Part 2: LOCAL AREA NETWORK pot

Broadcast the image of organization web server • Interconnecting large systems: • Main-frame • Storage network - SAN Topology Physical & Logical Topologies • Physical topologies –Define

Computer Network

40 units (45 minutes/unit)

References:

1 Data- Computer Communication handbook- William Stallings

2 TCP/IP Illustrated, Volume I - W.R Stevens

3 CCNA- semester1-2-3-4

Part 2- LOCAL AREA NETWORK

1 LAN Features & Purposes

2 Topology

3 Layered Model of LAN

4 Media Access Control: CSMA/CD | Ethernet

5 LAN Devices

LAN Features & Purposes

1 Belongs to own an organization

2 Building and development depending on organization’s requirements and budget

3 No cost to users but being controlled

Purposes

1 Sharing resources

• Hardware (CPU; RAM; Storages; primters…)

• Soft ware (applications: MS-offices; antivirus, OS)

• Organization’s Information -> database (file server)

• Communication facilities

– ADSL / Wireless / RAS / mail…

2 Communication to customers, partners, staff…

3 E- management (Lotus note)

• Business Process Reengineering

• CRM- Customer Relationship Management

4 Broadcast the image of organization (web server)

• Interconnecting large systems:

• Main-frame

• Storage network - SAN

Topology

Physical & Logical Topologies

• Physical topologies

–Define the actual layout of the wire (media)

• Logical topologies

–Define how the media is accessed by the hosts

•The path that the data flow transmitted and received

Physical Topologies Bus

Ring

Star

Extended Star Hierarchical

Mesh

Physical Topology: Bus

connected to backbone

bus must be properly terminated

Physical Topology: Ring

connected to each

of its neighbors

Physical Topology: Star

connected to a central point

usually a hub or a switch

Physical Topology: Extended Star

star topologies together.

star is a hub or a switch.

and size of the network.

Physical Topology: Hierarchical

star except a computer controls traffic (not a hub or

a switch).

Physical Topology: Mesh

connection to every other host.

where communication must not be

interrupted.

Logical Topologies Broadcast

Token Passing

Logical Topology: Broadcast

(or broadcasts its data) to every

other host.

Logical Topology: Token Passing

electronic token.

the right to pass data to its

• Layered Model of LAN

Layered Model of LAN Data-link and Physical layers

– 802.1d: Spanning tree.

– 802.2: LLC.

– 802.3: MAC ~ Ethernet.

– 802.5: MAC ~ Token ring

– 802.11: Wireless LAN.

Physical Layer in LAN Model

• Media with wire- link and wireless

– Wire-link: cable types

•Twisted pair (shielded and unshielded)

•Co-axial cable: think and thick.

•Fiber optical cable

– Wireless: the frequency used for transmission

•802.11a/ 802.11b/ 802.11g…

• Media with relevant connectors

– RJ-45

– AUI-15 / AUI-9

– RS-232C – 25pin

“Thick-net” Connections Twisted Pair Media

Any wire passing an electrical signal creates

a surrounding magnetic field

If the wires are aligned parallel, interference can occur called Crosstalk

Twisted Pair Media

To help eliminate crosstalk, wires are twisted together

Fast Ethernet The original fast Ethernet cabling.

Gigabit Ethernet

IEEE 802.11 Architecture

• IEEE 802.11 is MAC protocol and physical medium specification for wireless LANs

–Infrastructure network

–Ad- hoc network

•

802.11 Ad Hoc Networking

• 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

802.11 infrastructure Networking

•Station (STA)

– terminal with access mechanisms to the wireless medium and radio contact to the access point

•Basic Service Set (BSS)

– group of stations using the same radio frequency

•Access Point

– station integrated into the wireless LAN and the distribution system

•Portal

– bridge to other (wired) networks

•Distribution System

– interconnection network to form one logical network (ESS: Extended Service Set) based

on several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS 2

802.11 LAN

BSS1 Access Point STA1

STA2 STA3 ESS

MAC

Media Access Control

Ethernet introduction

• Ethernet is the most widely used local area network (LAN) technology

• Ethernet was designed to carry data at high speeds for very limited distances

• Ethernet is well suited to applications where a local communication medium must carry

sporadic traffic , occasionally heavy traffic at high peak data rates.

Ethernet history

• 1960s, the University of Hawaii

• 1970s, Xerox developed the first system

• 1980, IEEE released 802.3 specification.

• 1980s, Digital, Intel and Xerox jointly

developed and released an Ethernet

specification (v2.0), Compatible with 802.3

• Today, the term Ethernet is often used to

refer to Ethernet specifications, including

IEEE 802.3.

Layer 2

group the bits

identify computers

layers

transmit.

Shared media environment

• Multi-access

• Base-band

– => collision problem

• MAC- address- 48bits flat address

– Ex: 3a:23:4e:ef:10:33

Contention- Collisions

• More than one node attempts to transmit at the same time.

• Collisions occur in broadcast topology (physical layer):

– Ethernet LAN technology.

Collisions

a point of media at the same time.

Ethernet technology:

– Base-band environment allow only one data packet to access the cable at any one time.

