Service Providers &Data Link & Physical layers

Week 4 Lecture 1

Internet Architecture

Application layer – HTTP, SMTP etc

Transport layer – TCP, UDP

Network layer - IP

Data link & Physical layers – the territory of the Lan & Telcos

W3C

IETF

ITUIEEE

Local Area Networks (LAN)

• Nearly all organisation have a LAN in each office

• All devices are connected to the LAN

• LANs are then connected to the WAN via a router and firewall

• LANs provide file and print services, and application and database servers

WAN

Workstations

Router &Firewall

File server

Application & Database Servers

Printers

Main characteristics

• Owned & Operated by the Organisation

• Single geographic area, usually a building

• Relatively fast media

• Most common protocol is Ethernet using CSMA-CD

Bus topology using Co-axial cable

Hub topology using UTP

Floor 15

Floor 14

Floor 13

Machine roomRouter

Servers

WAN

All wires would be100mbps UTP Cat 5

Backbone could be Optic fibre

LAN Protocols• Most likely to be CSMA/CD at the data link layer• Carrier Sense Multiple Access/Collision Detection• Adaptor listens to see if other devices are transmitting – if

not, it sends• Adaptors looks for own packets and accepts• If a collision occurs (propagation delay), the sending

adaptors back-off for a random amount of time and then re-transmits

• CSMA/CD is a connectionless, unreliable service• The IP address is converted to the adaptor address by the

Address Resolution Protocol module on each Internet host and Router

Ethernet speeds

• Ethernet - 10mb

• Fast Ethernet - 100mb

• Gigabit Ethernet - 1000mb

• 10 Gigabit Ethernet - 10000mb

TCP/IP

Frame Relay/ATM

ATM/Fibre

CSMA/CD CSMA/CD

Routers

Local ISP POP

City carrier hubs

Devices in the network

• Network Adaptors

• Hubs

• Bridges

• Switches

• Routers

• Firewalls

Network adaptors

• Adaptor, NIC or PCMCIA card – connects the device to the network– Implements the data link layer and physical

layer in hardware– Chip set makes adaptors relatively cheap– It is semi-autonomous - accepts and hands over

IP packets – full responsibility for transmission & error control

– Each adaptor has a unique 6 byte address. IEEE allocates address ranges to manufacturers

Hubs

• Essentially a repeater – when it receives a bit it sends it down the other links to other adaptors and hubs – thus includes all devices in the one collision zone

• Physical layer devices

• Carries out some network management functions – if an adaptor malfunctions and floods the Ethernet it can internally disconnect the link – collects some statistics

• Extends the length of the LAN as each link has a discrete limit - 200 metres

Bridges & Switches

• They operate on Ethernet frames and are layer 2 devices– they are store and forward devices but use LAN addresses – LAN

devices

• Acts as a switch and only sends frames down a link on which the destination address is. Thus it can break a LAN into discrete collision zones – single tree

• Uses MAC addresses• It also connects links at different bandwidths i.e. 10mbps and

100mbps• They are plug & play devices and self learn what devices are

on what links. • But if there are multiple paths they could generate duplicate

packets, so they disable duplicate paths.

Floor 15

Floor 14

Floor 13

Machine roomRouter

Servers

WAN

All wires would be100mbps UTP Cat 5

Optic fibre

Network Devices

Application

Transport

Network

Link

Physical

Link

Physical

Network

Link

Physical

Application

Transport

Network

Link

Physical

Host Bridge orSwitch

Router Host

Physical

Hub

Routers

• Routers send packets on best path to the destination. This is necessary for path redundancy

• Because they operate at layer 3 they are inherently slower than bridges but more complex technologies are compensating – Route on IP addresses can use different paths

• They maintain separate network segments• LAN or WAN devices• Can be used as simple firewalls by filtering out packets

within an organisation• Vary between core trunk routers to SOHO routers for less that

$1500.

Router design

• Bus with CPU software switching• Bus with intelligent line cards• Non blocking cross bar switching with up to

5 TBPS switching. Cisco 12000 series is an example. Juniper Networks, Avici and Lucent have similar capacity routers

• Switching decisions on 100m packets per second

Application ApplicationProcess

Application layer

Message

Application ApplicationProcess

Transport layer

Message

Transport TransportSegmentHost

Application ApplicationProcess

Network layer

Message

Transport TransportSegmentHost

Network Network NetworkHost & Router

Datagram Datagram

Application ApplicationProcess

Link layer

Message

Transport TransportSegmentHost

Network NetworkHost & Router

Datagram Datagram

Link Link

Host, router& switch

Frame Frame

LinkLink LinkLink

LinkNetwork