Test2

Advantages of Packet Switching

(1) it allows for arbitrary, asynchronous communication between many devices. (2) it requires no dedicated communication channel so no set-up is needed before communication begins. (3) performance may be enhanced by the use of statistical multiplexing. (4) It is also less expensive since all devises share the same transmission medium.

Non-Persistent CSMA

- If idle, they transmit. - If busy, wait random amount of time.

1-Persistent CSMA

- If idle, transmit immediately. - If busy, wait till it becomes idle. - If collision, wait random amount of time.

P-Persistent CSMA

- If idle, transmit with probability P. - If busy, wait till it becomes idle. - If collision, wait random amount of time.

Packet switching

- Networks use packet switching for exchange of data between computers. - In packet switching, no dedicated line is necessary and communication can be asynchronous (like exchanging email). - Many pairs of devices can communicate over a single line at anytime. - All computers on a network share a single transmission medium (e.g.,wire) and can send "at the same time". - Each sender must divide its message into "packets" of data before sending it to the recipient.

What is the primary responsibility of the data link layer?

- Protocols in the data link layer specify communication over a single network - How neighboring computers on the same network communicate. - Includes: --------- hardware addressing (MAC addresses) --------- maximum packet size that a network can support --------- how communication medium (wires or air) are accessed/shared by all the devises connected to the network.

Collision Detection

--> each station monitors the cable during transmission. If the signal on the cable differs from the signal that the station is sending, it means that a collision has occurred. - The technique is known as collision detection. - When a collision is detected, the sending station aborts transmission.

A collision can occur if...

.......two stations wait for a transmission to stop, find the cable idle, and both start transmitting. --> Even at the speed of light, time is required for a signal to travel down the cable. So a station at one end of the cable cannot know instantly when a station at the other end begins to transmit.

Following a collision.....

.....transmission does not abort until enough bits have been sent to guarantee that the collided signals reach all stations. --> Stations must wait for an interpacket gap (9.6 sec for a 10 Mbps Ethernet) to insure that all stations sense an idle network and have a chance to transmit.

Ethernet Uses 3 Mechanisms to Handle Collisions

1) Carrier Sense 2) Collision Detection 3) Binary Exponential Back-off

3 General Properties define a circuit switched paradigm:

1) Point-to-point communication ---• means that a circuit is formed between exactly two endpoints 2) Separate steps for circuit creation, use, and termination. ---• distinguishes circuits that are switched (i.e., established when needed) from circuits that are permanent 3) Performance equivalent to an isolated physical path. ---• communication between two parties is not affected in any way by communication among other parties. ---• circuit switching must provide the illusion of an isolated path for each pair of communicating entities.

3 Controlled Access Protocols

1) Reservation 2) Polling 3) Token Passing

IEEE divides Layer 2 of the protocol stack, into two conceptual sub-layers; what are they?

1) The Logical Link Control (LLC) • sublayer specifies addressing and the use of addresses for demultiplexing. 2) The Media Access Control (MAC) • sublayer specifies how multiple computers share underlying medium.

How do multiple, independent computers coordinate access to a shared medium? (3 ways)

1) They can use a modified form of multiplexing. 2) They can engage in a distributed algorithm for controlled access. 3) They can use a random access strategy.

3 Steps to compare a switched circuit to a phone call:

1- a circuit is established between two parties 2- the two parties use the circuit to communicate 3- the two parties terminate use

What is Port?

A "logical connection place" and , using the protocol, TCP/IP, the way a client specifies a particular server program on a computer in a network. Some applications that use TCP/IP such as the Web protocol, Hypertext Transfer Protocol, have ports with preassigned numbers (0-1023). Others are given port numbers dynamically for each connection. When a service (server program) initially is started, it is said to bind to its designated port number. As any client program wants to use that server, it also must request to bind to the designated port number.

CSMA/CD Flow

A --> B --> C --> D --<-----<----<----< A sends to D ----but B also sends to A There will be a collision. The senders will recognize a collision, and to prevent D from sending, the senders will send a jamming signal. All senders on that line will not send. NOTE: there is a math example that Ra went over. Wait Tb time = Tb = (R x Tp) or (R x Tfr)

Polling (CAP)

A controlled access protocol. Polling - centralized controller repeatedly polls stations and allows each to transmit one packet. Uses a centralized controller, that cycles through stations on the network and gives each the opportunity to transmit a packet. Controller can select which station to poll at a given time. 2 General Polling Policies:--------------- -------------------------------------------- 1)___Round Robin - each station has equal opportunity to transmit packets. 2)___Priority - some stations may have more opportunity to send. Ex. may assign a telephone to have higher priority than a computer. 2 images, slides 16, 17

Token Passing (CAP)

A controlled access protocol. Stations circulate at token; each time it receives the token, a station transmits one packet.

