 is addressed to station C, which occupies it as it circulates past. — The frame eventually returns to A, which is absorbed. — Meanwhile, B seizes the token issued by A and transmits F 2 followed by a token — This action could be repeated any number of times, so that at any one time there may be multiple frames circulating the ring — Each station is responsible for absorbing its own frames based on the source address field Dr. L. Christofi 71

FDDI Dual Ring Operation (a) FDDI consists of 2 counterrotating rings. Dr. L. Christofi (b) In the event of failure of both rings at one point, the two rings can be joined together to form a single long ring 72

FDDI Physical Layer specifications • The FDDI standard specifies a ring topology operation at 100 Mbps Dr. L. Christofi 73

4. Wireless LANs • Wireless LANs dispense with cables and use radio or infrared frequencies to transmit signals through the air. • WLANs are growing in popularity because they eliminate cabling and facilitate network access from a variety of locations and for mobile workers. • The most common wireless networking standard is IEEE 802. 11, often called Wireless Ethernet or Wireless LAN. Dr. L. Christofi 74

Wireless Ethernet (IEEE 802. 11) Key Points • The principal technologies used for wireless LANS are — infrared — spread spectrum — narrowband microwave. • The IEEE 802. 11 standard defines a set of services and physical layer options for wireless LANs. • The IEEE 802. 11 services include managing associations, delivering data, and security. • The IEEE 802. 11 physical layer includes infrared and spread spectrum and covers a range of data rates. Dr. L. Christofi 75

Technology • Infrared (IR) LANs: Individual cell of IR LAN limited to single room. — IR light does not penetrate opaque walls. • Spread spectrum LANs: Mostly operate in ISM (industrial, scientific, and medical) bands. — No Federal Communications Commission (FCC) licensing is required in USA. • Narrowband microwave: Microwave frequencies but not use spread spectrum — Some require FCC licensing Dr. L. Christofi 76

Infrared Wireless LANs • Less flexible than IEEE 802. 11 WLANs because, as with TV remote controls that are also infrared based, they require line of sight to work. • Infrared Hubs and NICs are usually mounted in fixed positions to ensure they will hit their targets. • The main advantage of infrared WLANs is reduced wiring. • A new version, called diffuse infrared, operates without a direct line of sight by bouncing the infrared signal off of walls, but is only able to operate within a single room and at distances of only about 50 -75 feet. Dr. L. Christofi 77

Terminology • Access Point (AP): Provides access to the distribution system via the wireless medium. • Basic Service Set (BSS): A set of stations controlled by a single coordination function. • Coordination function: Determines when a station is operating. • Distribution System (DS): Interconnection between BSS and LANs to form an ESS. • Extended Service Set (ESS): A set of interconnected BSSs and LANs. • MPDU / MSDU: MAC Protocol/Service Data unit. Dr. L. Christofi 78

Wireless LAN Topology • WLAN topologies are the same as on Ethernet: physical star, logical bus. • Wireless LAN devices use the same radio frequencies, so they must take turns using the network. • Instead of hubs, WLANs use devices called access points (AP). Maximum transmission range is about 100 -500 feet. Usually a set of APs are installed making wireless access possible in several areas in a building or corporate campus. • Each WLAN computer uses an NIC that transmits radio signals to the AP. • Because of the ease of access, security is a potential problem, so IEEE 802. 11 uses 40 -bit data encryption to prevent eavesdropping. Dr. L. Christofi 79

Access Point Dr. L. Christofi 80

Wireless LAN Applications (1) • (1) LAN Extension: — Saves installation of LAN cabling. — Eases relocation and other modifications to network structure. • However, increasing reliance on twisted pair cabling for LANs as newer buildings are prewired with Cat 5. • Wireless LAN to replace wired LANs has not happened. • Buildings with large open areas: role for the wireless LAN (Airports, Manufacturing plants, stock exchange trading floors, warehouses). Dr. L. Christofi 81

