Layered Communication

Building Blocks: Hardware

Layers in LAN

Multi-layer Network Models

The process of transferring a message between sender and receiver is more easily implemented by breaking it down into simpler components.Instead of a single layer, a group of layers are used, dividing up the tasks required for network communications.The best known network model is the OSI models.

The OSI Networking Reference Model

OSI= Open Systems Interconnect. Created by the International Standards Organization (ISO) in 1984 as a network standards framework.The model’s seven layers from high to low are:

7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

Application Layers

The application layers are the user’s connection to the network and include the application software and other software used to connect the application to the network: 7. Application: provides a set of utilities

used by application programs. 6. Presentation: formats data for

presentation to the user, provides data interfaces, data compression and translation between different data formats.

5. Session: responsible for initiating, maintaining and terminating each logical session between sender and receiver.

Internetwork Layers

The internetwork layers connect applications to the network and as well as determine the best route for sending messages between sender and receiver. 4. Transport: deals with end-to-end

issues such as segmenting the message for network transport, and maintaining the logical connections between sender and receiver.

3. Network: responsible for making routing decisions.

Hardware Layers

The hardware layers move messages from one computer or device to another. 2. Data Link: deals with message

delineation, error control and network medium access control.

1. Physical: defines how individual bits are formatted to be transmitted through the network.

How the layers fit together in practice

Message Transmission Using Layers

Network model layers use protocols, i.e., sets of rules to define how to communicate at each layer and how to interface with adjacent layers.Generally, outgoing messages travel down all network layers.Before sending a message to the next layer, each layer places it in an envelope of overhead information related to that layer (encapsulation).At the receiving end, messages travels up through the network layers, each layer removing the envelopes added when the message was sent.

Message transmission using layers

Why Standards?

Standards provide a fixed way for hardware and/or software systems to communicate.For example, USB enables two pieces of equipment to interface even though they are manufactured by different companies.By allowing hardware and software from different companies to interconnect, standards help promote competition.

Some Major Standards Making Bodies

ISO: International Organization for Standardization (http://www.iso.ch)ITU-T: International Telecommunications Union —Telecom Group (http://www.itu.int)ANSI: American National Standards Institute (http://www.ansi.org)IEEE: Institute of Electrical and Electronic Engineers (http://standards.ieee.org)IETF: Internet Engineering Task Force (http://www.ietf.org)

Layer Common Standards5. Application layer HTTP, HTML (Web)

MPEG, H.323 (audio/video)IMAP, POP (e-mail)

4. Transport layer TCP (Internet)SPX (Novell LANs)

3. Network layer IP (Internet)IPX (Novell LANs)

2. Data link layer Ethernet (LAN)PPP (dial-up via modem)

1. Physical layer RS-232c cable (LAN)Category 5 twisted pair (LAN)V.92 (56 kbps modem)

Layers & Standards

Topology

Topology refers to the geometric layout of the network.A logical topology is how the network works conceptuallyPhysical topology refers to how the network is physically connected.

Topology

Topology determines type of equipment to purchase and how to manage networkWhen designing a network, you must understand different topologiesConsider growth and security requirementsGood design grows and adapts as needs change

Standard Topologies

Today’s network designs are based on three topologies: Bus consists of series of computers

connected along a single cable segment Ring connects computers to form a

loop Star connects computers via central

connection point

Terminated Bus Network

Cable Break

Ring Network

Star Topology

Ethernet

Ethernet’s two forms, shared and switched Ethernet, use bus and star logical topologies, respectively.

Hub Operation

D4-47-55-C4-B6-9F

A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65

C3-2D-55-3B-A9-4F

Ethernet Hub

Station A1-44-D5-1F-AA-4C transmits a bit.

Hub Operation

D4-47-55-C4-B6-9F

A1-44-D5-1F-AA-4C B2-CD-13-5B-E4-65

C3-2D-55-3B-A9-4F

Ethernet Hub

Hub broadcasts the bitout all ports.

Switches

Networking devices that manage network connections between any pair of star-wired devices on a networkDoes not broadcast; forwards the message to the intended computerOffer greater bandwidth (not shared)

Switch Operation

D4-47-55-C4-B6-9FOn Switch Port 13

A1-44-D5-1F-AA-4COn Switch Port 10

B2-CD-13-5B-E4-65On Switch Port 11

C3-2D-55-3B-A9-4FOn Switch Port 13

Ethernet Switch

Station A1-44-D5-1F-AA-4C transmits a bit.