Reservation (CAP)

A controlled access protocol. Stations submit a request for the next round of data transmission.

CRC

A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.

Programming differences between a server and a client

A server is a program, or machine, that waits for incoming requests. In simplest form, a server is a connection point for several clients, that will handle their requests. A client is a program, or machine, that sends requests to servers. It connects to the server to perform actions. The client provide a user interface that allows users to carry out actions. It forwards these requests to the server, which carries out the action and returns a response.

What is CSMA/CD ?

Abort transmission as soon as detecting collision. -->Then wait random time (B.E.Back-off) and then try to send again. Max time for collision detection = (2*tau) Does not work well in wireless networks.

Binary Exponential Back-off

After a collision occurs, a computer must wait for the cable to become idle before transmitting a frame. All computers choose a random time to delay before sending again. The standard says a maximum delay, d, and each station to choose a random delay less than d. When two stations each choose a random value, the station that chooses the smallest delay will proceed to send a packet and the network will return to normal operation. In the case where two or more computers happen to choose nearly the same amount of delay, they will both begin to transmit at nearly the same time producing a second collision.

Star Topology

All computers attach to a central point. The center of a star network is often called a hub. A typical hub consists of an electronic device that accepts data from a sending computer and delivers it to the appropriate destination. A star helps protect the network from damage to a single cable because each cable connects only one machine.

User Datagram Protocol

Analogy - Sending mail through the postal system. You are relying on good faith of the system the mail will be delivered but you have no verification it ever got to the right place with no errors.

LAN Topologies - Each network is classified into a category according to its topology or general shape. What are the four basic topologies?

Bus Ring Mesh Star

What is little endian?

Byte ordering places the least significant byte first. This method is used in Intel microprocessors, for example.

What is big endian?

Byte ordering places the most significant byte first. This method is used in IBM® z/Architecture® and S/390® mainframes and Motorola microprocessors, for example.

Be able to evaluate the performance of CSMA/CA (...as compared to blah)

CSMA/CA performance is sub par compared to CSMA/CD because of the fact that CSMA/CA is used over wireless communication mediums. This opens up the potential for more lost packets and collisions. CSMA/CD is used over LANs and over a hardline medium (cat5) reducing the potential for attenuation and allowing each party to detect a collision. When a collision has been detected(CSMA/CD ONLY) the sender can start retransmission. If you cannot detect a collision because of the medium being used, you have to send (ACK) messages for each of the transmissions. This incurs overhead. The requirement of redundant triggering in this mechanism serves as a safeguard against wasting energy by trapping objects with no nutritional value, and the plant will only begin digestion after five more stimuli to ensure it has caught a live bug worthy of consumption.

CSMA/CA

Carrier Sense Multi-Access with Collision Avoidance The basis for WiFi wireless networks. Used over wireless mediums → more potential for packet loss and collisions Has more overhead, must send ACK for each transmission

CSMA

Carrier Sense Multiple Access

CSMA/CD

Carrier Sense Multiple Access w/Collision Detection

CDMA

Code Division Multi-Access

What is CDMA?

Code Division Multi-Access - We are a station, transmitting our data at the same time as other stations, on the same channel. CDMA allows the different stations to transmit over this channel, at the same time by encoding the transmissions mathematically, which will distinguish each transmission.

Circuit Switching

Communication mechanism that establishes a path between a sender and a receiver with guarantee isolation from paths used by other pairs of senders and receivers.

DUR/ID

Duration/Connection ID: If used as a duration field, indicates the time (in microseconds) the channel will be allocated for successful transmission of a MAC frame. In some control frames, this field contains an association, or connection, identifier.

Standards for Packet Format and Identification

Each packet sent across such a network must contain the identification of the intended recipient. All senders must agree on the exact details of how to identify a recipient and where to place the identification in a packet. Standards are when organizations create protocol documents that specify all details. Standards have been created by various organizations. Institute for Electrical and Electronic Engineers (IEEE) is most famous standard. In 1980, IEEE organized the Project 802 LAN/MAN Standards Committee to produce standards for networking

Carrier Sense

Ethernet requires each station to monitor the cable to detect whether another transmission is already in progress. --> this process is known as carrier sense. --> it prevents the most obvious collision problems. --> and substantially improves network utilization.

