Networks Flashcards
Mail server*
Acts as a virtual post office, for all incoming and outgoing mail. They route mail according to their database of local network user’s email address’. They also STORE MAIL UNTIL IT CAN BE RETRIEVED
TCP/IP
A set of protocols acting together as four layers connected, passing incoming and outgoing data up and down the layers during network communication. It enables communication with any other computer connected to the internet, regardless of its location
Each layer in TCP IP…
Only communicates with adjacent layers.
Envelopes/ unwraps with more packet data.
Has different roles, with various protocols
Application layer
Uses protocols relating to the application being used to transfer data over a network (e.g. Internet)
E.g. Browser may use ftp, or http or POP3 as the high level protocol
Transport layer
TCP protocol used to establish end to end connection with recipient computer.
Splits data into packets, labels with packet sequence number, total packet number and port number (ensuring correct application handles packets.)
If packets go astray, retransmission requested .
Receipt of packets acknowledged
Network layer is aka
IP layer/internet layer
Network layer*
Adds source and destination IP addresses (port number + IP address = socket)
Router operates on this layer, forwarding packet onto correct destination according to IP addresses
More ERROR checking
router is hardware that operates at this layer
Link layer
LINK LAYER PROTOCOL used e.g. Ethernet
Physical connection between network nodes
Mac addresses are added, identifying NICs of the source and destination computer (where the packets are to be sent to next)- initially of the first router being sent to, unless same network
So once correct network found, correct hardware then can be found
Sent down CABLE
Cable and NIC are hardware that operate at this layer
Receiving end of TCP IP
MAC addresses stripped by link layer
IP addresses stripped by network layer
Port numbers removed and packets reassembled at transport layer
Then passed to application and data presented to user
Routers in IP TCP
Operate at network layers, and so MAC addresses or source and destination change at each network node
Packets move up and down lower layers at each switch/ router between client and server
File transfer protocol definition
An efficient method used to transfer data across a network, often the internet
Describe FTP*
Works as a HIGH LEVEL PROTOCOL IN THE APPLIC LAYER USING APPROP SOFTWARE
User is presented with a file management screen with file and folder structure in remote website and local computer
Files transferred via drag and dropping
Often need username and password but can use anonymously
Uses of FTP
Uploading images to remote newspaper headquarters by photographers
Companies offering large updates
POP3, post office protocol*
Protocol used to retrieve mail from a mail server TEMPORARILY STORING INCOMING MAIL
It is transferred to local computer then deleted from server so can’t synchronise across devices
Developed before WWW when mail stored on hard drive
IMAP internet message access protocol
Email protocol designed to keep emails on mail server, ensuring mail is synchronised (synchronicity) across devices
Used in PUSH email (sending email without request)
SMTP
Used to transfer mail from one server to another and from a client to a server
Bit after @, domain, shows which mail server to direct mail to. First own mail server then recipient’s
Client server network*
1 or more computers: clients are connected to a (or many performing different tasks) powerful central computer called servers
Each client may hold its own files and resources e.g. software and can also access resources held centrally by server
Server also DEALS WITH REQUESTs
EG in schools, businesses, hospitals
Why have many servers in a client server network
For many different tasks
File server*
Holds and manages DATA for all the clients
Print server
Manages print requests
Web server
Manages requests to access the web
Database server
Manages database applications
Requests in client server network
Client sends request to the server which then processes the request and responds with the requested data providing the device has permission to access the data
Adv client server network
- better since all files stored in a central location and access rights are managed by the server centrally, as well as authentication
- no need for individual backups of data because done centrally. If breakdown and data is lost- recovery procedures enable restoration
- data and other resources can be shared eg network printers and other peripheral devices
- can access server from any workstation in network and even at home with VPN
- downloading software and patching done centrally, saving time and bandwidth when keeping up to date
- one license rather than individual licenses for clients
- security and antivirus handled centrally
Disadvantages client server network
Expensive to install and manage OS and servers
Need professional IT staff to maintain servers and run network
Server failure = massive disruption to whole network
Peer to peer network*
No CENTRAL SERVER. Individual computers are connected to each other and share FILES which are stored on the hard disk. Can be over LANs or WANs. Particularly good if in one site e.g. Small office or home. All computers are 1) of equal importance 2) act as both a client and a host. Hosts can decide access rights for resources
Advantages peer to peer
Cheap to set up
Not difficult to maintain
Enables users to share resources e.g. Printer or router
can be more reliable, not dependent on companies server to distribute all requests from all users
Uses of peer to peer*
Video on demand and downloading videos.
