Local Area Networks

Network technologies are in classified in

Local Area Networks LAN roomy building

Metropolitan Area Network MAN City

Wide Area Network WAN group of city

LAN

- Many technologies

+ Ethernet, Appletalk, token ring, pronet, x25

- High throughput

- Relative low cost

- Limited short distance

- Often rely on shared media

LAN Principle

A computer is more likely to communicate with computers nearby than with computers that are distant.

This is also called the locality of principle

Topology of Network

- Specifies the general “shape” of the network

- Primarily refers to interconnection between computers

- Hides the actual hardware devices

Star Topology

Star network topology

(http://compnetworking.about.com/library/weekly/aa041601a.htm)

- Central component of the network

- There is a separate connection for end computer in the hub

Ring Topology

Ring network topology

- No central facility

- Connections go from one computer to the other

- Computers take terms to send messages to each other using a passing to hen

Bus Topology

Bus network topology

- Shared cable

- Shared medium connects the computers

Example of Bus Network : Ethernet

- Most popular LAN

- IEEE standard 802.3

- There are three generations of internet

10 Mbps 100 Mbps 16 Giga bit Ethernet

Ethernet Fast Ethernet

- It uses same frame format regardless of speed as well as thee same MTU (Maximum transfer unit, Max data size) of 1500 bytes

- Ethernet uses a shared medium

- Computers take turns to transmit only one station transmits at a time

- Media access control (MAC) policy ensures fairness.

Example of a transmission

Old Ethernet transceiver Ethernet interface

Messages send by a computer are received by all other computers, including itself

- Signal propagates across entire cable

- All stations receive transmission

- The Ethernet interface will filter the packets and will pass to the OS only the messages that are going to that computer.

- Destination address is 6 bytes

- Each interface has a different Ethernet address given by the manufacturer.

- A technique called CSMA/CD is used to control the access to the shared bus

- Carrier sense multiple access / collision detection

- Multiple access (MA)

+ Multiple computers share the same media

+ All of them used the same access algorithm

- Carrier sense (CS)

+ A computer transmits until the medium is idle.

- Collision Detection (CD)

+ Simultaneous transmissions are possible and they will interfere with each other.

+ The interface will listen to the medium during transmission.

+ If the received signal is different than the signal sent, then there is a collision

+ If the is the case, then the transmitter steps transmission (Back off) and tries again later.

- Back off after collision

+ Wait random time t1, such that 0< t1 < d

+ Use CSMA and try again

+If a second collision occurs, wait a random time t2, such that 0 <= t2 <= 2d

+ Double range after each successive collision

+ This is called “exponential back off”

Sept 17^th

Ethernet

CSMA/CD

Carrier Sense

Multiple Access

Collision Detection

Exponential back off

Wait random time, wait 0 <= t1 <= d

If second collision, wait 0 <= t2 <= 2d

Wireless Networks

- Limited range. Not all the stations receive all the transmissions

- The power of the transmitters is limited to allow more networks without interference with other.

- If maximum transmission range is “d” then station 2 can receive packets from 1 and 2. However, station 1 and 3 do not receive packets from each other.

- We cannot use collision detection only because

+ In Ethernet we assume that all stations receive the transmitted packets.

+ In a wireless network, there is no guarantee that all the stations will receive the packets sent.

Ex) If 1 sends a packet, 3 will not receive it.

- More about why we use limited power in the transmitter to allow more networks

Venn Diagram

Power = P1 Power = P2

P 1 > P 2

+ If we had collision detection and 1 sends a packet to 2 and 3 sends a packet to 2 at the same time.

1 will not be able to detect the collision since 3 is more than “d” from 1 and also 3 will not detect the collision since 1 is also more than d from 3.

- Instead of using collision detection, wireless networks use “collision avoidance” (CA)

CSMA/CA (Wireless nets)

Carrier sense, multiple access, collision avoidance.

- Both sides send a small message before data transmission

- The small message informs all the stations in range the channel will be used

1. Send a message

“1 is about to send packets to 2” : 2 receives message.

2. Send a message

“2 is about to receive packets from 1” : 1 and 3 receive message.

3. Message is sent from 1 to 2.

* This protocol does not prevent collisions 100% of the time but it limits the number of collisions.

Identifying a destinations

- All stations on a shared media receive all the transmissions

- Each station has an assigned unique number called “the station address”

Ethernet

+ Address is 48 bits (6 bytes).

+ Address is assigned by manufacturer

+ Network interface card is also called NIC.

- All stations receive packets and the stations will discard packets addressed to another station.

- Interface hardware, not software, checks address. If the address matches the NIC, the packet is passed to OS by interrupting CPU.

- This prevents CPU overhead due to excessive traffic.

Sept 19^th

- The same packet goes to all the computer in the network. If a packet is sent from B to A, all the computers will receive the packet (A, B, C, D). However, the network interface will filter the packets that are not destined to that computer.

- In this case, only in A, the packet will cause an interrupt in the OS but the same packet does not cause an interrupt in C, D and B.

- The filtering is done by hardware.

- There is a special mode in the Ethernet interface called “Promiscuous mode” where all packets regardless of their destination are passed to the OS.

- This mode is used by network analyzers and packet suffer programs

- The program needs to have root privileges to be able to set interface in promiscuous mode.

- Snoop programs may use a lot of CPU because there is an interrupt for every packet that goes through the net.

Types of Address

- Single destination (Unicast, used most of the cases).

- All stations in the network (broadcast, not used very often because it causes a lot of overhead in the computers CPU in the network)

- A subset of stations (multicast) the stations in the subset add a common address to the list of multicast addresses in the Ethernet in the interface.

Broadcast in the Ethernet

- An is in destination address specifies broadcast

- The sender places the broadcast address in the packet. It transmits a copy of the packet and keeps a copy of the packet to itself.

- All stations receive a copy

- A receiver always accepts a packet that contains as destination

- The stations’ unicast address

- Or the broadcast unicast address

- Or Multicast address that exists in the stations network interface.

Format of Ethernet Packet

Preamble 6 bytes 6 bytes 2 bytes 46 – 1500 bytes 4 bytes

8 bytes

The minimum packet size is necessary to be able to detect collisions in an Ethernet network that uses a long wire.

Internet header is fixed for all kinds of Ethernet.

Ethernet Frame type contains the types of data sent type

0x800 -> Data is an IP packet

Ox8137, 8138 -> Data is an IPX packet (Novel)

0x806 -> ARP

…

Appletalk

Evolution of Ethernet

Original Ethernet

- It used a heavy coaxial cable

- Formal name : 10 base 5

- Also called Thicknet

Terminator

Second Generation

- Thin Ethernet

- Formal name : 10 base 2