Switch Operation

D4-47-55-C4-B6-9FOn Switch Port 13

A1-44-D5-1F-AA-4COn Switch Port 10

B2-CD-13-5B-E4-65On Switch Port 11

C3-2D-55-3B-A9-4FOn Switch Port 13

Ethernet Switch

A switch sends a frameout a single port—

the one to the receiver

Switch Operation with Multiple Simultaneous Conversations

D4-47-55-C4-B6-9FOn Switch Port 13

A1-44-D5-1F-AA-4COn Switch Port 10

B2-CD-13-5B-E4-65On Switch Port 11

C3-2D-55-3B-A9-4FOn Switch Port 13

Ethernet SwitchMultiple simultaneous

conversations are possible

Star Topology

Star (Modern Ethernet) Extended Star or Hierarchy(Modern Ethernet)

RootSwitch

Only one possible path between two

stations

Switch

Switch

Switch

Mesh Topology

AB

CD

Multiple alternative paths between two

stations

PathABD

PathACD

Network Interface Cards

Network interface cards, also called network adapters NICs are part of both the physical and data link layer and include a unique data link layer address (MAC address), placed in them by their manufacturer.Before sending data onto the network, the network card also organizes data into frames and then sends them out on the network. Notebook computers often use NICs that are plugged into the PCMCIA port.

NIC Allows the Computer and the Network Cable to Communicate

Note: Gigabit Ethernet uses parallel transmission

Ethernet NIC

Cables

Each computer is physically connected to the network using a cable.The cables used on Ethernet LANs are either twisted-pair or optical fiber cables.Data can flow through cables in one of three modes: One way only (simplex) Both ways, one way at a time (half-duplex) Both ways at the same time (full-duplex)

Simplex, half-duplex, and full-duplex transmissions

General Cable Characteristics

Bandwidth rating (Mbps)Maximum segment lengthInterference susceptibility: EMI (electromagnetic interference), RFI (radio frequency interference)

Connection hardwareCable grade: cabling requirements for building and fire codes

Bend radiusMaterial costsInstallation costs

Twisted-Pair Cable

TP is two or more pairs of insulated copper wires twisted around each other Improves resistance to interference Limits crosstalk (EMI generated by wire

pairs) The more twists, the better

Two primary types of TP cable Unshielded twisted-pair (UTP) Shielded twisted pair (STP)

STP and UTP Cable

Ethernet Standards

Standardization

Institute of Electrical and Electronics Engineering (IEEE)

802 LAN/MAN Standards Committee (802 Committee) creates LAN standards

802.3 Working Group creates Ethernet standards

So Ethernet standards are also known as 802.3 standards.

Ethernet Standards

Physical Layer Ethernet Standards Using UTP 802.3 10Base-T

10 Mbps 802.3 100Base-TX

100 Mbps 10/100 operation thanks to auto-sensing

802.3 1000Base-T Gigabit Ethernet

Purchasing and Installing UTP

Wiring Quality Categories Governed by the TIA/EIA-568 standard Categories 3, 4, 5, 5e (enhanced), and 6

Higher numbers indicate better quality CAT 5e is recommended for new buildings CAT 5—for 100Base-TX CAT 5e—for 1000Base-TX (Gigabit

Ethernet) CAT 6—for 10 Gbs networks

Fiber-Optic Cable

Uses pulses of light rather than electrical signalsImmune to interference; very secure; eliminates electronic eavesdroppingExcellent for high-bandwidth, high-speed, long-distance data transmissionsSlender cylinder of glass fiber called core surrounded by cladding and outer sheath

Fiber-Optic Cable

Fiber-Optic Cable

Each core passes signals in only one directionMost fiber-optic cable has two strands in separate cladding May be enclosed within single sheath or

jacket or may be separate cables

More difficult to install and more expensive than copper media

Fiber-Optic Cables

Two primary types: Single-mode cables: cost more; span

longer distances; work with laser-based emitters

Multimode cables: cost less; span shorter distances; work with light-emitting diodes (LEDs)

Used for network backbone connections and with long-haul communications carrying large amounts of voice and data traffic

Full-Duplex Optical Fiber Cord

Switch Router

Fiber Cord

Fiber Cord

A pair of fibers is needed for full-duplex (simultaneous 2-way) transmission.Each fiber carries a signal in only one direction.

Optical Fiber Cabling

STConnectors(Popular)

SCConnectors

(Recommended)

Two fiber cords for full-duplex (two-

way) transmission

Summary

Layered communication: OSI modelLAN topologiesNetwork interface cardsCable characteristicsEthernet standard: 802.3Twisted pairFiber optic