Komunikasi Data Dan Jaringan Komputer

Komunikasi Data dan Jaringan Komputer (Bagian 3)

Dr. Tb. Maulana Kusuma [email protected] http://staffsite.gunadarma.ac.id/mkusuma

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LAN Generation First  Carrier Sense Multiple Access (CSMA) / Collision Detection (CD) and Token Ring  Terminal to host and client server  Moderate data rates Second  Fiber Distributed Data Interface (FDDI)  Backbone  High performance workstations Third  Asynchronous Transfer Mode (ATM)  Aggregate throughput and real time support for multimedia applications Magister Teknik Elektro

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Third Generation LAN Support for multiple guaranteed classes of service  

Live video may need 2Mbps File transfer can use background class

Scalable throughput 

Both aggregate and per host

Facilitate LAN / WAN internetworking

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LAN Applications (1) Personal computer LANs  Low cost  Limited data rate Back end networks and storage area networks  Interconnecting large systems (mainframes and large storage devices) High data rate High speed interface Distributed access Limited distance Limited number of devices Magister Teknik Elektro

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LAN Applications (2) High speed office networks  

Desktop image processing High capacity local storage

Backbone LANs    

Interconnect low speed local LANs Reliability Capacity Cost Magister Teknik Elektro

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LAN Architecture Protocol architecture Topologies Media Access Control (MAC) Logical Link Control (LLC)

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Protocol Architecture Lower layers of OSI model IEEE 802 reference model Physical LLC MAC

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IEEE 802 v OSI

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802 Layers Physical Encoding/decoding Preamble generation/removal Bit transmission/reception Transmission medium and topology

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802 Layers Logical Link Control Interface to higher levels Flow and error control

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802 Layers Media Access Control Assembly of data into frame with address and error detection fields Disassembly of frame  Address recognition  Error detection Govern access to transmission medium  Not found in traditional layer 2 data link control For the same LLC, several MAC options may be available Magister Teknik Elektro

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LAN Protocols in Context

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Topologies Tree Bus 

Special case of tree One trunk, no branches

Ring Star

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LAN Topologies

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Bus and Tree Multipoint medium Transmission propagates throughout medium Heard by all stations  Need to identify target station Each station has unique address Full duplex connection between station and tap  Allows for transmission and reception Need to regulate transmission  To avoid collisions  To avoid hogging Data in small blocks - frames Terminator absorbs frames at end of medium Magister Teknik Elektro

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Frame Transmission - Bus LAN

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Ring Topology Repeaters joined by point to point links in closed loop  Receive data on one link and retransmit on another  Links unidirectional  Stations attach to repeaters Data in frames  Circulate past all stations  Destination recognizes address and copies frame  Frame circulates back to source where it is removed Media access control determines when station can insert frame Magister Teknik Elektro

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Frame Transmission Ring LAN

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Star Topology Each station connected directly to central node 

Usually via two point to point links

Central node can broadcast  

Physical star, logical bus Only one station can transmit at a time

Central node can act as frame switch

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Media Access Control Where  Central Greater control Simple access logic at station Avoids problems of co-ordination Single point of failure Potential bottleneck  Distributed How  Synchronous Specific capacity dedicated to connection  Asynchronous In response to demand Magister Teknik Elektro

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Asynchronous Systems Round robin  Good if many stations have data to transmit over extended period Reservation  Good for stream traffic Contention  Good for bursty traffic  All stations contend for time  Distributed  Simple to implement  Efficient under moderate load  Tend to collapse under heavy load Magister Teknik Elektro

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Logical Link Control Transmission of link level PDUs between two stations Must support multiaccess, shared medium Relieved of some link access details by MAC layer Addressing involves specifying source and destination LLC users  Referred to as service access points (SAP)  Typically higher level protocol Magister Teknik Elektro

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Bus LAN Signal balancing  Signal must be strong enough to meet receiver’s minimum signal strength requirements  Give adequate signal to noise ration  Not so strong that it overloads transmitter  Must satisfy these for all combinations of sending and receiving station on bus  Usual to divide network into small segments  Link segments with amplifies or repeaters Magister Teknik Elektro

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Transmission Media Twisted pair  Not practical in shared bus at higher data rates Baseband coaxial cable  Used by Ethernet Broadband coaxial cable  Included in 802.3 specification but no longer made Optical fiber  Expensive  Difficulty with availability  Not used Few new installations  Replaced by star based twisted pair and optical fiber Magister Teknik Elektro

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Baseband Coaxial Cable Uses digital signaling Manchester or Differential Manchester encoding Entire frequency spectrum of cable used Single channel on cable Bi-directional Few kilometer range Ethernet (basis for 802.3) at 10Mbps 50 ohm cable Magister Teknik Elektro

