Flow Control

A mechanism that ensures the rate at which a sender is transmitting is in proportion to the receiver. Since in data communications multiple different devices are used, and each device may bring about different transmission parameters, this may lead to conflict if the sender starts transmitting data faster than the receiver can receive. By using flow control, this makes sure that the sending/receiving capabilities of both nodes are managed efficiently and there are no concerns.

Latency

Latency is the “lag” in a system. High latency creates congestion in any network communication. It prevents the data from fully utilizing the network pipe and decreases the communication bandwidth.

Causes of Network Latency

Total latency is the sum of all delays, usually in ms.

Congestion

• • Network state where a node or link carries so much data it may deteriorate (make/get worse) the network server quality - queuing delay, frame or data packet loss & blocking new connections.

  • Congestion occurs when bandwidth is insufficient and network data traffic exceeds capacity.

  • Congestion has been described as fundamental effect of limited network resources, especially router procession time throughput. Intermediate routers may actually discard data packets when they exceed its handling capacity. In this case, additional data packets may be sent to make up unreceived packets, which exacerbates the problem.

    • • Routers capable of reordering or dropping data packets when received rates reach critical level

      • Flow control mechanisms that respond appropriately when data flow rates reach critical levels.

  • Xon-Xoff is an example of flow control protocol: that syncs (happens at same time) the sender with the receiver. It transmits a transmit off signal when the receiver no longer has space in its buffer and transmit on signal when the receiver can resume taking data. The sending node buffers its data until it receives an on signal from the destination node.  

Deadlock

• • A buffer is a storage in a router on a packet switching network that can hold or store packets until the packet can be sent or resent if there is an error.

  • If a network is congested and all the buffers are full, obviously it can’t accept any further data, therefore ignores and rejects additional frames.

  • If two adjacent routers are sending packets to each other, since both are waiting for each other to accept a packet neither can proceed, and all buffers are full and no packets can be accepted, neither can proceed.

  • Deadlock occurs in a network 

    • • when packets are unable to act because of waiting for each other to release some resource. 

      • when two more competing actions are waiting for others to finish and thus neither ever does.

Internet Protocols

TCP/IP

• • Transmission Control Protocol/Internet Protocol refers to protocols and utility programs used for low-level network communication. Name reflects the nature of the relationship TCP on top and IP at the bottom.

  • TCP/IP is a packet switching protocol that divides the transmission into packets of information, suitable for retransmission across the internet. Depending on the scheme it moves through the internet on different routes to reach the intended address.

  • IP software deals with the routing of packets through the web of interconnected networks to their final destination.

  • TCP software breaks messages into packets hands them off to IP software for delivery and then orders and reassembles them back together at the destination. 

  • TCP deals with any errors like a packet never arriving.

Other Protocols

• • UDP - User Datagram Protocol. Alternative to TCP. Main difference TCP is highly reliable but slower while UDP is faster but less reliable.  

  • High Level Protocols - Deals with specific types of network connections (SMTP, FTP, HTTP). Built on the foundation of TCP/IP

Domain Name Systems (DNS)

• • A distributed system for managing hostname resolution

  • A hierarchical distributed system of Domain Name servers to maintain a directory of domain names and translate them to corresponding IP addresses. Used to go on web addresses and such.

  • Each web address has a corresponding IP address, and your operating system will either have already stored the IP address for the web address or query it form a series of DNS servers.

File Transfer Protocol (FTP)

• • Protocol to allow transfer of files from 1 computer to another.

  • May need to be authenticated using usernames and passwords.

  • Anonymous FTP allows users to access files programs and other data from the Internet without the need for a user ID or password.

  • FTP allows for easy transfer of files from a computer to the Web.

E-mail Protocols (POP3, IMAP, SMTP)

• • Simple Mail Transfer Protocol (SMTP)

    • Protocol that specifies the sending and transmission of electronic mail. Used when email is delivered from an email client, like outlook, to an email server or when an email is delivered from one email server to another

  • Post Office Protocol (POP3)

    • A protocol that extracts and retrieves emails from a remote mail server for access by a client device. Creates local copies and deletes them from the server.

  • Internet Message Access Protocol (IMAP)

    • Protocol for accessing email on a remote server from a local client. Users don’t have to tied to a single email client allowing flexibility as if the emails were on the cloud. This allows for users to view email on phones computers etc. Very popular as compared to POP3 as IMAP allows for multiple devices to view while POP3 doesn’t.

Data Integrity

Refers to the overall completeness, accuracy and consistency of data. Must be followed through throughout the sending process and can be achieved through error checking and correction protocols.

Error Detection

Class of techniques for detecting damaged messages.

Schemes include:

• • • Parity Bits

    • Checksums

      • • Each message is accompanied by a numerical value based on the number of set bits in the message. The receiving station then applies the same formula and checks to make sure the accompanying numerical value is the same. If not then the message is corrupt.

        • UDP and TCP use this

        • Advantage is their size and ease of computation, requires very little computation and the cost is negligible.

        • Basic Checksums may be the number of bytes in a file, but unreliable because bytes may be switched around but the total number will be the same.

    • Advanced Checksums

      • • Different more advanced checksum algorithms like

          • • Cyclic Redundancy Check

            • Cryptographic Hash Functions

Wireless Access Point (WAP)

• • Hardware device or configured node on a LAN that allows wireless capable devices and wired networks to connect through a wireless standard, including Wi-Fi or bluetooth. A WAP is also known as a hotspot.

  • Access points allow wireless networks to join an existing wired network, usually deployed when growing a network that already has routers installed.

Wireless Network Software

• • User interface used to provide connectivity to the network and configure the wireless router or access point. This will give the user options to configure authentication for the network.

Notes from ‘The Dawn of the Net’ video

• • Any packet that is to be sent to the Internet will be given an ‘address’ that is to be read by a proxy server

    • The proxy opens the packet and looks for the address (or URL)

    • If the address is acceptable, then it is sent to the Internet

  • The information is sent along a LAN through packet with a header

  • The router redirects

  • Proxy ⇒ the firewall

    • Blocks ‘nasty things’ from the Internet/network

    • Prevents sensitive corporate information from entering the Internet

  • Firewall ⇒ the Internet

    • Routers + switches establish links between networks

    • Is much larger and much less protected than a LAN

  • The Internet ⇒ Second firewall

    • Only lets in packets that meets its criteria

  • Request is then granted if packets meet the network criteria