Data Communications is the Real World OSI Layers 1 & 2 a.k.a TCP/IP Network Interface Layer

What is a Local Area Network  WikiPedia: a computer network covering a small physical area….  WhatIs.com: a group of computers and associated devices that share a common communications line or wireless link.  Most verbal definitions follow these two main definitions: close in area, or on a common link.

Ginter Defines LANS:  Any group of computer that can communicate at layers 1 & 2 of the OSI reference model.  This means that all of these computer share a common broadcast domain.  This also means that all of the systems are on the “inside” of a router if the LAN connects to a greater network.  What does all of this mean?

LAN Addressing  All Network Interface Cards have a unique address burned into them by the manufacturer.  This address has a lot of names: Hardware Address Hardware Address Physical Address Physical Address Ethernet Address Ethernet Address Media Access Control (MAC) Address Media Access Control (MAC) Address

LAN Communication Uses Frames BytesNameDescription 7Preamble Bit-pattern to ID Frame Start 1FDS IDs Frame Start 6DestinationMAC MAC Address of the Destination MAC 6 Source MAC My MAC 2Length/Type IDs number of bytes/Frame Type Data The Payload 4FCS Frame Check Sum

What about Errors  As the data is added to the frame a formula is calculated using the data.  At the receiving end, the same formula is used.  If there is an error: one of the following: 1. An automatic repeat request (ARQ) 2. Cancel the error-packet (ignore or pass on)

LAN Communication  Host A sends message to Host B. Media Access Control: Can Host A Send? Media Access Control: Can Host A Send? Addressing: Who am I, Who Gets this. Addressing: Who am I, Who Gets this. Queuing of Data Packets Queuing of Data Packets  Host B receives a Packet Checks to see if Destination Address is mine? Checks to see if Destination Address is mine? If so, Check the frame type and pass it up. If so, Check the frame type and pass it up. If not, it goes to the bit-bucket. If not, it goes to the bit-bucket.

LAN Equipment  LAN Equipment works at Layers 1&2  Hubs: Wired as a Star, but acts like Bus.  Switches: Wired as a Star and acts it.  Wireless Access Points  Bridges

They all Look the Same

Hubs  Connect as Star, Looks Like Bus.  When a host transmits (on transmit lines), Hub sends out on ALL ports Receive Lines  This behavior is just like a BUS connection.  Hubs have Contention.  Access Control is Carrier Sense, Multiple Access, Collision Detect (CSMA/CD)

CSMA/CD  Carrier Sense Listen First, then talk Listen First, then talk  Multiple Access Just means many can connect Just means many can connect  Collision Detect Listen to the receive lines. Listen to the receive lines. If the signal is different than what I sent then there is a collision (contention) If the signal is different than what I sent then there is a collision (contention)  This is like being at a Party where only two people are allowed to talk at one.  Similar to have a Phone on a “Party Line”

CDMA Logic Diagram

Switches  Like Phone Switches (after 1964).  Everyone gets a “Private Line”  Switches only send received data to the port that has the destination address attached.  Forwarding Methods: Store and Forward (buffer, err chk, forward) Store and Forward (buffer, err chk, forward) Cut Through (read destination addr, forward) Cut Through (read destination addr, forward) Fragment Free (errchk first 64B, forward) Fragment Free (errchk first 64B, forward) Adaptive: select between the previous three Adaptive: select between the previous three

Switching is Cool!!!!!  Virtually Eliminates Contention.  Allows Full-Duplex Connections Can Send and Receive at the same time Can Send and Receive at the same time  Allows for management of ports Quality of service Quality of service Traffic monitoring Traffic monitoring Virtual LAN Virtual LAN Use of redundant connections for reliability (Spanning Tree Protocol). Use of redundant connections for reliability (Spanning Tree Protocol).  This is a Party where Everyone can talk at once.

Wireless Access Points  Wireless Access Points are like Hubs without Wires.  WAPs use CSMA/CA Collision Avoidance disables the ability of a node to transmit if another one already is. Collision Avoidance disables the ability of a node to transmit if another one already is.  WAPs have speeds of 11 Mbs, 54 Mbs, and (about) 300 Mbs on b, g, and n respectively.

Review  Data Communication in the Real World (OSI Layers 1 & 2, TCP/IP Layer 1)  What is a LAN?  LAN Addressing  Frames  Error Detection and Handling  Hubs  Switches  Wireless Access Points