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OSI reference Model
(7) application
(6) presentation
(5) session
(4) transport
(3) network
(2) data link
(1) physical
Physical Layer (1)
sends and receives bits across the network.  includes cable (10base2, 10base5, etc), interfaces, network topologies (star, mesh, ring, bus) data rate specifications, Hubs, repeaters and ISDN.
Star topology
each individual node is directly connected to a switch, hub, or concentrator.  Devices become single point of failure.  Most common topology.
Mesh
All systems are interconnect to provide multiple paths to other resources.
Ring topology
A closed loop that connects end devices in a continuous ring.  Devices are connected to a Multistation Access Unit (MAU).  Common in token-ring and FDDI networks. communication travels in a single direction.
Bus
All devices are connected to a single cable (backbone) and terminated at both ends.  The backbone is the single point of failure.
Twisted Pair Cable Categories
Cat 3 - Data (10 Mbps) and voice.  Ethernet and telephone.
Cat 5 - Data (100Mbps).  Fast Ethernet
Cat 6 - Data (1000Mbps at 250 Mhz). Gigabit Ethernet
Cable types and Characteristics
RG58 (thinnet): 10base 2 (185 m)
RG8/11 (thicknet): 10base5 (500 m)
UTP/STP: 10baseT, 100baseTX, 1000baseT 10GbE (100 m)
Fiber: 100BaseF (2000 m)
repeater
non-intelligent device that amplifies a signal to compensate for attenuation.
Hub
Used to connect multiple LAN devices together; servers, workstations. Two basic types of hubs are:
Passive (data enters one port and exits all other ports without any signal amplification and regeneration)
Active (combines features of a passive hub and repeater.  Known as a multi-port repeater)
Data link Layer (2)
Ensures that message are delivered to the proper device across a physical network link.  Defines the networking protocol (ethernet, token ring, L2f, PPTP, L2PT,etc.)  Consists of the Logical Link Layer and MAC.  Bridges and switches are used.
Logical Link Control (LLC)
Used to format messages from layers above into frames for transmission, point to point synchronization and error control; can perform link encryption.  Monitors flow control of data between devices.
Media Access Control (MAC)
Operates below the LLC in the Data Link Layer.  Responsible for framing, error control, identifying MAC addresses, and controls media access (contention - devices vie control of the physical network medium, token-passing - devices wait for special frame (token) before transmitting data, Polling - device (secondary host) polled by primary host to see if they have data to be transmitted)
Ethernet 802.3
Operates at Layer 2. transports data to physical LAN by using CSMA/CD (designed for sporadic traffic). Implemented with twisted pair.
Token-Ring 802.5
Operates at Layer 2.  Transports data to the physical medium by using token passing.  All nodes are attached to  Multistation Access Unit (MSAU/MAU) in a logical ring (physical star). One station is an Active Monitor ensuring only on token passes on the network. If transmitting fails the Active Monitor removes the token and creates a new one.
FDDI
Uses dual token ring at 100MBps. works as a dual counter-rotating ring in which only one ring is active at a time.
Address Resolution Protocol (ARP)
Operates at Layer 2.  It maps Network IP addresses to get MAC address. MAC address is 48 bit and IP is 32 bit.  Stores IP and MAC address into a dynamic table.
Reverse Address Resolution Protocol (RARP)
Operates at Layer 2.  It uses the MAC address to get the devices IP address.  Used on a diskless machine.
Wireless LANs (WLAN)
Primarily operates at Layer 2. First encrypted using WEP and now use WPA (WiFi protected access). Protocols include Point to Point links (L2F, L2TP, Leased lines, PPP, PPTP, SLIP), circuit-switched Links (DSL and Inegrated Services Digital Network (ISDN), and Packet switched networks (Asynchronous Transfer Mode (ATM), Frame Relay, MPLS, SONET and SDH, SMDS, and X.25
Layer 2 Forwarding (L2F)
A tunneling (data encapsulation) used to implement VPNs.  No encryption or confidentiality.
Layer 2 Tunneling Protocol (L2TP)
Tunneling protocol used to implement VPNs. Derived from L2F. Uses IPsec for encryption because L2TP doesn't encrypt or provide confidentiality.
Point to Point Protocol (PPP)
A successor of SLIP. Provides router to router and host to network connections over sychronous and asynchronous circuits.
Point to Point Tunneling protocol (PPTP)
Tunneling protocol developed by Microsoft.  Does not provide encryption or confidentiality, instead it relies on PAP, CHAP. and EAP.
Circuit-switched Networks
A dedicated physical circuit path is established, maintained, and terminated between sender and receiver across carried network.  Used for telephone company networks. Provides an "always on" connection.
Digital Subscriber Line (DSL)
Uses existing analog lines (telephone lines) to deliver high bandwidth connectivity to remote customers.
Integrated Services Digital Network (ISDN)
Communications protocol that operates over analog phone (telephone lines) and converted to used digital signaling. Transmit both data and voice.  Service levels include BRI (2 B channels and 1 D channel) and PRI (one 64 Kbps D-channel and 23 64-kbps B-channel, max 1.544 Mbps (T1)).  B-Channels are data and voice, D-channel for control and signalling.
Packet-switched Networks
Connectionless-oriented devices sharing bandwidth on communications links to transport packets between sender ands receiver.  More resilient to error and congestion than circuit-switched.
