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Welcome to CS 334/534. Network of networks. BHM. CHL. NO. ATL. “Fig 1.5” – An internet. 4 Ethernet LANs linked by a WAN. Comer Figure 1.1 – Growth of the Internet. WORLD TOTALS. ► Population 2010: 6,845,609,960. ► Internet Users Dec 31 2000: 360,985,492.

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CS 334/534

Network of networks





“Fig 1.5” – An internet

4 Ethernet LANs linked by a WAN


► Population 2010: 6,845,609,960

►Internet Users Dec 31 2000: 360,985,492

►Internet Users June 30 2010: 1,966,514,816 (+444.8 %)

►Penetration of population: 28.7 %

August 2010: “ Sometime this month, the 5 billionth device will plug into the Internet”

“Today, there are over 1 billion computers that regularly connect to the Internet.”

“But cellular devices, such as Internet-connected smartphones, have outstripped that total and are growing at a much faster rate.”

2.2 Two Approaches to Network Communication

* circuit-switched networks(telephone)

3 phases:

establish connection between end points

use connection

relinquish connection

disadvantage: cost independent of use

* packet-switched networks(post office)

at source, data divided into packets

packets individually sent from source to destination

data reassembled at destination

advantage: can share transport facilities

disadvantage: traffic spike may overload

2.4 Ethernet Technology

Comer Figure 2.1 Ethernet using twisted pair wiring (with HUB)

2.4.5 Properties of an Ethernet

Ethernet was “designed to be”

i.e. “classical” or “original” Ethernet

■ shared bus

■ broadcast technology

■ best-effort delivery

■ distributed access control

- shared bandwidth

- only one station transmitting at a time

- “half duplex”

(station transmits XOR receives)

- all stations receive all messages

- Like Post Office


2.4.8 Ethernet Hardware Addresses

6 bytes total - globally unique

High-Order 3 bytes:assigned to manufacturer by IEEE

Low-Order 3 bytes:serial number assigned by


Destination address as filter

An Ethernet station receiving packet

checks destination address

ignores packet if not intended for this station

Ethernet Addresses – continued

Types of Destination address

An address can be used to specify

■ a single, specific station

on this network (“unicast address”)

■ all stations on this network

(“broadcast address”)

■ a subset of stations on this network

(“multicast address”)

Interface Modes of Operation

■ normal mode

Interface processes only packets with destination

* its own unicast address

* the network broadcast address

■ promiscuous mode

Interface process all received packets

(including those addressed to other stations)

Figure 2.1 (with hub)

Figure 2.2 Format of an Ethernet frame (packet)

► Bridge is “store and Forward” device, operating at frame level

►2 interfaces operting in promiscous mode,

frame buffer for each interface

►receives frame, checks for validity before forwarding –

no “runts”

►” An (almost) arbitrary number of Ethernets can be connected together with bridges”

►”A set of bridged segments acts like a single Ethernet”


► “Most bridges . . . Make intelligent decisions about which frames to forward” -- No “runts”

► Special case when bridge first powered up -- “flooding”

switch connected together with bridges”

► No waiting to transmit connected together with bridges”

► not CSMA/CD

► If we upgrade switch with fast backplane, we can have multiple transmissions at same time

► Special case – station can be transmitting and receiving at same time - Full Duplex

2.4.5 connected together with bridges”Properties of an Ethernet

Ethernet was “designed to be”

i.e. “classical” or “original” Ethernet

■ shared bus

- shared bandwidth

- only one station transmitting at a time

- “half duplex”

(station transmits XOR receives)

■ broadcast technology

- all stations receive all messages

■ best-effort delivery

■ distributed access control


Properties of a “switched” Ethernet connected together with bridges”

■ not shared bus

- point-to-point connections

- not shared bandwidth

- “full duplex”

(station can be transmitting and receiving)

■ not broadcast technology

- stations receive only their own messages

■ best-effort delivery

■ no access control needed

- private frame buffer

- no entrance collisions

- not CSMA/CD

- exit port collision

Most new wired Ethernet installations are switched

Return to section 2.4.7 connected together with bridges”Wireless Networks and Ethernet

IEEE 802.11 standards for wireless LANs

We have 802.11g in the lab

(Independent) Basic Service Set connected together with bridges”

(ad-hoc network)

Figure 1

Extended Service Set

(infrastructure network)

New components: Distribution System each BSS has an Access Point

Figure 2

Figure 3 – Hidden Station Problem connected together with bridges”

