ITC242 – Introduction to Data Communications Chapter 10

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Last Week

Internet Operation • Describe the characteristics of an Internet Address • Describe the different classes of IP addresses • Explain the purpose of subnet masks.

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Last Week

LAN architecture and protocols • Define the various types of Local Area Networks (LANs) • Discuss the different types of transmission media commonly used in LANs.

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Topic 10 - Ethernet

• Learning Objectives • Describe the characteristics of Ethernet networks • Discuss the operation of CSMA/CD • Discuss the operation of bridges, hubs, and switches • Describe the characteristics of fast Ethernet standards.

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The Link Layer

Application Presentation Session Transport Network Link Physical The 7-layer OSI Model TCP IP UDP LAN-LINK The 4-layer Internet Model 5

Link Layer: Introduction

Some terminology: • hosts and routers are

nodes

• communication channels that connect adjacent nodes along communication path are

links

– wired links – wireless links – LANs • layer-2 packet is a

frame ,

encapsulates datagram

data-link layer

has responsibility of transferring datagram from one node to adjacent node over a link 6

Link layer: context

• datagram transferred transportation analogy by different link protocols over different • trip from Melbourne to Brisbane – car: Melbourne to Albury links: – train : Albury to Sydney – e.g., Ethernet on first – plane : Sydney to Bridbane link, frame relay on • tourist = datagram intermediate links, • transport segment = 802.11 on last link communication link • each link protocol • transportation mode = link layer provides different protocol services • travel agent = routing algorithm – e.g., may or may not provide reliable data transmission over link 7

• •

Link Layer Services

framing, link access:

– encapsulate datagram into frame, adding header, trailer – channel access if shared medium – “MAC” addresses used in frame headers to identify source, dest • different from IP address!

reliable delivery between adjacent nodes

– we learned how to do this already!

– seldom used on low bit-error link (fiber, some twisted pair) – wireless links: high error rates • Q: why both link-level and end-end reliability?

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Link Layer Services (more)

•

flow control:

– pacing between adjacent sending and receiving nodes • •

error detection

: – errors caused by signal attenuation, noise. – receiver detects presence of errors: • signals sender for retransmission or drops frame • error correction: – receiver identifies

and corrects

bit error(s) without resorting to retransmission

half-duplex and full-duplex

– with half duplex, nodes at both ends of link can transmit, but not at same time 9

Where is the link layer implemented?

• in each and every host • link layer implemented in “adaptor” (aka

network interface card

NIC) – Ethernet card, PCMCI card, 802.11 card – implements link, physical layer • attaches into host’s system buses • combination of hardware, software, firmware application transport network link link physical cpu

host schematic

controller memory physical transmission

host bus (e.g., PCI) network adapter card

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Adaptors Communicating

datagram controller datagram controller

sending host frame

• sending side: – encapsulates datagram in frame – adds error checking bits, rdt, flow control, etc.

datagram

receiving host

• receiving side – looks for errors, rdt, flow control, etc – extracts datagram, passes to upper layer 11 at receiving side

Multiple Access Links and Protocols

Two types of “links”: • point-to-point – PPP ( point-to-point protocol) for dial-up access – point-to-point link between Ethernet switch and host • broadcast a copy (shared wire or medium): multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. Any one node transmits a frame, the channel broadcasts the frame and each of other nodes receives – old-fashioned Ethernet – 802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical) 12

Multiple Access protocols

• single shared broadcast channel • two or more simultaneous transmissions by nodes: interference – collision if node receives two or more signals at the same time

multiple access protocol

• distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit • communication about channel sharing must use channel itself! 13

MAC Protocols: a taxonomy

Three broad classes: • Channel Partitioning – divide channel into smaller “pieces” (time slots, frequency, code) – allocate piece to node for exclusive use • Random Access – channel not divided, allow collisions – “recover” from collisions • “Taking turns” – nodes take turns, but nodes with more to send can take longer turns 14

Random Access Protocols

• When node has packet to send – transmit at full channel data rate R.

– no

a priori

coordination among nodes • two or more transmitting nodes ➜ “collision”, • random access MAC protocol specifies: – how to detect collisions – how to recover from collisions (e.g., via delayed retransmissions) • Examples of random access MAC protocols: – CSMA, CSMA/CD, CSMA/CA 15

CSMA/CD Protocol

All hosts transmit & receive on one channel Packets are of variable size.

When a host has a packet to transmit: 1. Carrier Sense: Check that the line is quiet before transmitting.