Asynchronous Transfer Mode (ATM)
High speed and low delay technology that uses switching and multiplexing technology that uses 53 byte fixed size cells instead of frames.  Suited for Fiber optic networks (bursty)
Frame Relay
Packet-switched that utilizes a simplified framing approach that has no error correction.  used on Switched Virtual Circuits (SVC - temporary connection to transmit data then disconnected) and Permanent Virtual Circuit (PVC - built for dedicated circuit and bandwidth). Equipment includes Data Circuit-Terminating Equipment (DCE) and Data terminal equipment (DTE).
Synchronous Optical Network (SONET)
High availability, high-speed, multiplexed, low-latency technology on Fiber Optic
x.25
first packet switching network used at Layer 2.  uses SVC and PVC.  slower than frame relay because of all the error detection and correction functions.  Uses Link Access Procedure Balanced (LAPB) for error detection and correction functions.
bridge
A semi-intelligent repeater to connect two or more networks. Maintains the ARP cache that contains the MAC address of individual devices. Forwards packets and filters. Forwards broadcast traffic but not collision traffic.  A networking issue include a broadcast storm (flooding a network).
switch
An intelligent hub that uses MAC addresses to route traffic.  Transports data only to the port connected to the MAC address. Transmission method creates network segment and effectively increases data transmission rates. Can be used to implement VLANs.
Network Layer (3)
Provides routing and related functions that enable data to be transported between systems on networks and interconnected networks.  Protocols include RIP. ICMP, OSPF and BGP, IP, BOOTP and IPX.  Routers and gateways are used in layer 3.
Static routing
Requires an admin to manually create and update routes on the router. only practical in small networks.  has low bandwidth requirements and security (destinations are only available if they are specified in the routing table)
Dynamic routing
Determine the best route possible to a given destination. The routing table is periodically updated with current routing information.  Can also be link-state and distance-vector protocols.
distance vector routing protocol
Makes routing decisions based on the distance (hop count or other metric) and vector (the egress router interface). Convergence (the time it takes for routers to update their routing tables) is a disadvantage.
Link state routing protocol
Requires every router to calculate and maintain a complete map or routing table. Transmit updates that contain information about adjacent connections (link states). Can calculate most efficient route based on link speed, delay, load, reliability and cost. Convergence occurs rapidly. Uses Open shortest Path first (OSPF).
Routing Information Protocol (RIP)
A UDP distance vector routing protocol that uses hop count.  Prevents routing loops by implementing a 15 hop limit. Implements split horizon (prevents router from advertising a route back out), route poisoning (sets the hop count on a bad route to 16) and holddown timers ( timer is started when the router first receives info that a destination is unreachable.  updates about that destination will not be accepted until the timer expires).
Open Shortest Path First (OSPF)
A link-state routing protocol used in large networks. Considered an Interior Gateway Protocol (IGP) because it performs routing within a single autonomous state ((AS) group of contiguous IP address ranges under the control of a single Internet entity). Encapsulated directly in IP datagrams.  Networks are divided into areas identified by 32-bit area identifiers.
Internet Protocol (IP)
Connectionless, best effort protocols containing addressing information that enables packets to be routed. Fragmentation and reassembly of datagrams.  Faster than TCP
Internet Control Message Protocol (ICMP)
Reports errors and other information back to the source regarding the processing of transmitted IP addresses. Common ICMP messages include: Destination unreachable, Echo Request and Reply, Redirect and Time Exceeded.  Uses PING technology.
Gateways
Created with software running on a computer or router.  Link dissimilar programs and protocols by examining the entire packet to translate incompatibilities.  Example:  Can link and IP network to and IPX network.
Transport Layer (4)
Provides transparent, reliable data transport and end to end transmission control. Hides the details of the lower layer functions from the upper layers.  The functions include:
Flow control, Multiplexing, Virtual circuit management, and error checking and recovery.  Protocols include: TCP, UDP. SPX, SSL/TSL
TCP
A full duplex, connection-oriented protocol that provides reliable delivery of packets across a network. Connection is accomplished with a three way handshake (receiving device acknowledges packets, are packets are transmitted if an error occurs). Charactersitics and features include: Connection-oriented, reliable, and slow.
UDP
A connectionless protocol that provides the fastest best-effort delivery of datagrams across a network. Does not guarantee packet delivery, attempt to establish a connection with destination prior to sending a packet, acknowledge received datagrams, perform re-sequencing and perform error-checking or recovery.  Used in DNS, SNMP and streaming audio and video.
Sequenced Packet Exchange (SPX)
used to guarantee data delivery in Novell Netware.  Sequences transmitted packets, re-assembles received packets, confirms all packets are received, and request retransmission of packets that arent received.
SSL/TLS
Provides session based encryption and authentication for secure communications between clients and servers on the internet.  Provides server authentication Uses RSA asymmetric key system, IDEA, DES and 3DES symmetric keys and MD5 hash functions.
Session Layer (5)
Establishes, coordinates, and terminates communication sessions (service requests and service responses) between networked systems. Divided into 3 phases: Connection establishment (simplex, half-duplex, and full duplex), data transfer, and connection release. Protocols include NetBIOS, Network File System (NFS), Remote Procedure Call (RPC), Secure Shell, Session Initiation Protocol, and Structured Query Language
Simplex
A one-way communication path is established with a transmitter at one end of the connection a receiver at the other end. An analogy is an AM radio.
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