Networking ITTB16 – Flashcards
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How many layers are in the OSI network model and what are their names?
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OSI network model has 7 layers L7:Application L6:Presentation L5:Session L4:Transport L4:Network L2:Data Link L1:Physical
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For each OSI Layer describe briefly what the layer defines or what happens at that layer?
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L7: interface between network and application software authentication L6: formatting and organizing of data encryption;compression L5: establishment and management of flows between endpoints transactional flows L4: segmentation of data connection establishment and termination; flow control; error control; L3: logical addressing;routing;path determination L2: preparation of data frames for transmission rules for using the medium; physical addressing L1: definition of the medium media,cables,connectors
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What are the names of the PDUs as generally referred to for TCP/IP for each OSI layer form L4 down to L1?
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OSI Layer name - PDU name - TCP/IP layer name Application - Data - Application Presentation - Data - Application Session - Data - Application Transport - Segment - Transport Network - Packet - Internet Data Link - Frame - Network Access Physical - Bit - Network Access
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What is decapsulation?
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The decapsulation process is the opposite of the encapsulation process ant it occurs at the receiver.
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What is encapsulation
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Encapsulation occurs at the transmitter/sender (Picture)..
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Name and explain some important L2 (Data Link) tasks?
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Framing -Where does each frame begin and end? Error control - Does the bit stream have errors? Flow control - How should the data flow be regulated? Medium Access Control - Who can use the medium when? Addressing - If the link is not point to point, then what are the source and destination?
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What is the PDU at L2 (Data Link)
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The Layer 2 (Data Link Layer) PDU is the FRAME encapsulates data coming form L3 into frames and passes each frame to L!, where the bits of the frame are encoded for transmission
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Draw a basic generic FRAME and name the 3 main parts?
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1) Header 2) Data( the payload) 3) Trailer
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What is the main function of the FRAME trailer?
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The main function of the trailer is error control. The trailer is thus also known as the frame check sequence (FCS)
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What is framing?
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Frames are the PDUs at L2(Data Link Layer) and the process of creating frames by the Data Link Layer is known as Framing.
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Name and explain some framing methods?
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1) Character count Field in header gives number of chars in frame. Char count includes the counting character itself 2) Flag bytes with byte stuffing Each frame starts with special start and end bytes (flag bytes). Here will imagine it as same byte, FLAG. After error, can always find start of next frame. 3) starting and ending flags, with bit stuffing Each frame begins and ends with bit pattern 01111110 (6 1's) If 5 1's in a row in data, stuff a 0 in so will never be 6 in a row. Stuff it in always - whether the next char was going to be a 1 or not. De-stuffer removes the 0's after any 5 1's.
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Why is code violation used intentionally?
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Thie method can only be used if there is some redundancy in the physical encoding, such as in biphase encoding
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What is ARQ?
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The purpose of ARQ (Automatic Repeat Request)is to ensure that the information is delivered in order and without duplication even when there are transmission errors; it is responsible for: -Error control - Flow control
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Name some different forms of ARQ?
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Stop-and-Wait ARQ -the sender sends one frame and then waits until it receives an acknowledgement (ACK) for that frame The receiver sends an ACK when it receives a valid frame -If the sender does not receive the ACK before a given timeout, the sender resends the frame Go-Back-N ARQ -Go-Back-N ARQ improves on Stop-And-Wait by not waiting - It continues to send frames -The channel is thus used more efficiently Selective-Repeat ARQ / Selective-Reject ARQ -the sender only retransmits frames that did not arrive or had errors
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What kind of addressing is used at L2? Where are L2 addresses relevant?
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The addressing at L2 is physical addressing - as apposed to the logical addressing found at l3 only in the local network -frames are re-encapuslated at the network borders
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At which OSI layers is Ethernet specified?
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Ethernet is a technology specified at L1 and L2
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What is the media access algorithm for Ethernet?
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Carrier Sense Multiple Acces with Collision Detection CSMA/CD -carrier sense before sending - collision detection before and after sending - jamming signal on collision detection - Exponential backoff algorithm
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How do the access method work?