2. Collision Detection: Detect collision as soon as possible. If a collision is detected, stop transmitting; wait a

random time

, then return to step 1.

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Token Passing

• A token rotates around a ring to each node in turn. • All nodes (computers, routers, etc.)

copy

all data and tokens, and

repeat

them along the ring. • When a node wishes to transmit packet(s), it

grabs

the token as it passes.

• It

holds

the token while it transmits.

• When it is done, it

releases

sends it on its way.

the token again and 17

“Taking Turns” MAC protocols

Polling: • master node “invites” slave nodes to transmit in turn • typically used with “dumb” slave devices • concerns: – polling overhead – latency – single point of failure (master) data slaves data poll master 18

“Taking Turns” MAC protocols

Token passing: • control

token

passed from one node to next sequentially.

• token message • concerns: – token overhead – latency – single point of failure (token) (nothing to send) T T data 19

MAC Addresses • 32-bit IP address:

–

network-layer

address – used to get datagram to destination IP subnet

• MAC (or LAN or physical or Ethernet) address:

– function:

get frame from one interface to another physically-connected interface (same network)

– 48 bit MAC address (for most LANs) • burned in NIC ROM, also sometimes software settable 20

LAN Addresses

Each adapter on LAN has unique LAN address 71-65-F7-2B-08-53 LAN (wired or wireless) 1A-2F-BB-76-09-AD ( hexadecimal) 58-23-D7-FA-20-B0 Broadcast address = FF-FF-FF-FF-FF-FF = adapter 0C-C4-11-6F-E3-98 21

LAN Address (more)

• MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address • MAC flat address ➜ portability – can move LAN card from one LAN to another • IP hierarchical address NOT portable – address depends on IP subnet to which node is attached 22

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: • 7 bytes with pattern 10101010 (“wake up”) followed by one byte with pattern 10101011 • used to synchronize receiver, sender clock rates 23

Ethernet Frame Structure (more)

• Addresses: 6 bytes – if adapter receives frame with matching destination address, or with broadcast address, it passes data in frame to network layer protocol – otherwise, adapter discards frame • Type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) • CRC: checked at receiver, if error is detected, frame is dropped 24

Ethernet: Unreliable, connectionless

• connectionless: No handshaking between sending and receiving NICs • unreliable: receiving NIC doesn’t send acks to sending NIC – stream of datagrams passed to network layer can have gaps (missing datagrams) – gaps will be filled if app is using TCP – otherwise, app will see gaps • Ethernet’s MAC protocol: unslotted CSMA/CD 25

Ethernet CSMA/CD algorithm

1. NIC receives datagram 4. If NIC detects another from network layer, transmission while creates frame transmitting, aborts and 2. If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle, 5. During aborting, after

m

th collision, NIC then transmits 3. If NIC transmits entire frame without detecting sends jam signal ( to make sure all other transmitters are aware of collision; 48 bits ) chooses

K

at random from {0,1,2,…,2

m

-1}.

NIC waits K ·512 bit times ( Bit time: .1 microsec for 10 Mbps Ethernet ; another transmission, NIC is done with frame !

for K=1023, wait time is about 50 msec returns to Step 2 ), 26

The Original Ethernet

10Mb/s

l

 1500

m

Repeaters every 500m Thick copper coaxial cable In practice, minimum packet size = 512 bits.

• allows for extra time to detect collisions.

• allows for “repeaters” that can boost signal.

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• 802.3 Ethernet Standards: Link & Physical Layers

many

different Ethernet standards – common MAC protocol and frame format – different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps – different physical layer media: fiber, cable application transport network link physical MAC protocol and frame format 100BASE-TX 100BASE-T4 100BASE-T2 100BASE-SX 100BASE-FX 100BASE-BX copper (twister pair) physical layer fiber physical layer 28

802.3 Medium Notation

• Notation format: • e.g 10Base5 provides 10Mbps baseband, up to 500 meters • T and F are used in place of segment length for twisted pair and fiber 29

802.3 10BaseX Media Options

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An Ethernet Network

Problem:  Shared network limits throughput.

 Lots of collisions reduces efficiency.

Router Outside world 31

Ethernet Switching

Ethernet Switch Benefits:   Number of collisions is reduced. If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link).

Capacity is increased: the switch can forward multiple frames to different computers at the same time.