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10Base5 Ethernet and 802.3 originally used 0.4 inch diameter cable at 10Mbps Max cable length 500m Distance between taps a multiple of 2.5m 

Ensures that reflections from taps do not add in phase

Max 100 taps 10Base5 Magister Teknik Elektro

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10Base2 Cheaper 0.25 inch cable       

More flexible Easier to bring to workstation Cheaper electronics Greater attenuation Lower noise resistance Fewer taps (30) Shorter distance (185m) Magister Teknik Elektro

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Repeaters Transmits in both directions Joins two segments of cable No buffering No logical isolation of segments If two stations on different segments send at the same time, packets will collide Only one path of segments and repeaters between any two stations Magister Teknik Elektro

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Baseband Configuration

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Ring LAN Each repeater connects to two others via unidirectional transmission links Single closed path Data transferred bit by bit from one repeater to the next Repeater regenerates and retransmits each bit Repeater performs data insertion, data reception, data removal Repeater acts as attachment point Packet removed by transmitter after one trip round ring Magister Teknik Elektro

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Ring Repeater States

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Listen State Functions Scan passing bit stream for patterns  

Address of attached station Token permission to transmit

Copy incoming bit and send to attached station 

Whilst forwarding each bit

Modify bit as it passes 

e.g. to indicate a packet has been copied (ACK) Magister Teknik Elektro

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Transmit State Functions Station has data Repeater has permission May receive incoming bits 

If ring bit length shorter than packet Pass back to station for checking (ACK)



May be more than one packet on ring Buffer for retransmission later

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Bypass State Signals propagate past repeater with no delay (other than propagation delay) Partial solution to reliability problem (see later) Improved performance

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Star LAN Use unshielded twisted pair wire (telephone)  Minimal installation cost May already be an installed base All locations in building covered by existing installation Attach to a central active hub Two links  Transmit and receive Hub repeats incoming signal on all outgoing lines Link lengths limited to about 100m  Fiber optic - up to 500m Logical bus - with collisions Magister Teknik Elektro

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Hubs and Switches Shared medium hub  Central hub  Hub retransmits incoming signal to all outgoing lines  Only one station can transmit at a time  With a 10Mbps LAN, total capacity is 10Mbps Switched LAN hub  Hub acts as switch  Incoming frame switches to appropriate outgoing line  Unused lines can also be used to switch other traffic  With two pairs of lines in use, overall capacity is now 20Mbps Magister Teknik Elektro

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Switched Hubs No change to software or hardware of devices Each device has dedicated capacity Scales well Store and forward switch  Accept input, buffer it briefly, then output Cut through switch  Take advantage of the destination address being at the start of the frame  Begin repeating incoming frame onto output line as soon as address recognized  May propagate some bad frames Magister Teknik Elektro

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Hubs and Switches

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Wireless LAN Wireless LANs are growing in popularity because they eliminate cabling and facilitate network access from a variety of locations. The most common wireless networking standard is IEEE 802.11, often called Wireless Ethernet or Wireless LAN. Broadband wireless (IEEE 802.16) is now growing in popularity Magister Teknik Elektro

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Wireless Communications In wireless communications signals travel through space instead of through a physical cable. Two general types of wireless communications are:  

Radio transmission Infrared transmission

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Why Wireless LAN? Mobility Flexibility Hard to wire areas Reduced cost of wireless systems Improved performance of wireless systems

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Types of Wireless LANs IEEE 802.11a IEEE 802.11b IEEE 802.11g Infrared Bluetooth

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IEEE 802.11b Two forms of the IEEE 802.11b standard currently exist: Direct Sequence Spread Spectrum (DSSS) systems transmit signals through a wide range of frequencies simultaneously. The signal is divided into many different parts and sent on different frequencies simultaneously. Data rate: ~ 11Mbps. Frequency Hopping Spread Spectrum (FHSS) divides the frequency band into a series of channels and then use each frequency in turn. FHSS changes its frequency channel about every half a second, using a pseudorandom sequence. Magister Teknik Elektro

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FHSS is more secure, but is only capable of data rates of 1 or 2 Mbps. IEEE 802.11a is another Wireless LAN standard developed around the same time as 802.11b. It operates in the 5 GHz band and is capable of data rates of up to 54 Mbps. IEEE 802.11g combines the best of both 802.11a and 802.11b. 802.11g supports bandwidth up to 54 Mbps, and it uses the 2.4 Ghz frequency for greater range. 802.11g is backwards compatible with 802.11b, meaning that 802.11g access points will work with 802.11b wireless network adapters and vice versa. Magister Teknik Elektro

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IEEE 802.11a vs 802.11b vs 802.11g 802.11a is the most expensive. It fits predominately in the business market, whereas 802.11b better serves the home market. 802.11a supports bandwidth up to 54 Mbps and signals in a regulated 5 GHz range. Compared to 802.11b, this higher frequency limits the range of 802.11a. The higher frequency also means 802.11a signals have more difficulty penetrating walls and other obstructions.