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CS (carrier sense) : "Listen before talk" MA (Multiple access): Like ALOHA, the network was open to access by multiple devices CD (collision detection): "Liste after talk" - if there is a collision, send a jam signal so all stations notice the collision, the stop sending and set a timer, which is binary exponential with every repeated back-of, for retransmission
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What are the names of two standardized forms of Ethernet and Ethernet frames which are still in use today?
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DIX(Digital, Intel, Xerox) - Ethernet I - Ethernet II IEE802.3 defined by IEEE Both Ethernet II and IEEE802.3 are in use today and compatible which each other
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How do the frames of DIX and IEEE802.3 differ?
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Ethernet II(DIX) - 8 bytes PREAMBLE - 2 byte TYPE field PREAMBLE+DESTINATION+SOURCE+TYPE+DATA+PAD+CHECKSUM IEEE802.3 - 7 bytes PREAMBLE + 1 byte start frame delimiter - 2 byte LENGTH field PREAMBLE+STARTE FRAME DELIMITER+DESTINATION+SOURCE+LENGTH+DATA+PAD+CHECKSUM
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Two main sublayers in L2 that are defined in IEEE802.3?
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Medium Acces Control (MAC) Logical Link Control (LLC)
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What at which OSI layer does switching operate?
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switching operate at L2
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What at wich OSI layer does routing operate
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routing operate at L3
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How does a switch learn MAC addresses?
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Like bridges switches learn MAC addresses form the source destination address in incoming frames. It knows at which port the frame came in, so it knows that the node with this MAC address resides in the segment attached to this port. It adds the address and the matching port to its MAC address table called CAM(Content Addressable Memory) table Switches make their forwarding decision on the basis of the destination MAC address in each frame.
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What is a collision domain?
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A collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters or hubs, particularly when using a early version of Ethernet.
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When does a network collision occur?
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It occurs when more than one device attempts to send a packet on a network segment at the same time.
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Name devices that separate collision domains and some that don't?
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Hubs,Repeaters => one collision domain -shared media Bridges,Switches => separate collision domains -with full duplex and one host per switch port, there are no collisions. Thus this is a point to point connection where no collisions can occur
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Possibilities a switch habe when making a decision what to do with a frame
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Filter - do not send the frame Forward - send the frame out a particular segment/port Flood - send out all ports except the port where frame came in
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What is a broadcast domain?
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A broadcast domain is a logical division of a computer network in which all nodes can reach each other by broadcast at the data link layer
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Which devices separate broadcast domains?
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Switch: creates a entire broadcast domain Switch with several VLANs: every VLAN is a broadcast domain Routers: separate broadcast domains;don't forward L2 broadcast
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What is different in Ethernet of today from legacy Ethernet?
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-Switches are used instead of Repeaters,Hubs and Bridges -100 Mbit/s(Fast Ethernet) and 1 Gbit/s(Gigabit Ethernet) -Fully switched,full duplex network with one node per switch port
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What is the main protocol used for addressing at L3?
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IP (Internet Protocol) logical addressing - end to end addressing
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Name some functions performed by IP?
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Fragmentation Packet transport Logical Addressing
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What are some functions that IP does not include?
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Error control Flow control Acknowledgements Reliability Retransmission
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What describes an IPv4 address?
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network portion host portion 32 bits, dotteddecimal notation
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Why is IP fragmentation important, why is it needed?
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If the IP packet is bigger than the MTU(maximal transmission unit) than the packet is divider into smaller packets.
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What does ICMP stand for
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Internet Control Message Protocol -can be used to report errors
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command which uses ICMP?
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traceroute - ping
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Which protocol resolves IP addresses to MAC addresses?
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Address Resolution Protocol (ARP= -when an Ethernet hosts wants to send an IP packet, it needs a L2 MAC address
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If the destination IP is in the local subnet, which MAC address does the host need?
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The MAC added of the destination host
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If the IP is not in the local subnet which MAC address does the sending host need?