3 Channelization Protocols

FDMA - frequency division multi-access TDMA - time division multi-access CDMA - code division multi-access Channelization protocols extend the multiplexing techniques.

What UDP can offer?

Fast, connectionless, little overhead, but is unreliable. No congestion control UDP = Multiplexing/demultiplexing + light error checking

FC

Frame Control: Indicates the type of frame (control, management, or data) and provides control information. Control information includes whether the frame is to or from a DS, fragmentation information, and privacy information.

FDMA

Frequency Division Multi-Access

What is FDMA?

Frequency Division Multi-Access - When we have to extend frequency division multiplexing, this lets us as an independent station to choose a carrier frequency, that won't interfere with the stations that have chosen other carriers.

What is TCP timeout?

If no ACK is received, sends a second copy(/retransmits)

IEEE

Institute for Electrical and Electronic Engineers

IETF

Internet Engineering Task Force

Why do need network byte ordering?

Internet protocol (IP) uses 'network byte order' (big-endian) because it was agreed that it would do so (see RFC 1700. IP 139.156.32.45 in big-endian binary is 10001011.10011100.00100000.00101101 and this is how it would be transferred over the network. Depending on your architecture this address can be stored in a big-endian or little-endian format. If you are using X86 architecture (little endian), the bits (per byte) would be reverted, so they would end up being stored as 11010001.00111001.00000100.10110100. In a SPARC CPU, which uses big-endianness the bits would be stored like the IP protocol uses them.

What is Carrier Sense Multiple Access?

It is a network protocol that listens to, or senses network signals on the carrier/medium before transmitting any data. --Principle: Only one device can transmit on a network at a time. --Device listens before transmission. --If channel sensed idle, transmit entire frame. --If channel sensed busy, defer transmissions. However, if multiple devices access it simultaneously and a collision occurs, they both have to wait for a specific time before reinitiating the transmission process. i.e. Listen before speaking (carrier sensing) - If someone else is talking at the same time, you stop talking. (collision detection) COLLISION: entire packet transmission time wasted spatial layout of nodes. NOTE: role of distance & propagation delay in determining collision probability.

What is socket?

It is a one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to. Ex. Server has socket bound to port....listening. Client identifies themselves, and requests a connection. If server accepts, a new socket is bound to the local port. Needs a new socket, so it can use the original socket to keep listening for connection requests. A socket consists of three things: ---An IP address ---A transport protocol ---A port number 3 types of sockets (UDP, TCP, Raw)

What is a Stream Socket?

It is like a phone call -- one side places the call, the other answers, you say hello to each other (SYN/ACK in TCP), and then you exchange information. Once you are done, you say goodbye (FIN/ACK in TCP). If one side doesn't hear a goodbye, they will usually call the other back since this is an unexpected event; usually the client will reconnect to the server. There is a guarantee that data will not arrive in a different order than you sent it, and there is a reasonable guarantee that data will not be damaged. So you use a stream socket when having information in order and intact is important. File transfer protocols are a good example here. You don't want to download some file with its contents randomly shuffled around and damaged!

What is a Datagram Socket?

It is like passing a note in class. Consider the case where you are not directly next to the person you are passing the note to; the note will travel from person to person. It may not reach its destination, and it may be modified by the time it gets there. If you pass two notes to the same person, they may arrive in an order you didn't intend, since the route the notes take through the classroom may not be the same, one person might not pass a note as fast as another, etc. You'd use a datagram socket when order is less important than timely delivery (think VoIP or game protocols), when you don't want the higher overhead of a stream (this is why DNS is primarily a datagram protocol, so that servers can respond to many, many requests at once very quickly), or when you don't care too much if the data ever reaches its destination.

What TCP can offer?

It is reliable, but with more overhead

What is flow control?

It is the management of data flow between computers or devices or between nodes in a network so that the data can be handled. Too much data arriving for a device, causes data overflow, and data is either lost, or must be retransmitted.

What is a MAC address used for?

It is unique, and is used to make sure that a packet destined for a given computer is received by that computer only (i.e., there is no ambiguity as who should receive a packet).

Why are random access protocols expected to give better performance than a channel partitioning scheme?