Different computers in different LOCATIONS hold parts of the video so share the load so faster- main principle behind torrent websites: enabling sharing of files, often containing copyright material
internet*
a worldwide network of networks set up to allow computers to communicate with each other globally, regardless of their location, by using the same protocol
how was internet developed
via ARPANET created in 60s for US defence military project enabling distant departments working together to communicate without physical travel
WWW*
world wide web: a collection of webpages (i.e. LINKED HYPERTEXT) that reside on computers connected to the internet. the internet is the service used to communicate information on these pages
intranet
a private network using internet protocols
who developed WWW
Tim Berners-Lee, developed concept of WWW and browsing for information
physical structure of the internet
each continent has backbone cables connected by transcontinental leased lines fed across SEA BEDS and ISPs connect directly to this backbone and distribute and sell internet connection to smaller providers which distribute to individual homes and businesses
URL*
uniform resource locator: FULL address of a location of an internet resource inc file type and resource name. BROWSER REQUESTS FROM WEBSITE SERVER
http://www.domainname.com/folder/subfolder.html#element
http is the protocol being used, www is the hostID, folder is the pathname, and #element is the resource
URN
uniform resource name, unique resource name allowing you to find a resource even if moved to a new site/new page on the same site e.g. isbn (namespace identifier) 02498459 (namespace specific string)
URI
uniform resource identifier made up of URN and URL used to identify the target of a hyperlink
Internet registrar*
private companies which must be accredited by governing registries, it hold LISTS and RECORDS of all existing website names and details of domains available to purchase currently
internet registries*
5 global organisations governed by ICANN (internet corporation for assigned names and numbers) with WORLDWIDE DATABASES holding records of all domain names currently ISSUED to individuals and companies, and their DETAILS e.g. name, type, registered mailing address, registrar that sold and data of registry
They allocate IP address’ and keep track of corresponding domain name as part of the DNS
what does a domain name do* and how its set up
identifies the area/domain a webpage resides in. Theres a HIERARCHY of smaller domains, written as a string with full stops, dictated by RULES of DNS
first level domains
generic top level: .com, .edu, .gov
country top level: .uk, .fr
second level domains
.sch, .co, .gov
third level domains
.bbc, .lidl, .ebay
root domain
www.ROOTDOMAIN.com
subdomain
WWW.domain.com
www is the subdomain
DNS*
CATALOGUES all domain names and IP addresses in a series of GLOBAL DIRECTORIES, which domain name servers access in order to find the correct IP address location for a resource
without DNS
have to type in IP address in URL bar instead of domain name
process of DNS*
webpage requested by URL entered by user, browser requests for IP address from a local DNS (firstly ISP with preprogrammed database of IP related to domains), if not found, search extended up hierarchy to larger DNS database and when found, user’s computer sends data request to that IP address location to find webpage data VIA IP/TCP PROTOCOL
FQDN
fully qualified domain name containing host server name e.g. mail.google or www.google, depending on server resource is hosted on
IP address*
internet protocol address, a unique address assigned to a NETWORK DEVICE indicating where a packet of data is to be sent/has been sent from. routers use this to direct packets
IP addresses of domain names*
the address is of the server the site is residing on
IP addresses are made of
four numbers IPv4 with number between 0-255 and now IPv6 has 6 numbers because so many IP address being used
272.15.63.144
272 is the geographical location
272.15.63 is the network ID identifying a network
144 is the hostID identifying hosts (individual devices) on a network
computer network
a number of computers linked together to allow the sharing of resources e.g. software, hardware, data
server
a powerful central computer able to provide one or more services to a network and its users e.g. file storage
WAN
wide area network, a network of interconnecting networks inc LANs that spread over a large geographical locations (e.g. continents) can be connected using internet, satellite links and leased lines (rely on third party carriers eg british telecom) e.g. internet, bank cash dispensers.
LANs use routes to connect to these other networks
LAN*
local area network, a number of a COMPUTING DEVICES single site/building consisting of computers, printers and a central server for example. Users SHARE hardware and data and can COMMUNICATE to each other. data travels quickly over these short distances
physical bus topology
all computers connected to a single cable and the end of cable is connected to a terminator
adv of physical bus
inexpensive to install as less cable and no additional hardware
effective for small networks
disadv of physical bus
low security, all other computers can see data transmissions
traffic means degraded performance
if main cable fails then network data can’t be transmitted to any of the nodes
troubleshooting individual devices issues is hard
collisions
physical star topology
has a central node, a switch or computer which acts as a router to transmit messages.