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Although slower than 802.11a, the range of 802.11b is about 7 times greater than that of 802.11a. Because 802.11a and 802.11b utilize different frequencies, the two technologies are incompatible with each other. Some vendors offer hybrid 802.11a/b network gear, but these products simply implement the two standards side by side. 802.11g offers the best of both worlds and allow for greater flexibility.

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IEEE LAN Standard

Pros

Cons

802.11a

fastest maximum speed; highest cost; •shorter range signal that is supports more simultaneous users; more easily obstructed less signal interference from other devices

802.11b

lowest cost; signal range is best and is not easily obstructed

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slowest maximum speed; supports fewer simultaneous users; appliances may interfere on the unregulated frequency band 47

IEEE Lan Standard 802.11g

Pros fastest maximum speed; supports more simultaneous users; signal range is best and is not easily obstructed Magister Teknik Elektro

Cons costs more than 802.11b; appliances may interfere on the unregulated signal frequency

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Wireless LAN Applications LAN Extension Cross building interconnection Nomadic access Ad hoc networks

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LAN Extension Buildings with large open areas  

Manufacturing plants Warehouses

Historical buildings Small offices May be mixed with fixed wiring system

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Single Cell Wireless LAN

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Multi Cell Wireless LAN

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Cross Building Interconnection Point to point wireless link between buildings Typically connecting bridges or routers Used where cable connection not possible 

e.g. across a street

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Nomadic Access Mobile data terminal 

e.g. laptop

Transfer of data from laptop to server Campus or cluster of buildings

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Ad Hoc Networking Peer to peer Temporary e.g. conference

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Wireless LAN Configurations

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Wireless LAN Requirements Throughput Number of nodes Connection to backbone Service area Battery power consumption Transmission robustness and security Collocated network operation License free operation Hand-off / roaming Dynamic configuration Magister Teknik Elektro

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Wireless LAN Technology Infrared (IR) LANs Spread spectrum LANs Narrow band microwave

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Wireless LAN standard – IEEE 802.11 The IEEE 802.11 standard for wireless LANs was finalized in 1997. The standard defines three different physical layer specifications - 2 are radio frequency-based and one is infraredbased:   

Direct Sequence Spread Spectrum Frequency-hopping spectrum Infrared Magister Teknik Elektro

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Wireless LAN Components The smallest building block of a wireless LAN is called the Basic Service Set (BSS). BSS is a number of stations executing the same MAC protocol and using the same shared medium. A BSS may be isolated or connected to a backbone distribution system via an access point. The distribution system is usually a wired backbone LAN. Magister Teknik Elektro

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Wireless LAN Components (cont’d)

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Wireless LAN Components (cont’d) Signals from wireless computers are transmitted via built-in antennas on the NIC to the nearest access point, which serves as a wireless repeater. Because of the ease of access, security is a potential problem. The IEEE 802.11 has specified a data link security protocol called Wired Equivalent Privacy (WEP), which is designed to make the security of WLAN as good as that of wired LANs. Magister Teknik Elektro

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Medium Access Control The MAC protocol used in 802.11 LANs is called Distributed Foundation Wireless MAC (DFWMAC). This protocol provides a distributed access control mechanism with an optional centralized control built in. A distributed access mechanism distributes the decision to transmit over all the nodes, using a carrier sense mechanism, like CSMA/CD. Magister Teknik Elektro

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Medium Access Control (cont’d) A centralized access mechanism involve regulation of transmission by a centralized manager. It is particularly useful for timesensitive or high priority data. The MAC layer is divided into 2 sub-layers: The lower layer is called the Distributed Coordination Function (DCF), operates similar to CSMA/CD. Used for ordinary traffic.

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Medium Access Control (cont’d) The upper layer is called the Point Coordination Function (PCF). PCF is a centralized MAC algorithm used for contention-free service. All implementations must support DCF, but PCF is optional. When DCF is employed, 802.11 uses a protocol called CSMA/CD to regulate access to the medium. Magister Teknik Elektro

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Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA ) Wireless LANs use CSMA/CA . Like CSMA/CD, stations listen before they transmit and if the line is free, they transmit. Detecting collisions is more difficult in wireless networks, so wireless LANs try to avoid collisions to a greater extent than traditional Ethernet. Two different WLAN MAC techniques are now in use: the Physical Carrier Sense Method and the Virtual Carrier Sense Method. Magister Teknik Elektro