Least amount of overhead, highest possible performance Channel partitioning means dividing a communication channel either into fixed time slots or fixed frequency bands. (TDM / FDM) With random access protocols, any node has full use of the channel but only one node at a time may use it. There are no scheduled time slots as in TDM. Because there is no schedule on who can transmit when, random access protocols have a chance of collision when multiple nodes decide to transmit at the same time. In a random access protocol, a transmitting node always transmits at the full rate of the channel, namely, R bps. When there is a collision, each node involved in the collision repeatedly retransmits its frame until the frame gets through without a collision. But when a node experiences a collision, it doesn't necessarily retransmit the frame right away. Instead it waits a random delay before retransmitting the frame. Each node involved in a collision chooses independent random delays. Because after a collision the random delays are independently chosen, it is possible that one of the nodes will pick a delay that is sufficiently less than the delays of the other colliding nodes and will therefore be able to sneak its frame into the channel without a collision.

LAN

Local Area Network - Least expensive; spans a single room or a single building. LAN technologies allow multiple computers to share a medium in such a way that any computer on the LAN can communicate with any other. - We use the term multi-access to describe this and we say that a LAN is a multi-access network. - In general, LAN technologies provide direct connection among communicating entities. Professionals say that LANs connect computers with the understanding that a device such as a printer can also connect to a multi-access LAN

Packet switched networks are classified by size. What are the main three classifications?

Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN)

LLC

Logical Link Control • Sub-layer specifies addressing and the use of addresses for de-multiplexing

MAC address Vs. IP address

MAC addresses "hardware address" are used for network communication over a single network. IP addresses are used for communication across multiple networks, for example on the Internet.

MAC

Media Access Control

MAC

Media Access Control 32 Bit IP address Network-Layer address. Used to get datagram to destination IP network. ex. 06 : 01 : 02 : 01 : 2C : AD First 3 bytes assigned by manufacturer. Last 3 bytes represent the manufacturer. IP address will change if you go to a new Internet provider. The MAC address will remain the same for your device regardless. MAC = Social Security Number IP = Postal Address

MAN

Metropolitan Area Network - Medium expensive; spans a major city or metroplex.

What is Pure ALOHA?

Original - 1970 - N. Abramson --Whenever a station has data to send it transmits it without waiting. --Time is always continuous. --The probability of successful transmission is S=G*e^-2G --The time of sender and receiver is not globally synchronized. --The maximum throughput occurs at G=1/2 which is 18 %

Ring Topology (Pros & Cons)

PROS Easy for computers to coordinate access and to detect whether the network is operating correctly. CONS An entire ring network is disabled if one of the cables is cut.

BUS TOPOLOGY (pros & cons)

PROS For small networks, BUS topology is the appropriate topology. Implementation and handling the topology is easy. CONS Since it suits small networks, the length of the cable is small and this defines the length of the network in which the stations are connected. ---Only limited nodes can be functioned with the help of this topology. ---Network is disabled if someone accidentally cuts the main cable.

Star Topology (Pros & Cons)

PROS Isolation of network devices is achieved in this star topology. The operation of data transmission is simpler due to its nature of centralization. CONS The main functionality of the network devices in this topology is achieved by the central hub. If the central hub fails the data transfer in the entire network gets affected

MESH Topology (Pros & Cons)

PROS The main advantage of the network is it transmits data from one node to multiple nodes. CONS ---Formula for number of mesh connections is (n^2 - n) / 2 where n is the number of computers in the network. Essentially, the number of connections required grows faster than the number of computers added to the network. ---Expensive. Also, redundancy of the network connection happens in this topology because when one node is connected to multiple nodes the purpose of many connections in the network between nodes is of no purpose.

How does a sender transfer a large file in packet switching?

Packet switching requires that a sender divides the message into small packets "no larger than the maximum packet size needed in a given computer" Packets from computer are then transmitted over the shared medium using statistical multiplexing.

ALOHA

Part of Random Access Protocols - Early network in Hawaii - Ground based, radio broadcasting. - Data Link Layer

Example: Port vs Socket

Port = 1030 Socket = (10.1.1.2 , TCP , port 1030)

LAN Topologies - Ring

Ring topology arranges for computers to be connected in a closed loop. A cable connects the first computer to a second computer, another cable connects the second computer to a third, and so on, until a cable connects the final computer back to the first. Ring topology requires a computer to connect to a small device that forms the ring. This is needed for the ring to continue operation even if some of the computers are disconnected

What is Slotted ALOHA?