Figure 4 – CSMA/Collision Avoidance connected together with bridges”

Independent Basic Service Set connected together with bridges”(IBSS)

Station Service (SS)

must be provided by all stations:

(a) Authentication

(b) Deauthentication

(c) Privacy

(d) Data Unit Delivery

Extended Service Set (ESS)

Additional services that must be provided by the access point/distribution system:

(a) Association

(b) Distribution

(c) Disassociation

(d) Reassociation

Figure 5 connected together with bridges”

AP acts like a bridge

Figure 6 connected together with bridges”

Network, BSS, and Station Identification connected together with bridges”

In the Network Lab:

BSSID is 00:06:25:49:B3:B2

(MAC address of Access Point)

Each station identification is its MAC address

ESSID is netlab_w

Wired Ethernet Frame Format connected together with bridges”

Wired: All frames are data frames

Wireless: Management, Control, and Data frames

Figure 6 - 802.11 frame format

Usage of Address Fields in 802.11 connected together with bridges”

Address 1 identifies the immediate receiver

(the unit that will process the frame)

Address 2 identifies the transmitter

(the unit that transmits the frame and will receive the acknowledgment)

Usage of other addresses is situation-dependent.

Another IBSS! connected together with bridges”

Example 1 – IBSS

For frames traveling within an IBSS:

Address 1 is the destination address

Address 2 is the source address

Address 3 is the BSSID

(used as a filter, since IBSSs may overlap)

Example 2 – ESS with 802.3 (wired) DS, client-server transaction

On 802.11 segment

Client request

Addr 1 - immediate destination - AP

Addr 2 – client address

Addr 3 – ultimate destination (DA)

Server reply

Addr 1 – client

Addr2 – immediate source (AP)

Addr3 – original source (server)

Example 3 – ESS with 802.11 (wireless) DS transaction


Addr 1 – AP2

Addr 2 – AP1

Addr 3 – ultimate dest

Addr 4 – original source

“Fig 1.5” – An internet transaction

4 Ethernet LANs linked by a WAN

Net 2 transaction

Net 1




Figure 3.1

Net 3

Net 1

Net 2

B1 ?

B2 ?



Figure 3.2

Comer figure 3.3 (a) user’s view transaction

(b) structure of physical networks and routers

“Fig 1.5” – An internet transaction

4 Ethernet LANs linked by a WAN

Comer section 3.8: All Networks are Equal

We regard each of the links in the WAN as a network

0 transaction


| | |

0 | | |


10 | | |


110 | | |


IPv4 transaction

Figure 4.1 The original classful IP addressing scheme

IP addresses specify network connections

A router must have at least two IP addresses, with different network parts

4.11 Dotted Decimal Notation transaction

1 0 0 0 1 0 1 0 0 0 0 1 1 0 1 0 0 1 0 0 0 0 1 0 0 0 0 0 0 1 1 0

138 . 26 . 66 . 6

4.14 transactionInternet Addressing Authority

Figure 4.5 Logical connection of transaction

Two networks to the Internet backbone transaction

Figure 4.6 Example IP address assignment

BHM transaction




“Fig 1.5” – An internet

Final router has to deliver packet to final destination over Ethernet network.

Final Router has to deliver packet over Ethernet network. transaction

The ONLY way data can move over an Ethernet is in the payload of an Ethernet frame.


IP Packet

Destination Ethernet Address

Figure 2.2 Format of an Ethernet Frame

From the incoming packet final router knows the destination IP address.

We have to find the Ethernet address corresponding to the destination IP address.

Ch 5: transactionMapping Internet Addresses to Physical Addresses

Incoming IP Packet



Comer Section 5.10 ARP Implementation transaction

■ action when sending an ARP request

detain outgoing data message in queue

until ARP reply received

■ action when receiving an ARP message

either request or reply contain mapping(s)

in either case

look in ARP cache to see if receiver already has an entry for the sender.

if yes, overwrite physical address (quickest way) and reset timer

if no, make new entry and start timer

further action depends on two sub-cases:

* incoming ARP message was a request

look at target IP address; if it’s for this machine, generate ARP reply

* incoming ARP message was a reply

since reply is unicast,this machine earlier sent an ARP request

for the IP address in the reply

so release outgoing data message from

queue, incorporate packet into outgoing frame and transmit.

5.11 transactionARP Encapsulation and Identification



Figure 2.2 Ethernet Frame Format

5.12 transactionARP Protocol Format

ARP Message