Computer Networks & The Internet University of Management & Technology Lecture 2 Imran Ahmed University of Management & Technology

Agenda Network & its types What’s the Internet? What’s a protocol? History Network edge Network core Access net, physical media Internet/ISP structure Performance: loss, delay Protocol layers, service models

A closer look at network structure: network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links

The network edge: end systems (hosts): client/server model run application programs e.g. Web, email at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; email client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Gnutella, KaZaA

Network edge: connection-oriented service Goal: data transfer between end systems Handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human protocol Set up “state” in two communicating hosts TCP – Transmission Control Protocol Internet’s connection-oriented service TCP service [RFC 793] Reliable, in-order byte-stream data transfer Loss: acknowledgements and retransmission Flow control: Sender won’t overwhelm receiver Congestion control: Senders “slow down sending rate”, when network congested

Network edge: connectionless service Goal: data transfer between end systems Same as before! UDP – user Datagram Protocol [RFC768]: Connectionless Unreliable data transfer No flow control No congestion control App’s using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP: Streaming media, teleconferencing, DNS, Internet telephony

Agenda Network & its types What’s the Internet? What’s a protocol? History Network edge Network core Access net, physical media Internet/ISP structure Performance: loss, delay Protocol layers, service models

The Network Core mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks”

Network Core: Circuit Switching End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required

Circuit Switching It’s the method used by the telephone network. A B Source Destination It’s the method used by the telephone network. A call has three phases: Establish circuit from end-to-end (“dialing”), Communicate, Close circuit (“tear down”). Originally, a circuit was an end-to-end physical wire. Nowadays, a circuit is like a virtual private wire: each call has its own private, guaranteed data rate from end-to-end.

Circuit Switching Telephone Network Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000 calls. Destination “Callee” Source “Caller” Central Office “C.O.” Central Office “C.O.” Trunk Exchange

Packet Switching It’s the method used by the Internet. Source Destination R1 R3 R4 It’s the method used by the Internet. Each packet is individually routed packet-by-packet, using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at the same time, therefore a packet switch has buffers.

Packet Switching Simple router model Link 1, ingress Link 1, egress “4” Choose Egress Link 2 Link 2, ingress Choose Egress Link 2, egress R1 “4” Link 1 Link 3 Link 3, ingress Choose Egress Link 3, egress Link 4 Link 4, ingress Choose Egress Link 4, egress

Why does the Internet use packet switching? Efficient use of expensive links: The links are assumed to be expensive and scarce. Packet switching allows many, bursty flows to share the same link efficiently. “Circuit switching is rarely used for data networks, ... because of very inefficient use of the links” - Gallager Resilience to failure of links & routers: ”For high reliability, ... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum Breaking message into packets allows parallel transmission across all links, reducing network latency. In summary the benefits that of packet switched networks can be summarized as follows: They use the bandwidth efficiently, meaning that a trunk link uses less resources than the sum of its tributaries, as they multiplex and conserve bandwidth They have little state in the intermediate nodes They are robust, some claim that they were designed to withstand a nuclear attack They do not have a central authority from whom we need permission to run experiments

Some Definitions Packet length, P, is the length of a packet in bits. Link length, L, is the length of a link in meters. Data rate, R, is the rate at which bits can be sent, in bits/second, or b/s.1 Propagation delay, PROP, is the time for one bit to travel along a link of length, L. PROP = L/c. Transmission time, TRANSP, is the time to transmit a packet of length P. TRANSP = P/R. Latency is the time from when the first bit begins transmission, until the last bit has been received. On a link: Latency = PROP + TRANSP. 1. Note that a kilobit/second, kb/s, is 1000 bits/second, not 1024 bits/second.

Packet Switching: Store-and-forward

Packet Switching Host A Host B A B Source Destination TRANSP1 “Store-and-Forward” at each Router TRANSP2 R1 PROP1 TRANSP3 R2 PROP2 TRANSP4 R3 PROP3 Host B PROP4

Agenda Network & its types What’s the Internet? What’s a protocol? History Network edge Network core Access net, physical media Internet/ISP structure Performance: loss, delay Protocol layers, service models

Access networks and physical media Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks Keep in mind: bandwidth (bits per second) of access network? shared or dedicated?

Internet access technologies Previously, most people use 56K dial-up lines to access the Internet, but a number of new access technologies are now being offered. The main new access technologies are: Digital Subscriber Line (DSl,ADSL) Cable Modems Local Area Networks Wireless Networks

Access networks: DSL & ADSL Digital Subscriber Line (DSL) is one of the most used technologies now being implemented to significantly increase the data rates over traditional telephone lines. Historically, voice telephone circuits have had only a limited capacity for data communications because they were constrained by the 4 kHz bandwidth voice channel. Most local loop telephone lines actually have a much higher bandwidth and can therefore carry data at much higher rates.

Access networks: DSL & ADSL DSL services are relatively new and not all common carriers offer them. Two general categories of DSL services have emerged in the marketplace. Symmetric DSL (SDSL) provides the same transmission rates (up to 128 Kbps) in both directions on the circuits. Asymmetric DSL (ADSL) provides different data rates to (up to 640 Kbps) and from (up to 6.144 Mbps) the carrier’s end office. It also includes an analog channel for voice transmissions.

DSL Architecture Customer Premises Local Carrier End Office DSL Modem Line Splitter Main Distribution Frame Voice Telephone Network Local Loop Hub Telephone ISP POP ATM Switch Computer DSL Access Multiplexer Computer ISP POP Customer Premises ISP POP ISP POP Customer Premises

Access networks: Cable modems One potential competitor to DSL is the “cable modem” a digital service offered by cable television companies which offers an upstream rate of 1.5-10 Mbps and a downstream rate of 2-30 Mbps. A few cable companies offer downstream services only, with upstream communications using regular telephone lines.

Cable Modem Architecture Customer Premises Cable Company Fiber Node Cable Company Distribution Hub TV Video Network Cable Modem Cable Splitter Downstream Combiner Optical/Electrical Converter Upstream Hub TV Router Shared Coax Cable System Cable Company Fiber Node Cable Modem Termination System Computer Computer ISP POP Customer Premises Customer Premises Cable Modem Architecture

Access networks: Local area networks Company/univ. local area network (LAN) connects end system to edge router Ethernet: Shared or dedicated link, connects end systems and router 10 Mbs, 100mbs, Gigabit Ethernet etc. Details will be available in future

Access networks: Wireless networks Shared wireless access network connects end system to router via base stations aka “access point” Wireless Lans: 802.11b (WiFi): 11 Mbps Wide-area wireless access: Provided by telecommunication companies WAP/GPRS etc. Satellite: Up to 50 Mbps channels or multiple smaller channles

Physical Media Bit: propagates between transmitter/rcvr pairs Physical link: What lies between transmitter & receiver Guided media: Signals propagates in solid media; copper, fiber, coax. Unguided media: Signals propagates freely, e.g., radio Twisted Pair (TP) Two insulated copper wires: Category 3: traditional phone wires, 10 Mbps (Ethernet) Category 5: 100 Mbps (Ethernet)

Physical Media Coaxial cable: Fiber optic cable: Two concentric copper conductors Bidirectional Baseband: Single channel on cable Broadband: Multiple channel on cable Fiber optic cable: Glass fiber carrying light pulses, each pulse a bit High-speed operation: High-speed point-to-point transmission (e.g., 5 Gps) Low error rate: repeaters spaced far apart; immune to electromagnetic noise

Summary We studied about network edge: end systems etc. Circuit switching & packet switching. Internet access technologies: DSL, cable modem etc. Physical medias: Twisted pair, coaxial pair & fiber optics.