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Physical Carrier Sense Method In the physical carrier sense method, a node that wants to send first listens to make sure that the transmitting node has finished, then waits a period of time longer. Each frame is sent using the Stop and Wait ARQ, so by waiting, the listening node can detect that the sending node has finished and can then begin sending its transmission. With Wireless LAN, ACK / NAK signals are sent a short time after a frame is received, while stations wishing to send a frame wait a somewhat longer time, ensuring that no collision will occur. Magister Teknik Elektro

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Virtual Carrier Sense Method When a computer on a Wireless LAN is near the transmission limits of the AP at one end and another computer is near the transmission limits at the other end of the AP’s range, both computers may be able to transmit to the AP, but can not detect each other’s signals. This is known as the hidden node problem. When it occurs, the physical carrier sense method will not work. Magister Teknik Elektro

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Virtual Carrier Sense Method (cont’d) The virtual carrier sense method solves this problem by having a transmitting station first send a request to send (RTS) signal to the AP. If the AP responds with a clear to send (CTS) signal, the computer wishing to send a frame can then begin transmitting.

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Infrared Wireless LAN Infrared WLAN is 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 WLAN 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.

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Infrared Wireless LAN (cont’d)

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Bluetooth Bluetooth is a 1 Mbps wireless standard developed for piconets, small personal or home networks. It may soon be standardized as IEEE 802.15. Bluetooth is designed to facilitate networking of different hand-held and mobile devices. For example:  linking a wireless mouse to a computer, a telephone headset to a base unit, or a Palm handheld computer to your car to lock or unlock the door.  3-in-1 phone concept  automatic synchronizer : automatically synchronizes a user’s desktop PC, mobile PC and mobile phone. Magister Teknik Elektro

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Bluetooth was designed to operate within a very small area (up to 30 feet). May be extended. Devices are small and cheap. A Bluetooth network consists of no more than eight devices, but can be linked to other piconets to from larger networks. Although Bluetooth uses the same 2.4 GHz band as Wireless LANs, it is not compatible with the IEEE 802.11 standard and so cannot be used in locations that use the Wireless LANs.

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Ethernet (CSMA/CD) Carriers Sense Multiple Access with Collision Detection Xerox - Ethernet IEEE 802.3

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IEEE802.3 Medium Access Control Random Access 

Stations access medium randomly

Contention 

Stations content for time on medium

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CSMA Propagation time is much less than transmission time All stations know that a transmission has started almost immediately First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time (round trip plus ACK contention) No ACK then retransmit Max utilization depends on propagation time (medium length) and frame length  Longer frame and shorter propagation gives better utilization Magister Teknik Elektro

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If Busy? If medium is idle, transmit If busy, listen for idle then transmit immediately If two stations are waiting, collision

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CSMA/CD With CSMA, collision occupies medium for duration of transmission Stations listen whilst transmitting If medium idle, transmit If busy, listen for idle, then transmit If collision detected, jam then cease transmission After jam, wait random time then start again  Binary exponential back off Magister Teknik Elektro

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CSMA/CD Operation

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Collision Detection On baseband bus, collision produces much higher signal voltage than signal Collision detected if cable signal greater than single station signal Signal attenuated over distance Limit distance to 500m (10Base5) or 200m (10Base2) For twisted pair (star-topology) activity on more than one port is collision Special collision presence signal Magister Teknik Elektro

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Gigabit Ethernet Configuration

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Gigabit Ethernet - Differences Carrier extension At least 4096 bit-times long (512 for 10/100) Frame bursting

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Gigabit Ethernet - Physical 1000Base-SX 

Short wavelength, multimode fiber

1000Base-LX 

Long wavelength, Multi or single mode fiber

1000Base-CX 

Copper jumpers <25m, shielded twisted pair

1000Base-T 

4 pairs, cat 5 UTP

Signaling - 8B/10B Magister Teknik Elektro

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Token Ring (802.5) MAC protocol  Small frame (token) circulates when idle  Station waits for token  Changes one bit in token to make it SOF for data frame  Append rest of data frame  Frame makes round trip and is absorbed by transmitting station  Station then inserts new token when transmission has finished and leading edge of returning frame arrives  Under light loads, some inefficiency  Under heavy loads, round robin Magister Teknik Elektro

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Token Ring Operation

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Priority Scheme

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Dedicated Token Ring Central hub Acts as switch Full duplex point to point link Concentrator acts as frame level repeater No token passing

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FDDI 100Mbps LAN and MAN applications Token Ring

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FDDI Operation

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ATM LANs Asynchronous Transfer Mode Virtual paths and virtual channels Preconfigured or switched Gateway to ATM WAN Backbone ATM switch  Single ATM switch or local network of ATM switches Workgroup ATM  End systems connected directly to ATM switch Mixed system Magister Teknik Elektro

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Example ATM LAN

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ATM LAN HUB

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Compatibility Interaction between end system on ATM and end system on legacy LAN Interaction between stations on legacy LANs of same type Interaction between stations on legacy LANs of different types

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