Introduction and Vocabulary

Introduction (1): A bit of history Ubiquitous & mobile computing are not the original concepts –Networking started with very few computers –Hosts were connected with wires –Hosts did not move around relative to each other Networks were cooperative efforts –Security was not an issue Network design reflected these assumptions –We have a huge installed base of computers using networking that is based on old assumptions 2

Introduction (2) Networks are complex because they consist of a combination of multiple technologies –One must understand both the individual technologies and how they are combined as a working whole Computer networks can be confusing because –Various organizations have created networking standards and conceptual models –The models and standards from different organizations are not compatible with each other –The technologies are diverse and change rapidly –Terminology is used inconsistently and often confused by the vendor names for their products 3

Vocabulary (1) Network: a communications channel for sharing information resources between two or more devices –The communications channel includes a physical channel that limits network performance This could be guided (wired) or unguided (wireless) To move information some energy must propagate on the physical channel –The communications channel also includes virtual channels created by software The software objects interact via protocols and interfaces 4

Vocabulary (2) Internetwork: a collection of interconnected networks WAN, MAN, CAN, LAN, PAN: Wide, Metro, Campus, Local, Personal Area Network Stations, hosts, systems: all names used to describes the computers interconnected by the network 5

Vocabulary (3) Unicast: a message from one host to one host Broadcast: a message from one host to all other hosts in a broadcast domain –The limits of the broadcast domain will become clear in a subsequent lecture Multicast: a message from one host to many hosts –Unlike broadcast - a multicast does not have to be delivered to all hosts in the broadcast domain 6

Vocabulary (4) Subnet: an overloaded term. The original meaning was all of the network except for the stations on the edges. The other meaning relates to address assignments. Segment: a section of the network typically using a single set of protocols and technologies. A segment often has some network device that connects it to another segment. It often refers to a physically distinct piece of the network – a single cable for example – but it may also be used to refer to a logical part of the network. 7

Vocabulary (5) Point-to-Point (P2P) segment: a segment that interconnects only two stations. Long distance segments are often P2P. Having only two stations on a segment often allows for simpler protocols Shared segment: a segment with multiple hosts Contention: some shared segments require the hosts to contend with other hosts in order to use the network Collision: collisions occur when contending hosts try to use the network at the same time. Hosts whose traffic can collide are in the same collision domain 8

Vocabulary (6) Simplex: a segment where data traffic is always in only one direction ( a one-way street) Half-duplex: a segment where traffic can go both directions but only one direction at a time (perhaps a one-way street where the direction depends on time of day– or a railroad track) Full-duplex: a segment where traffic can flow in both directions simultaneously – often used for two simplex segments in opposite directions (a regular street) 9

Vocabulary (7) Topology: the “shape” of some aspect of the network –Physical topology –Logical topology –These may be different –Common shapes include mesh, partial mesh, bus, ring, star The star is also called a hub and spoke topology Network behavior often is described using graph theory – with edges and nodes –A tree is a graph without loops 10

Vocabulary (8) 11 If all of the endpoints have a P2P link to each other then we have a fully-connected mesh as in the examples above. A fully-connected mesh of N devices uses N*(N-1)/2 separate P2P links. This grows approximately as the square of the number of nodes for large N. We say that the “Big O” of the number of connections is N 2.

Vocabulary (9) 12 A partially-connected mesh (shown below) reduces the number of P2P connections. But if communication is limited to the physical paths then B can no longer talk to A, etc. E C B D A If node C could move information from its BC line to its CA line then B could indirectly communicate with A. That would create a virtual channel between A and B.

Vocabulary (10) Bus topology: also called a backbone –All attached hosts connect to the backbone –As a physical topology this often means a backbone wire that snakes through the building –Each host must be near the backbone to connect Ring topology: can be thought of as a specific form of a partially connected mesh –Each connected device is attached to two neighbors –Typically the traffic flows inbound from one neighbor and outbound to the other neighbor – i.e., the ring has upstream and downstream neighbors and a direction for data flow 13

Vocabulary (11) Star topology: also called a hub and spoke topology –For physical topology it comes from collapsing the backbone or ring into a “hub” in a communications closet –The attached hosts are said to be “home run” to the closet –The result is called “structured wiring” –Often used with bus (Ethernet) or ring (FDDI) logical topologies 14

Vocabulary (12) 15 Bus or backbone Collapsed backbone Ring Collapsed ring hub

Vocabulary (13) A network with multiple segments and virtual channels that span multiple segments must have a network element to connect the segments –If the element does not make decisions about the traffic moving between segments then it is called a repeater –A repeater recreates the signal it receives on one interface as a new signal on all of its other interfaces This allows the connected segments to function as a larger virtual segment –The device originally used at the center of hub and spoke topologies was a multiport repeater called a hub 16

Vocabulary (14) A network element that can selectively forward between segments is generically called a switch –The attached segments form a switched network –We will study a layered model in which there is switching at multiple layers A bridge is a data-link layer switching device A router is a network layer switching device Forwarding in a switch depends on addresses assigned to hosts Any network with virtual channels requires sharing of physical segments 17

Vocabulary (15) Multiplexing: combining multiple data streams so they share a limited network resource – –time division multiplexing, frequency division multiplexing, wavelength division multiplexing, code division multiplexing, and spatial multiplexing De-multiplexing: splitting the multiple data streams apart from a multiplexed stream Statistical multiplexing: relying on the network- usage statistics to allow oversubscription of a network resource 18

Vocabulary (16) Link: a term often used in the “data-link layer” as a shortened version of data link. The link is distinguished from a segment because a link often includes multiple segments and their intermediate switches. If the link uses media for which broadcast is the natural connection pattern then the link is equivalent to the broadcast domain –Therefore the link is a section of the network whose attached devices can communicate without forwarding through a router 19

Vocabulary (17) The most common type of LAN wired link uses a broadcast physical medium –Some of the energy sent from the sender is received at every one of the other devices –The devices are in the same broadcast domain –The information is given a destination address so that each receiver can decide whether to keep or discard (filter) the received information –This addressing and filtering allows a broadcast medium to also support multicast and unicast –Adding more devices on the bus requires more energy at the sender 20

Vocabulary (18) 21