Objectives • Introduce LAN • Study OSI model • Look at LAN Media • Investigate LAN Architecture and Components • Study standard LAN Architectures
What is a Local Area Network? • LAN is a combination of hardware & software tech. that allows computers to share a variety of resources e.g. printers, storage devices, Data, Applications, etc. • It allows messages to be sent between attached computers Enable users to work together electronically = “Collaborative computing”
What is a Local Area Network? • Generally, LANs are confined to an area no larger than a single building or a small group of buildings • It can be extended by connecting to other similar or dissimilar LANs, to remote users, or to mainframes computers = “LAN Connectivity” or “Internetworking” • Can be connected to other LANs of trading partners = “Enterprise Networking” • The computers themselves are not part of the LAN !!!
Categorizing LAN Architecture:OSI Model • Consists of 7 layers that loosely group the functional requirements for communication between two computing devices. • Each layer relies on lower layers to perform more elementary functions and to offer total transparency to the intricacies of those functions. At the same time, each layer provides the same transparent service to upper layers.
OSI Model • Physical Layer: responsible for the establishment, maintenance, & termination of physical connection between communicating devices “Point-to-Point data link”. • Data-Link Layer: responsible for the providing protocols that deliver reliability to upper layers for Point-to-Point connections established by physical layer protocols. To allow the OSI model to closely adhere to the protocol structure, & operation of a LAN, Data-Link layer was splitted into two sublayers.
Data-Link Sublayers • Media Access Control (MAC): interfaces with the physical layer & is represented by protocols that define how the shared LAN media is to be accessed by the many connected computers. • Logical Link Control (LLC): interfaces to the network layer. • The advantage of splitting the Data-Link layer & of having a single common LLC protocol is that it offers transparency to the upper layers while allowing the MAC sublayers protocols to vary independently.
OSI Model cont’d • Network Layer: responsible for the establishment, maintenance, & termination of end-to-end network links. Network layer protocols are required when computers that aren’t physically connected to the same LAN must communicate. • Transport Layer: responsible for providing reliability for the end-to-end network layer connections. It provide end-to-end recovery & flow control. It also, provide mechanisms for sequentially organizing network layer packets into a coherent message.
OSI Model cont’d • Session Layer: responsible for establishing, maintaining, & terminating sessions between user application programs. • Presentation Layer: provide an interface between user applications & various presentation-related services required by those applications. An example is data encryption/decryption protocols. • Application Layer: it includes utilities that support end-user application programs but it does not include end-user application programs.
Encapsulation/De-encapsulation • Encapsulation: in this process, each successive layer of the OSI model adds a header according to the syntax of the protocol that occupies that layer. • De-encapsulation: in this process, each successive layer of the OSI model removes headers &/or trailers & processes the data that was passed to it from the corresponding layer protocol on the source client. • These two processes describe how the various protocol layers interact with each other to enable an end-to-end communications session.
LAN Media • Not Twisted Pair • Unshielded Twisted Pair (UTP) • Shielded Twisted Pair (STP) • Coaxial Cable (Coax) • Fiber Optic
Not Twisted Pair • Phone wire • RYBG • Flat Gray Modular Wiring • 4, 6 and 8 wires
Unshielded Twisted Pair • No Shielding • EIA Cat (1 – 5) • AWG • Attenuation: Loss of signal volume and power over a long distance • NeXT: a strong signal overpowering a weaker signal on an adjacent pair
Shielded Twisted Pair • Shielding is metallic foil or copper braid • Shielded from EMI and RFI
Coaxial Cable (coax) • Reliable High speed data transmission over relatively long distance • Used in Ethernet and comes in different thickness
Fiber Optic • Untappable and Immune to EMI and RFI • Glass Vs. Plastic • Multimode Step Index: 200Mbps < 1Km • Multimode Graded Index: 3Gbps several Kms • Single mode: light rays are more focused only one wavelength can pass at a time. (most expensive)
How is a LAN Implemented ? • Appropriate networking hardware & software must be added to every computer or shared peripheral device that is to communicate via the LAN. • Some type of network media must physically connect the various networked computers and peripheral devices to converse with each other.