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the MAC of the gateway
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What can happen if there is a switching loop?
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-nobody can access the network -the network can become full of circling frames =>broadcast storm -switch may record wrong mac address table information and rewrite continually the table
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What protocol prevents switching loops?
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STP -(Spanning Tree Protocol) - 802.1 D RSTP - Rapid Spanning Tree Protocol - 802.1W
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4 STP states?
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Blocking Listening Learning Forwarding
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What is a bridge ID and what role does it play in STP?
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Switches have an ID called bridge ID, it is made up of a Bridge Priority ant the base MAC address of the switch. the bridge id(BID) is required for the election process of the root bridge and is part of the Bridge-PDU. Bridge-PDU is exchanged between bridges to maintain a loop free topology
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What are the basic steps in STP to decide which ports are forwarding and which are blocking?
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1) Elect a rood bridge - the bridge with the lowest route bridge ID becomes the root bridge 2) Determine the root ports - the root port is the port on a non-root bridge switch that has the least-cost-path to the root bridge 3) determine for each LAN segment one designated port - the port the advertises the lowest cost Hello onto a LAN segment becomes the designated port for the segment Root ports and designated ports go into forwarding state All other ports are put into blocking state
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What is RSTP?
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Rapid Spanning Tree Protocol (RSTP) - can converge in 10 seconds (STP 50 seconds - RSTP can revert to STP -standardized in 802.1W
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What is VLAN?
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VLAN is a virtual LAN. In technical terms, a VLAN is a broadcast domain created by switches. VLANs logically segment switched networks With VLANs, a switch can put different switch ports into different broadcast domains
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What is VLAN trunking?
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VLAN trunking allows switches to pass frames form multiple VLANs over a single physical connection.
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Between which devices or where is trunking found?
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Switches and Switch Router and Switch Firewalls and Switches Servers and Switches
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What should de relationship between VLANs an IP be?
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the devices in a VLAN should be configured to be in the same subnet!! devices in different VLANs should be in different subnets
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What device is needed for inter-VLAN communication?
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A router is required
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What is TDM?
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Short for Time Division Multiplexing, a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel.
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What are the 2 types of TDM?
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Deterministic TDM also called synchronous TDM Statistical TDM , asynchronous TDM
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Disadvantages and advantages of Deterministic TDM?
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Advantages: -each user gets a constant bandwidth - no addressing information required - no buffer necessary -protocol transparent Disadvantages: -bad utilization of the trunk(because of bandwidth lost if not used) Characteristic -connection oriented -users have a fixed time to send
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Disadvantages and advantages of Statistical TDM?
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Advantages: - shared bandwidth - good utilization of trunk Disadvantages: -not protocol transparent - Multiplexer require buffer - address information required Characteristics -buffering needed -variable size time slots
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Switching technologies?
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Circuit switching based on - Deterministic TDM -minimal delay -protocol transparent -examples: PDH,SDH/SONET Packet switching based on - Statistical TDM -switch must analyze address information -technology differences -- datagram principle: ---global addresses,connections,routing ---IP,IPX,Appletalk --virtual call principle: ---local addresses, connection oriented, switching table ---X.25,Frame-Relay-ATM
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What are the PPP's Layer 2 and Layer 3 protocols
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Point to Point Protocols Layer 2: Network Control Protocol(NCP) Layer 3 : Link Control Protocol(LCP)
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Which WAN technology uses a form of packet switching called cell switching?
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ATM (Asynchronous Transfer Mode) is a cell switching technology.
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Name 2 or 3 technologies used for leased lines and Internet backbones?
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PDH and SDH/SONET are used for leased lines and for network backbones
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Name some WAN technologies beside Ethernet or VPN with broadband? Name the WAN technologies discussed?
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Based on Deterministic TDM: PDH, SDH/SONET, ISDN Based on Statistical TDM: Datagram Principle: Virtual Call Principle: IP, IPX, Apple Talk, OSI CLNP X.25, Frame Relay, ATM, MPLS
