CSEC 701 Computer Networks Review

network

a set of devices connected by communication links

local area network LAN

priavtely owned and links the devices in a single office, building, or campus, is a direct connection (ethernet)

wide area network WAN

long distance transmission over large geographic areas, an indirect connection (internet)

open system interconnection OSI model 

framework for implementing protocols and networking components in seven distinct layer, not a working model, control passed down layers and then passed bottom up through next machine

TCP/IP

transmission control and internet protocol

IETF: internet engineering task force

architectural design of the internet software system, RFC internet standard, ICANN

application

supporting network applications (FTP, SMTP, IMAP)

transport

host-host date transfer (TCP, UDP)

network

routing of datagrams from source to destination (IP, routing protocols)

link

data transfer between neighboring network elements (PPP, Ethernet)

physical

bits “on the wire'“

how does the protocol work

when your application generates data, it gets broken into small units called packets, that are sent out with a packet header (new header is attached at each layer)

cable attack

new undersea threat is starting to reshape naval wars

layer 1: physical layer 

mechanical and electrical specification of the interface and transmission medium, procedures and functions that physical devices and interfaces have to perform

layer 2: data link layer

transforms the physical layer to a reliable lin

DL

hop-by-hop delivery

DL layer functions

data link control (LLC): communication between two adjacent nodes; media access control (MAC): how to share link among multiple devices

media access control(MAC)

single shared broadcast channel (collision) and multiple access protocol 

MAC address

48 bit address burned in the adapter ROM, administered by IEEE, manufacturer buys portion of MAC address space

ethernet

dominant wired LAN technology, shared medium wire, prone to collision

CSMA/CD

sense if the line is free, then transmit data; if collision is detected the devices stop transmission and send a jamming signal  

preamble

8 bytes: 7 with pattern 10101010, 1 byte with pattern 10101011; used to synchronize receiver and sender clock rates, serves as start-of -frame

addresses

6 bytes (48-bit each), if adapter receives frame with matching destination address, it passes data to network layer, otherwise the adapter discards the frame

promiscuous mode (ethernet frame)

allows listening to all frames 

type (ethernet frame)

indicates the higher layer protocol

data payload (ethernet frame)

46-1500 bytes

FCS (ethernet frame)

CRC Checksum, checked at receiver, if error is detected then the frame is simply dropped

minimum Ethernet frame length

chosen as 64 bytes for proper operation of CSMA/CD _{if upper layer data is less than 46 bytes than padding is needed}

maximum ethernet frame length

1518 for the whole frame, to limit buffer memory requirements and to prevent monopolizing

maximum transmission unit MTU

size (in bytes) of the largest packet that a given layer of communications protocol can pass onwards, larger packets cause fragmentation 

length (type field in Ethernet II)

number of bytes in data field, if value is greater than 1536 that it is Ethernet II frame