Second - 1972 - N. Abramson --A user must wait until the next time slot to transmit data. --The time is discrete and divided into slots. --The probability of successful transmission is S=G*e^-G --The time of sender and receiver is globally synchronized. --The maximum throughput occurs at G=1 which is 37%

SC

Sequence Control: Contains a 4-bit fragment number subfield, used for fragmentation and reassembly, and a 12-bit sequence number used to number frames sent between a given transmitter and receiver.

Client vs Server

Server needs: -socket(), bind(), listen(), accept() Client needs: connect() TCP→ send(), revc() UDP → sendto(), recvfrom()

Random Access Protocol:

Some LANs do not employ a controlled access mechanism. Instead a set of computers attached to a shared medium attempt to access the medium without coordination. The term 'random' is used because access only occurs when a given station has a packet to send.

When using a sliding window of size N, how many packets can be sent without requiring a single ACK to be received?

There is not a limit other than the implied N. The initial sender can send N-packets, with N being related to the size of the sliding window. All N packets go to the receiver without the ACK. That said, once the sender sends one of the N-packets, while it does not prevent it from continuing with those N-packets, it is still waiting for an ACK in regards to that packet. Once it does reach that N-limitation, then the acknowledgement is required to proceed with further packets.

What is sliding window?

This is a packet-based data transmission protocol. This protocol is used for reliable, in-order delivery of packets, in both the Data Link Layer(OSI model) and in the Transmission Control Protocol (TCP). Each portion of the transmission (packets in most data link layers, but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to: ------Place received packets in the correct order. ------Discard duplicate packets. ------Identify missing packets. There is no limit on the size of the sequence number that can be required: ------BUT, by placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence number

What is a MAC address?

This is also referred to as "hardware address" is a unique address assigned to each computer (more accurately to each network interface card, e.g., Ethernet, wireless, etc.).

LAN Topologies - Mesh

This provides a direct connection between each pair of computers. The chief disadvantage of a mesh arises from the cost: A mesh network connecting n computers requires: ---Connections in a mesh network: ---------( n^2 - n ) / 2 IMPORTANT - the number of connections needed for a mesh network grows faster than the number of computers, and because connections are expensive, few LANs employ a mesh topology in special circumstances

LAN Topologies - BUS

This usually consists of a single cable to which computers attach. The ends of a BUS network must be terminated to prevent electrical signals from reflecting back along the BUS. Any computer attached to a bus can send on the cable, and all computers receive the signal. Because all computers attach directly to the cable, any computer can send data to any other computer. The computers attached to a BUS network must coordinate, to ensure that only one computer sends a signal at any time

TDMA

Time Division Multi-Access

What is TDMA?

Time Division Multi-Access - This is similar to FDMA. There are distinguished differences between when the stations can send, but these timed sections are assigned, and moderated after a station appears on the network. Dynamic allocation may be offered for TDMA

What is the sequence number?

To handle duplicate or out-of-order packet deliveries! Send attaches a sequence number to each packet. The receiver stores sequence number of the last packet in order. == Sequence # is next one to be expected _____________good == s.# has arrived out of order ____________protocol adds packet to list == s.# has already been delivered ____________software discards the new copy

What is ACK?

To help handle packet loss! Positive acknowledgment Reports successful reception

What is going to be happened if TCP timeout is too long or too short?

Too short: premature timeout → unnecessary retransmissions Too long: slow reaction to segment loss

What is TCP?

Transmission Control Protocol Analogy - Mailman delivering mail directly to house and getting recipients signature. Full duplex, p2p, pipelined, send & receives buffers, flow controlled

UDP

User Datagram Protocol

What is byte/bit stuffing? Why do we need it?

We have to be able to distinguish parts of transmitted data, such as sequences of bits that will represent specific data-based details such as the start and end of a frame. We have to make sure that we can represent, and distinguish both the data, and the control information. A sender must change control bytes from a frame delimiter, with a specific sequence, which the receiver will return back to the original value. Because of this repeated swap, we can use the frame to transfer arbitrary information, which won't be confused with the related control information.

WAN

Wide Area Network - Most expensive; spans sites in multiple cities.

W3C

World Wide Web Consortium

IEEE assigns a multi-part identifier of the form XXX.YYY.ZZZ What are those 3 layers?

XXX denotes the category of the standard YYY denotes a subcategory If a subcategory is large enough, a third level can be added