Single Cell Wireless LAN Dr. L. Christofi 82

Multi-Cell Wireless LAN Dr. L. Christofi 83

Wireless LAN Applications (2) • (2) Cross-Building interconnect: — Connect LANs in nearby buildings. — Point-to-point wireless link. — Connect bridges or routers. • (3) Nomadic Access: — Link between LAN hub and mobile data terminal like a Laptop or notepad computer. — Also useful in extended environment such as campus or cluster of buildings. — Users move around with portable computers. — May wish access to servers on wired LAN. Dr. L. Christofi 84

Infrastructure Wireless LAN Dr. L. Christofi 85

Wireless LAN Applications (3) • (4) Ad Hoc Networking (=special purpose networking) — Point-to-point wireless link, Peer-to-peer network. — Set up temporarily to meet some immediate need. — E. g. group of employees, each with laptop or palmtop, in business or classroom meeting. — Network for duration of meeting. Dr. L. Christofi 86

Ad Hoc Wireless LAN Dr. L. Christofi 87

Wireless LAN Requirements (1) • Throughput: MAC should make efficient use of the wireless medium to maximise capacity. • Number of nodes: Support hundreds of nodes. • Connection to backbone LAN: Use control modules to connect to both types of LANs. • Service area: Diameter of 100 to 300 m • Battery power consumption: Wireless LAN have features to reduce power consumption. Dr. L. Christofi 88

Wireless LAN Requirements (2) • Transmission robustness and security: The design must permit reliable transmission in noisy environment and protection from eavesdropping. • Collocated network operation: Two or more wireless LANs in same area. • License-free operation. • Handoff/Roaming: Mobile stations can move. • Dynamic configuration: Addition, deletion, and relocation of end systems without disruption to users. Dr. L. Christofi 89

Advantages / Disadvantages • • • Extremely high data rates. Use ceiling reflection to cover entire room. Does not penetrate walls or other opaque objects. More easily secured against eavesdropping than microwave. Inexpensive and simple • • Background radiation Sunlight, indoor lighting Noise, requiring higher power and limiting range Power limited by concerns of eye safety and power consumption. Dr. L. Christofi 90

MAC Frame • • • Dr. L. Christofi Frame Control: Control management or data. Duration/Connection ID: Time the channel is allocated in μs for a transmission of a frame. Addresses: Source, destination, transmitting station and receiving station. Sequence control: Contains data fragmentation and reassembly and the number of frames. Frame body: Contains an MSDU. Frame Check Sequence: CRC. 91

IEEE 802. 11 Services Service Provider Category Association Distribution system MSDU delivery Authentication Station LAN access and security Deauthentication Station LAN access and security Dissassociation Distribution system MSDU delivery Integration Distribution system MSDU delivery Station MSDU delivery Privacy Station LAN access and security Reassocation Distribution system MSDU delivery Dr. L. Christofi 92

IEEE 802. 11 xx standards • IEEE 802. 11 a: 5 GHz, Orthogonal FDM, data rates of 6 -54 Mbps. • IEEE 802. 11 b: 5. 5 Mbps and 11 Mbps. • IEEE 802. 11 g: Higher speed extension of 802. 11 b 20 Mbps. • IEEE 802. 16: Wi-Fi – Starbucks Wi-Fi spot 100 Mbps. • IEEE 802. 21 Wi. Max, over 100 Mbps, 50 km range, broadband speeds. Dr. L. Christofi 93

Review Questions • List and briefly define the four application areas of wireless LANs. • List and briefly define the key requirements for wireless LANs. • What are some key advantages and disadvantages of wireless LANs. • List and define some of the IEEE 802. 11 services. • List and describe the IEEE 802. 11 xx family. Dr. L. Christofi 94

References • W. Stalling, Local and Metropolitan Area Networks, 6 th edition, Prentice Hall, 2000 • B. A. Forouzan, Data Communications and Networking, 3 rd edition, Mc. Graw-Hill, 2004 • F. Halsall, Data Communications, Computer Networks and Open Systems, 4 th edition, Addison Wesley, 1995 • W. Stallings, Data and Computer Communications, 7 th edition, Prentice Hall, 2004 Dr. L. Christofi 95