host/end system (ES)----communications link-----host

host: PC, server, printer, WAP, IP phone, "appliance"(printer,camera,NAS etc)
host can be client and/or server (often large and single-purpose: web, file, print,)

link: medium (wire,fiber,radio) and bandwidth (bps).

if hosts not directly connected by a link, go thru intermediate systems (IS)/routers.

              |              |             |
              |              |             |
host--------routerA--------routerB--------routerC-----host
              |                          /   |
              |                         /    |
            routerD--------------------/
              |

more routers
path/route between the two hosts.
packets of message go on link from host to host. long message broken into short packets is more efficient.*
packet switching: each packet individually routed: could take different paths, and (even if same route) could take different time.
more efficiently share the communications infrastructure than circuit switching (path components (bandwidth and buffers along route) reserved for exclusive use, even when silent) of PSTN. Cheaper and simpler too.
packets may go missing, out of order, duplicated.

node/entity: ES or IS.
hosts and routers run protocols that control the sending and receiving of information.
protocol is rules of operation: format and order of messages, actions taken upon send or receive of message.

TCP/IP suite of protocols is Internet, or private intranet.
defined by IETF's RFCs.
Internet packet switching network edge (hosts) complex, core (routers) simple.
PSTN circuit switching network: edge (phone) simple, core (switches etc) complex.

network infrastructure enables applications (web,email,IM,games,streaming, remote login,games, P2P file sharing, VoIP, etc) on ESs to exchange data.
Provides message-passing services:
1. connection-oriented (TCP): ESs create a logical connection thru handshaking (exchange of packets that establishes connection). ISs oblivious. Data transfer is reliable (app can rely on data to be transferred).
2. connectionless (UDP): no handshaking. not reliable but simpler and faster.

Metcalfe's Law: the value of the network expands exponentially as the number of users increases.

* message switching: entire message sent from router to router which are store and forward, thus sequential. packets pipelined/parallel.
bit error: packet vs message retransmittal.
but overhead of header per packet.