The LAN Architecture Model • All network architecture are made up of the same logical components. • To accurately describe a given network architecture, one needs to know the following: • Access methodology. • Logical topology. • Physical topology
Access Methodology • Since many users is to send requests onto the shared LAN media at the same time, there must be some way to control access by multiple users to that media. These media-sharing methods are named “Access methodologies”. • Sharing the media is an important concept in LANs, which are sometimes called “media-sharing LANs”. • There is two access controlling methods: 1- CSMA/CD 2-Token Passing
CSMA/CD • It’s based on the philosophy: “Let’s just let everyone onto the media whenever they want & if two users access the media at the same second, we’ll work it out somehow.” • Carrier sense multiple access with collision detection • Carrier sense:the PC wishing to put data onto the shared media listens to the network to see if any other users are “on line” by trying to sense a neutral electrical signal known as the carrier. • If no transmission is detected, multiple access allows anyone onto the media.
CSMA/CD • If two user PCs should access the same media in the same time, a collision occurs & collision detectionlets the user PCs to know that their data wasn’t delivered & controls retransmission is such a way to avoid collisions. • Another factor of collisions is propagation delaying, which is the time it takes to a signal from a source PC to reach a destination PC. • Because of this delay, it’s possible for a workstation to sense if there is no signal on the shared media, when in fact another distant workstation has transmitted a signal that hasn’t yet reached the carrier sensing PC.
Token Passing • “Don’t you dare access the media until it’s your turn. You must first ask permission, & only if I give you the magic token may you put your data on the shared media”. • It ensures that each PC user has 100% of the network channel available for data requests & transfers by insisting that no PC accesses the network without processing a specific packet of data (Token). • The token is first generated by a specified PC known as active monitorand passed among PCs until one PC would like to access the network.
Token Passing • The requesting PC seizes the token, changes the token status from free to busy, puts its data frame onto the network, & doesn’t release the token until it’s assured that its data was delivered. • Successful data delivery is confirmed by the destination workstation setting frame status flagsto indicate a successful receipt of the frame. • Upon receipt of the original frame with frame status flag set to “destination address recognized, frame copied successfully” the sending PC rests the token status from busy to free & release it. • The token is passed along the next PC.
Logical Topology • After the data message ha reached the shared-media LAN, the next step is to determine how that message will be passed from workstation to workstation until the message reaches its intended destination. • This passing technology is known as “Logical Topology”. • There are two known logical topologies: 1- Sequential 2- Broadcast
Sequential Topology • Also known as “ring logical topology”. • The data is passed from one PC (or node) to another. • Each node examines the destination address of the data packet to determine if this packet is meant for it • If the data was not meant to be delivered at this node, the data packet is passed along to another node in the logical ring.
Broadcast Topology • A data message is sent simultaneously to all nodes on the network. • Each node decides individually if the data message was directed toward it. If not, the message is ignored. • No need to pass the message to a neighboring node.
Physical Topology • The clients & servers must be physically connected to each other according to some configuration & be linked by the shared media of choice. • The physical layout configuration can have a significant impact on LAN performance & reliability. • There are three physical topologies: 1- Bus 2- Ring 3-Star
Bus Topology • A linear arrangement with terminators on either end & devices connected to the “Bus” via connectors &/or transceivers. • A break or loose connection anywhere along the entire bus will bring the whole network down.
Ring Topology • Each PC connected via a ring topology is actually an active part of the ring, passing data packets in a sequential pattern around the ring. • If one of the PCs dies or a network adapter card malfunctions, the “sequence” is broken, the token is lost, & the network is down !
Star Topology • It avoids the drawbacks of both Bus & Ring topologies by employing some type of central management device. This central device may called a Hub, a wiring center, a concentrator, a MAU (multistation access unit), a repeater, or a switching hub. • By isolating each PC or node on its own leg or segment of the network, any node failure only affects that leg. • If this central device goes down, the whole network goes down too.