Router Outside world 32

• •

Switch

• link-layer device: smarter than hubs, take

active

role – store, forward Ethernet frames – examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment

transparent

– hosts are unaware of presence of switches

plug-and-play, self-learning

– switches do not need to be configured 33

One Ethernet Switch in “Packard”

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Star topology

• bus topology popular through mid 90s – all nodes in same collision domain (can collide with each other) • today: star topology prevails – active

switch

in center – each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) bus: coaxial cable switch star 35

Switch: allows

multiple

simultaneous transmissions

• • hosts have dedicated, direct connection to switch • switches buffer packets • Ethernet protocol used on

each

incoming link, but no collisions; full duplex – each link is its own collision domain

switching:

A-to A’ and B-to-B’ simultaneously, without collisions – not possible with dumb hub C’ B’ 6 5 A 1 2 4 A’ 3 B C

switch with six interfaces ( 1,2,3,4,5,6 )

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Switch Table

•

Q:

how does switch know that A’ reachable via interface 4, B’ reachable via interface 5?

•

A:

each switch has a switch table, each entry: – (MAC address of host, interface to reach host, time stamp) • • looks like a routing table!

Q:

how are entries created, maintained in switch table? – something like a routing protocol?

C’ B’ 6 5 A 1 2 4 A’ 3 B C

switch with six interfaces ( 1,2,3,4,5,6 )

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• switch

Switch: self-learning

learns

which hosts can be A A A’ C’ reached through Source: A Dest: A’ B which interfaces 6 – when frame received, switch “learns” location of sender: incoming LAN segment B’ – records sender/location pair in switch table MAC addr interface TTL

A 1 60

5 1 2 4 A’ 3 C

Switch table (initially empty) TTL: time-to-live

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Switch: frame filtering/forwarding

When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address

3. if

entry found for destination

then { if

dest on segment from which frame arrived

then

drop the frame

} else

forward the frame on interface indicated

else

flood

forward on all but the interface on which the frame arrived

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Self learning, forwarding: example

• frame destination unknown:

flood

• destination A location known:

selective send

C’ A A A’ 5 1 2 3 4 A’ A Source: A Dest: A’ B C B’ A’ MAC addr interface TTL

A A’ 1 4 60 60 Switch table (initially empty)

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Interconnecting switches

• switches can be connected together S 4 S 1 S 3 A B C S 2 D F E G H I • •

Q:

sending from A to F - how does S 1 forward frame destined to F via S 4 know to and S 3 ?

A:

self learning! (works exactly the same as in single-switch case!) 41

to external network

Institutional Network

mail server web server router IP subnet 42

Switches vs. Routers

• both store-and-forward devices – routers: network layer devices (examine network layer headers) – switches are link layer devices • routers maintain routing tables, implement routing algorithms • switches maintain switch tables, implement filtering, learning algorithms 43

Hubs

• Alternative to bus topology • Each station is connected to the hub by two lines (transmit and receive) • When a single station transmits, the hub repeats the signal on the outgoing line to each station.

• Physically a star; logically a bus.

• Hubs can be cascaded in a hierarchical configuration.

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Hubs

… physical-layer (“dumb”) repeaters: – bits coming in one link go out

all

other links at same rate – all nodes connected to hub can collide with one another – no frame buffering – no CSMA/CD at hub: host NICs detect collisions twisted pair hub 45

Interconnecting LANs

• Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router. • By partitioning a large LAN into multiple smaller networks, there are fewer collisions, and more parallel communications.

• Provide a number of advantages – Reliability: Creates self-contained units – Performance: Less contention – Security: Not all data broadcast to all users – Geography: Allows long-distance links 46

Key Aspects of Bridge Function

• Makes no modification to content or format of frames it receives; simply copies from one LAN and repeats with exactly the same bit pattern as the other LAN. • Should contain enough buffer space to meet peak demands. • Must contain addressing and routing intelligence. • May connect more than two LANs.

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Bridge Operation

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Bridge Functions

• Read all frames from each network • Accept frames from sender on one network that are addressed to a receiver on the other network • Retransmit frames from sender using MAC protocol for receiver • Must have some routing information stored in order to know which frames to pass 49

Summary comparison

traffic isolation plug & play hubs no yes optimal routing cut through no yes routers switches yes yes no yes no yes no yes 50

Two-Level Hub Topology

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Summary

Topic 10 - Ethernet • Describe the characteristics of Ethernet networks • Discuss the operation of CSMA/CD • Discuss the operation of bridges, hubs, and switches • Describe the characteristics of fast Ethernet standards.

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