switch has a record of MAC addresses of each device on a network and can identify particular computers on networks to send data to
adv of physical star*
- one cable FAILS, only one station affected
- consistent performance even if heavily used
- COLLISIONS aren’t a problem: each station has own cable to server
- higher transmission speeds: better performance
- more SECURE: messages sent directly to central node and harder to intercept
- easier to add new stations without disrupting networks
disadv of physical star
- costly installation e.g. cost of cable
- if central switch goes down, network data can’t be transmitted to any more nodes
- number of nodes restricted by capacity of central device
physical topology
the physical design layout of a network, important for when deciding wiring layout
logical topology
shape path of data travels in and describes how components communicate
topologies are…
independent of each other so a physical star topology can act logically like a bus topology using appropriate PROTOCOLS and physical SWITCHING
ethernet topology
logical bus topology regardless of physical layout
wifi*
local area wireless technology allowing devices to connect a device to a network resource or internet via a wireless network access point (usually 20m indoors but more outdoors)
1999 WiFi alliance
group of 100s of worldwide companies formed to establish international standards for interoperability and backward compatibility
enforces use of standards for device connectivity and network connections
mesh network topology
each nodes is connected to all other nodes by transmitting data over intermediate nodes
only one node requires internet access, others share connection
use of mesh network topology
more common with widespread use of wireless technology. can spread over whole cities
adv of mesh network topologies*
- no cabling COSTS
- self healing- if one connection breaks it can be circumvented by another route so more nodes = faster and more RELIABLE
- new nodes automatically incorporated into network
- FASTER communication as data packets don’t pass through central switch
disadv of mesh network topologies
- harder to manage: continuous supervision
- many devices needed = expensive
circuit switching
creates a direct link between two devices for the duration of the communication e.g. public telephone system. so you have all the bandwidth between two points and no other messages can use the connection
adv circuit switching
- you have all the bandwidth between two points and no other messages can use the connection
- exclusive connection means packets arrive in the same order that they are sent, simplifying the reconstruction process
disadv circuit switching
- bandwidth is being wasted when no data is being sent
- data has to be transmitted at the same RATE by both devices so only devices operating at same transfer rate can be connected
- switches connecting and disconnecting circuits cause electrical interference so data is lost and corrupt if path is used to transmit data: so leased line can be used instead if problem
packet switching*
method of communicating packets of data across a network whilst other network communications are happening simultaneously e.g. website data and emails are sent and RECEIVED as a series of PACKETS. relies on packets to travel via entirely different ROUTES (fastest and least congested)
what are data packets*
manageable chunks/packets of data BROKEN DOWN from data to be sent across a network. the size can be FIXED or VARIABLE: usually between 500-1500 bytes
payload of data packet
body of data being sent
trailer of data packet
cyclic redundancy check/checksum: a hash total is created and attached, calculated from packet data to detect transmission errors. calculated by number of 1s in transmission. CRC is RECALCULATED and MATCHED at end of transmission to verify data is unchanged- if totals differ packets refused with suspected data corruption and a NEW COPY requested from sender
header*
IP address of sender and recipient, PROTOCOL used for this packet type, packet sequence number, time to live/hop limit when packet expires and discarded
packets at other end
reassembled in correct order and packets requested for again if didn’t make it.
data extracted
advantages of packet switching
- network failures don’t stop messages
- easy to add more nodes
- secure, unpredictable route and packet split
- takes advantage of unused connections
- cheap as efficient use of bandwidth (only charged per unit of data rather than connection time)
disadvantages
- can be corrupted in transmission
- must be reassembled at end of transmission
end to end principle
endpoints control communication and security, midpoints just pass on packets
open architecture principle
anyone can design a network just through creating a linking computer called a gateway
MAC address*
media access control address. each device capable of connecting to a network has a wired or wireless NIC with a unique MAC address ASSIGNED to and hardcoded by a manufacturer uniquely identifying a device (48bits long, 12 hex digits)
protocol*
a set of rules defining common methods of DATA communication- standard rules allow devices to COMMUNICATE with each other
HTTP
standard protocol for browsers to render webpages
telnet*
- allows communication between one computer and another
- allowing a user on one computer to log into another computer on the SAME NETWORK
sending mail from one place A to another B
A uses SMTP to send message; this goes via her LOCAL EMAIL SERVER which checks the DOMAIN to which the email is addressed e.g.@domain.com; and forwards the message (often via other mail servers); to B webmail server. B logs in via his local ISP; and downloads his message using POP3/IMAP.
switches and router are used for?
handling traffic and ensuring requests and files get to the correct location
wifi workstations
when you connect to a network using a wireless access point
router
connects at least two networks: 2 LANs/WANs or LAN with ISP’s network. Reads the recipient’s IP address in each packet and forwards it on to recipient using fastest, least congested route to next router. This continues until destination is reached. routers use routing tables to store and update locations of other network devices and most efficient routes to them. Uses routing algorithm. When router is connected to internet, IP address of port connecting it must be registered with internet registry bc it must be unique across the internet
routing algorithm
Finds optimum route between network devices. This routing algorithm can become a bottleneck in network traffic since decision making process is complicated. Shortest path algorithm: Dijkstra’s algorithm.
hop
traversing between one router and another across a network
gateway
routes packets between networks if the protocols differ. strips header data, leaving raw data only. new data is added in the format of the new network before packet sent again.