– Sharing media environment with collision problem due contention causing for collision.

area where collisions occur

Collisions domain: Share access

• Ethernet Frame Format

Ethernet Frame Format Ethernet Frame Structure

• Sending interface encapsulates IP datagram (or other network

layer protocol packet) in Ethernet frame

• Preamble: 8 bytes

– 101010…1011

– Used to synchronize receiver, sender clock rates

• CRC: 4 bytes

– Checked at receiver, if error is detected, the frame is

simply dropped

• Type: 2 bytes

–Indicates the higher layer protocol, mostly IP

but others may be supported such as Novell IPX and AppleTalk)

Ethernet Frame Structure

• Addresses: 6 bytes

– Each adapter is given a globally unique address

at manufacturing time

•Address space is allocated to manufacturers

–24 bits identify manufacturer

–E.g., 0:0:15:* Æ 3com adapter

•Frame is received by all adapters on a LAN and dropped

if address does not match

– Special addresses

•Broadcast – FF:FF:FF:FF:FF:FF is “everybody”

•Range of addresses allocated to multicast

–Adapter maintains list of multicast groups node is interested in

CSMA/CD

• When a station wishes to transmit, it checks the

network to determine whether another station is

transmitting.

• If network is free, the station proceeds with the

transmission.

• While sending, the station monitors the network

to ensure that no other station is transmitting.

• If a transmitting node recognizes a collision, it

transmits a jam signal so that all other nodes

recognize collision.

• All transmitting nodes then stop sending for a

backoff time (randomly 0 2 n - 1 of 51.2μs).

CSMA/CD (cont.)

CSMA/CD Algorithm

Back…

Given t

Calculating Backoff

Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

Exponential Backoff:

• If deterministic delay after collision, collision will occur again in lockstep

• If random delay with fixed mean

– Few senders Æ needless waiting

– Too many senders Æ too many collisions

• Goal: adapt retransmission attempts to estimated current load

– heavy load: random wait will be longer

LAN DEVICES

Devices function at Layers

LAN Media Symbols

Token Ring

FDDI Ring

Ethernet Line

Serial Line

LAN Device: Transceiver

• Layer 1 device.

UTP

BNC

AUI

10Base-T: Transceiver

LAN Device: Repeater

• Expanding the capacity of communication distance.

• Regenerates and repeats the signal with the determinate level of voltage

• Layer 1 device.

10/100 Base-T: Repeater

• Regenerates signals:

• Enables cables to extend farther to reach longer distances.

• Increase number of nodes that can be connected

to a network.

• Cannot filter traffic.

LAN Device: Hub

• A multi-port repeater.

• Layer 1 device.

• Multi port repeater.

• Center of a star topology network.

• Allows many devices to be inter-connected.

• Receives signals from only a port then broadcasts

to others.

¾Shared bandwidth

¾Cannot filter traffic.

• NIC (Network Interface Card)

• Connect your computer with network.

• Provide MAC addresses to each connection.

• Implement CSMA/CD algorithm.

• Bridge

• Forward or filter frame by MAC address.

• Switch

• Multi-port bridge.

LAN Device: NIC

• Layer 2 device.

NIC

NIC (cont.)

• Provides ports for network connection.

• When selecting a network card, consider:

1 Type of network:

• Ethernet

• Token Ring

2 Type of media

• Twisted-pair

• Coaxial

• Fiber-optic

3 Type of system bus

• PCI

NIC: Layer 2 functions

upper layers in the computer

identifier

packaging the bits for transport

structured access to shared access media

the media

Bridge

• Connects network segments.

• Make intelligent decisions about whether to pass

signals on to the next segment

• Improve network performance by eliminating

unnecessary traffic and minimizing the chances of

collisions

• Divides traffic into segments and filters traffic based

on MAC address.

• Often pass frames b/w networks operating under

different Layer 2 protocols.

LAN Device: Bridge

based on physical addresses.

• Layer 2 device.

Segmentation with routers LAN Device: Switch

• Layer 2 device.

• No collision domain, because of micro-segmentation.

• Low latency levels and a high rate of speed for frame forwarding

• Increases the bandwidth available on a network

• Is performed in hardware instead of in software, it is significantly faster.

Issues of LAN Switch

• All hosts connected to the switch are still in the

same broadcast domain.

• Security problem within a broadcast domain.

• Solutions ?

• VLAN?

•How does it work?

VLAN & Implementation

• Each port can be assigned to a VLAN

• Ports assigned to the same VLAN share broadcasts

• Ports that do not belong to that VLAN do not share these broadcasts

• Two methods that can be used to assign a switch port to a VLAN:

• Static

• Dynamic

Transport of VLANs Benefits of VLAN

• Reduce administration costs – moves, additions changes.

• Controlled broadcast activity.

• Workgroup and network security.

• Save money by using existing hubs.

• Ports on switch that is statically assigned to a VLAN.

• Require administrator to make changes.

• Secure.

• Easy to configure.

• Straightforward to monitor.

• Works well in which moves are controlled and managed.

• Ports on switch automatically determine their VLAN assignments

• Based on MAC addresses, logical addressing etc…

• Less administration with in wiring closet when a user moves or new one added

• Centralized notification when an unrecognized user is added

• More administration is required to initially set up database