NETWORK ARCHITECTURES • Classic Architectures: • Ethernet • Token Ring • FDDI • High Speed Architectures • Family of Fast Ethernet • 100BaseT • 100VG-AnyLAN • Gigabit Ethernet (1000BaseT) • 10 Gigabit Ethernet • HSTR (High Speed Token Ring) • Fibre Channel • iSCSI • LAN-Based ATM • Home Network Architectures: • HPNA. • Bluetooth and PAN • Wireless spread spectrum technologies.
Ethernet • Origins: – Invented by Robert Metcalfe (founder of 3Com CO.). – Although Ethernet &IEEE 802.3 are different standards. – Ethernet is used to refer to IEEE 802.3 compliant network. • Functionality: – Access methodology: CSMA/CD. – Logical topology: broadcast. – Physical topology: traditionally, bus; currently, star.
Ethernet Figure 5-8 Ethernet and IEEE 802.3 Standards
Ethernet • Media related Ethernet standards:
Token Ring • Origin: – Olaf Soderbulm in 1969. – IBM standardized it as IEEE 802.5. • Functionality: – Access methodology: token passing. – Logical topology: sequential. – Physical topology: before, ring; now, star.
Token Ring • Standards: – IEEE 802.5 no speed specification. – Operate at speed of 4 &16 Mbps. – 24-bit data packet – The starting delimiter field alert the token ring card installed in workstation that a frame is approaching. – receive access control field. – Workstation distinguish btw tokens &MAC sub layer frames. – If token bit =0 then frame represents free token. – If token bit =1 then frame represents busy token. – Routing info used with source routing bridges that link multiple token ring LANs (LAN-to-LAN). – Sequential logical topology =message passing form neighbor to neighbor. – Token ring architecture = logical ring, physical star.
Active Monitor • Removes Dead frames • Replace lost or damaged token • Responsible for master clock • Makes sure there is only one active monitor • Provide buffer for token in small networks
FDDI • Origins: – Fiber Distributed Date Interface. – 100 Mbps network architecture. – Specified 1984 by ANSI(X3T9.5). – No IEEE standard. –supports IEEE802.2 protocol. It is most popular. • Functionality: – Access methodology:Modified token passing. – Logical topology:Sequential. – Physical topology:Dual counter-rotating rings.
FDDI • Built-in reliability &Longer distance: – Support 100Mbps of bandwidth. – High degree of reliability &security. – Reliability comes from fiber +EMI +RFI +design of physical topology of FDDI. – EMI (Electromagnetic Interference). – RFI (Radio Frequency Interference). – FDDI physical topology compromised of two separate rings in which data moves simultaneously in opposite directions. – 1st ring: Primary data ring. – 2nd ring: Secondary or backup data ring used in failure of primary ring or an attached workstation.
FDDI Figure 5-13 FDDI Network Architecture and Technology
FDDI – Both rings attached to a single hub or a concentrator. – Distance: FDDI LAN cover 500 nodes at 2km apart. – If repeaters used every 2km media can stretch up to 200km. – Interoperate with IEEE 802.3 10-Mbps Ethernet. – Interoperation needs FDDI-to-Ethernet bridge. – Bridge can connect many Ethernets. – PCs, and mainframes etc. must be equipped with either FDDI NIC or external FDDI controllers if they wish to access the FDDI LAN.
FDDI – To cut down costs &benefit of 100 Mbps bandwidth managers only connect one of the 2 FDDI fiber rings. – This is known as SAS (Single Attachment Stations). – Else if both fiber rings connected it is called DAS (Dual Attachment Stations). – The heart of the FDDI LAN is the FDDI concentrator or hub. – The design of the hubs is modular with backbone connections to both FDDI rings. – The dual counter rotating rings network architecture of FDDI has a self-healing capabilities.
FDDI Figure 5-14 FDDI’s Self-Healing Ability
FDDI • Standards: Two ways of modification to the token passing access methodology. – FDDI removes the token from the ring &transmit a full data frame. If the transmition is complete it releases a new token. Collision is avoided as only one station can have the free token at a time, and a station cannot put a data message onto the Network without a token. – A Station can send more than one message per token.