Name the three kinds of components in the "nuts and bolts" view of the Internet.
1) Connected computing devices: hosts = end systems running network apps. 2) Communication links (fiber, copper, radio, satellite); transmission rate = bandwidth. 3) Packet switches that forward packets: routers and switches.
Why is the Internet called a "network of networks"?
Because it is made of interconnected ISPs.
What do RFC and IETF stand for?
RFC: Request for comments. IETF: Internet Engineering Task Force. (Internet standards.)
Describe the Internet from the "service view".
Infrastructure that provides services to applications (Web, VoIP, email, games, e-commerce, social nets…); provides programming interface to apps (hooks to "connect" to Internet); provides service options, analogous to postal service.
Complete: "Protocols define …"
Format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt.
What belongs to the network edge, and what to the network core?
Edge: hosts — clients and servers (servers often in data centers); access networks and physical media are the links. Core: interconnected routers, network of networks.
Which two questions should you keep in mind about any access network?
Bandwidth (bits per second) of access network? Shared or dedicated?
DSL: how do voice and data share the line, and where does each go?
Voice and data are transmitted at different frequencies over a dedicated line to the central office. Data over DSL phone line goes to Internet; voice goes to telephone net.
DSL transmission rates?
< 2.5 Mbps upstream (typically < 1 Mbps); < 24 Mbps downstream (typically < 10 Mbps).
Cable access network: what is HFC, and how do homes use the access network compared with DSL?
HFC = hybrid fiber coax. Homes share access network to cable headend, unlike DSL, which has dedicated access to central office. Asymmetric: up to 30 Mbps downstream, 2 Mbps upstream.
How does the cable network carry different channels?
Frequency division multiplexing: different channels transmitted in different frequency bands.
What does a home network contain?
Wireless access point (54 Mbps), wired Ethernet (100 Mbps), router, firewall, NAT, cable or DSL modem — often combined in a single box.
Compare wireless LANs with wide-area wireless access.
Wireless LANs: within building (100 ft); 802.11b/g (WiFi): 11, 54 Mbps. Wide-area: provided by telco (cellular) operator, 10's km, between 1 and 10 Mbps; 3G, 4G: LTE.
Packet transmission delay formula, and the other names for R?
Transmission delay = L (bits) / R (bits/sec). R = link transmission rate, aka link capacity, aka link bandwidth.
Guided vs unguided media?
Guided: signals propagate in solid media — copper, fiber, coax. Unguided: signals propagate freely, e.g., radio.
Twisted pair: what is it, and what rates do Category 5 and 6 support?
Two insulated copper wires. Category 5: 100 Mbps, 1 Gbps Ethernet. Category 6: 10 Gbps.
Three properties of fiber optic cable.
Glass fiber carrying light pulses, each pulse a bit; high-speed point-to-point transmission (10's–100's Gbps); low error rate: repeaters spaced far apart; immune to electromagnetic noise.
Describe coaxial cable.
Two concentric copper conductors; bidirectional; broadband: multiple channels on cable; HFC.
Radio: which propagation environment effects are listed?
Reflection, obstruction by objects, interference.
Satellite radio link: rate and end-end delay?
Kbps to 45 Mbps channel (or multiple smaller channels); 270 msec end-end delay; geosynchronous versus low altitude.
What does store-and-forward mean, and what is the end-end delay over two links?
Entire packet must arrive at router before it can be transmitted on next link. End-end delay = 2L/R (assuming zero propagation delay).
When do packets queue, and when are they lost?
If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: packets queue, wait to be transmitted; packets can be dropped (lost) if memory (buffer) fills up.
Routing vs forwarding?
Routing: determines source-destination route taken by packets (routing algorithms). Forwarding: move packets from router's input to appropriate router output (using the local forwarding table).
Key characteristics of circuit switching.
End-end resources allocated to, reserved for "call"; dedicated resources: no sharing; circuit-like (guaranteed) performance; circuit segment idle if not used by call; commonly used in traditional telephone networks.
1 Mb/s link, each user 100 kb/s when active, active 10% of time. How many users for circuit switching and for packet switching?
Circuit-switching: 1000k/100k = 10 users. Packet switching: with 35 users, probability > 10 active at same time is less than .0004.
Advantages and problems of packet switching.
Great for bursty data: resource sharing; simpler, no call setup. But excessive congestion possible: packet delay and loss; protocols needed for reliable data transfer, congestion control.
Why not connect every access ISP directly to every other access ISP?
It doesn't scale: O(N²) connections.
How are competing global ISPs interconnected?
Through an IXP (Internet exchange point) and peering links.
What sits at the center of the Internet?
A small # of well-connected large networks: "tier-1" commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT) with national & international coverage, and content provider networks (e.g., Google).
What is a content provider network?
A private network (e.g., Google) that connects its data centers to Internet, often bypassing tier-1, regional ISPs.
Write the nodal delay equation and name each term.
d_nodal = d_proc + d_queue + d_trans + d_prop: nodal processing, queueing, transmission, propagation.
What happens during d_proc, and what does d_queue depend on?
d_proc: check bit errors, determine output link (typically < msec). d_queue: time waiting at output link for transmission; depends on congestion level of router.
d_trans vs d_prop: formulas and what each variable means.
d_trans = L/R (L: packet length in bits; R: link bandwidth in bps). d_prop = d/s (d: length of physical link; s: propagation speed in medium, ~2×10⁸ m/sec).
Caravan analogy: what do a car, the caravan, and the toll-booth service time represent?
Car ~ bit; caravan ~ packet; the 12 sec toll-booth service time ~ bit transmission time. Answer for the ten-car caravan: lined up before the 2nd booth after 62 minutes (120 sec + 1 hr).
What happens to a packet that arrives at a full queue, and who may retransmit it?
It is dropped (aka lost). A lost packet may be retransmitted by previous node, by source end system, or not at all.
Define throughput; instantaneous vs average.
Throughput: rate (bits/time unit) at which bits transferred between sender/receiver. Instantaneous: rate at given point in time. Average: rate over longer period of time.
Define bottleneck link, and give the per-connection throughput when 10 connections fairly share a backbone link R.
Bottleneck link: link on end-end path that constrains end-end throughput. Per-connection throughput = min(Rc, Rs, R/10); in practice Rc or Rs is often the bottleneck.
What does each layer do?
Each layer implements a service via its own internal-layer actions, relying on services provided by layer below.
Why use layering?
Explicit structure allows identification, relationship of a complex system's pieces; modularization eases maintenance, updating (a layer's implementation change is transparent to the rest).
List the five Internet-stack layers with their jobs.
Application: supporting network applications. Transport: process-process data transfer. Network: routing of datagrams from source to destination. Link: data transfer between neighboring network elements. Physical: bits "on the wire".
Which two OSI layers are "missing" from the Internet stack, what do they do, and where are their services implemented if needed?
Presentation: allow applications to interpret meaning of data (e.g., encryption, compression, machine-specific conventions). Session: synchronization, checkpointing, recovery of data exchange. If needed, these services must be implemented in the application.
Name the encapsulated unit at the transport, network and link layers.
Transport: segment (Ht M). Network: datagram (Hn Ht M). Link: frame (Hl Hn Ht M). Application: message M.