Week 6: Computer networks and wireless networks

1. LAN

• ltd coverage: smaller geographic area (ex: home, school)
• most common: Ethernet and wifi
• can incl. many devices: switches, firewalls, routers, etc.
• simple LANs: 1+ switches
• switch: connects devices in network
• complex LANs
• spanning tree protocol (??) to prevent loops

History

• evolution from late 1960s
• ex: Cambridge Ring: 1974; ethernet: 1973-5; ARCNET: 1976-7
• PCs (1970s) + DOS-based (80s) --> ++ computers
• share storage, printers
• issues: match physical layer and network protocol implementations
• ea. vendor own structures
• appearance of Novell, Windows NT/Workgroups, Unix-based workstations

Types of cabling

• early cabling based on coaxial cabling
• (((tubular conducting shield (( insulating layer ( inner conductor ) )) )))
• then shielded, unshielded twisted pair (Star LAN Cat3)
•  2 conductors of single circuit together to cancel external interference
•  unshielded = same for tel. systems
• 10Base-T, aka ethernet over twisted pair, etc.
• can mix diff. gens. of equipment: higher-speed implementations w/ lower-speed standards
• current: structured cabling
• smaller elements forming structures in building/campus
• ex.: wifi, fiber optic

Network topology

• arr. of links, nodes, etc. in network
• physical: placement of components (ex. devices, cables)
• logical: data flows
• most common: switched ethernet, IP (TCP/IP)
• bus
• node :: cable
• "singularity"/uniqueness, match :: match = can easily track failure to source
• linear bus: 2 endpoints
• distributed bus: 2+ endpoints
• mesh
• fully connected network: nodes connected to each other; not useful for large networks
• partially connected: nodes connected to 1+, but not all to each other; take advantage of redundancy but avoid complexity of fully connected
• ring
• circular, uni-directional
• ea. device = repeater; nodes work as server
• network dependent on ability to travel around
• one node breaks, entire network stops functioning
• star
• each network host :: central hub/switch via pt<>pt connection
• central hub = signal repeater; all traffic passes through
• easy to add addtl nodes
• hub = point of failure

2. Computer/data network

• Telecom network : computer << data >> computer
• network links est. via cable or wireless --> Internet
• network nodes
• create, route, terminate data, "hosts"
• ex. :  PC, phones, servers

History

• experiments and tests late 1950s-70s
• 1960s: ARPANET
• 1973: Ethernet
• 1976: ARCNET
• 1995: +++ speed capacity for Ethernet

Distributed computing

• network-wide resources for tasks (ex. P2P apps, progs)
• processor << messages >> processor
• each entity: autonomous, own memory
• "independent", localized failure

Network packet

• most info carried in packets (appropriately-sized blocks)
• 2 kinds of data in packet data
• control info: ex. network addresses
• user data (payload)
• network packet: formatted unit of data (bits/bytes) carried by packet-switched network
• framing
• network address
• error detection and correction
• hop counts
• hop: pt of path between source and destination
• hop count: intermediate device, ex. router; detect fault in network
• closed circuit > no action > congestion > failure > discard
• packet length
• class/priority
• payload

IP packet

• header and payload
• but often as payload w/in Ethernet frame  

Consultative Committee for Space Data Systems (NASA)

• packet length can vary
• transmitted b/w frames
• size fixed during dev.
• error-correcting codes
• principal type of data loss: deleted, undecodable whole frames (??)

Packetized elementary stream (MPEG)

• elem. stream / packets > MPEG transport or program stream (TS, PS) > distributed ("multiplexed" ??) 

Networked links

• electrical cable
• optical fiber: pulses of light ~~ data
• radio waves (wireless)
• price significant consideration

Networked nodes

• interface controller
• hardware accessing transmission media
• low-level info
• ex. Ethernet MAC address
• repeater and hub
• repeater: receives info > clean > regenerate
• hub: repeater
•  bridge: join segments to form 1 network
• local: direct connection
• remote: can be used for WAN
• wireless: join LANs, or remote devices to
•  switch
• fwds and filters b/w MAC-based physical ports
• "multi-port bridge"
• router
• processes routing info incl. in packets
• fwds packets b/w networks
• modem
• via wire: connect info not orig. for dig net traffic
• 1+ freq. modulated by dig signal >> analog
• analog sig >> can be modified for transmission (ex. telephony)
• firewall 
• network sec. and access

Communications protocols

• protocol suite
• definition of protocols
• protocol stack 
• software implementation
• HTTP : application
• TCP : transport
• IP : internet/network
• foundation of modern networking
• Ethernet : data link
• ? : physical

Scale

• personal, local, storage, campus, metropolitan, wide area, global area
• backbone (?)
• enterprise private network
• single orgs, maybe diff. locs
• VPN
• open connections, virtual circuits

Org. scope

• intranet
• extranet
• single admin control
• external connection to ex., business partners, etc.
• internetwork -- Internet
• darknet
• accessible via spec. software
• sharing is anonymous

Network service and performance

• services hosted on servers
• ex. www, email
• performance
• = grade of service
• congestion: deterioration
• resilience
• acceptable service level despite faults

Security and surveillance

• prevent and monitor unauthorized access, misuse
• controlled by network admin
• surveillance: data monitoring
• social control?
• Electronic Frontier Foundation, ACLU
• end-to-end encryption (E2EE)
• sender encrypts data for receiver decrypting
• confidentiality and integrity

3. Coyle, K. (2005). Management of RFID in Libraries. Journal of Academic Librarianship, 31(5), 486-489. doi: 10.1016/j.acalib.2005.06.001.

• RF: radio frequency
• ID: identifier
• similar to barcodes, but read by electromagnetic field
• RFID tag doesn't have to be visible to be read
• variety

 Implementation in libs: one tag, many fxns

• privacy issues
• useful for tracking inventory/circ fxns
• ID tag re-used multiple times --> justification of expense?
• payment systems?
• security mechanism not worse than other techs

ROI

• efficiency via automation (checking in/out items)
• pitfalls of self-checkout: lack of human interaxn
• check user satisfaction

Further issues

• RFID tags for "non-trad" items, shapes (ex. optical discs)
• if no RFID, alternative check-out system
• reprogramming tags