Edge & Core Networking Flashcards
Network edge
- Hosts (end-systems) : clients and servers
- Servers often in data centers
Access networks, physical media
Wired, wireless communication links
Network core:
- Interconnected routers
- Network of networks
How to connect end systems to edge router?
- Residential access nets
- Institutional access networks (school,company)
- Mobile access networks (WiFi, 4G/5G)
Access networks: Digital subscriber line (DSL)
- Use existing telephone line to central office DSLAM
- data over DSL phone line goes to Internet
- voice over DSL phone line goes to telephone net
- 24-52 Mbps dedicated downstream transmission rate
- 3.5-16 Mbps dedicated upstream transmission rate
Frequency division multiplexing
Frequency division multiplexing (FDM): different channels transmitted in different frequency bands
Access networks: cable-based access
- (HFC: hybrid fiber coax): asymmetric: up to 40 Mbps -1.2 Gbs downstream transmission rate, 30-100Mbps upstream transmission rate
- network of cable, fiber attaches homes to ISP router
- homes share access network to cable headend
Fiber To The Home (FH) and 5G Wireless
- Optical fiber path directly to home
- Can potentially provide very high Internet access rates
- 5G fixed wireless is beginning to be deployed
- No cabling from the telco’s CO to the home.
- Data is sent wirelessly from a provider’s base station to the a modem in the home.
Access networks: home networks
Access networks: enterprise networks
- Companies, universities, etc.
- Mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly)
- Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps
- WiFi: wireless access points at 11, 54, 450 Mbps
Accessn networks: data center networks
- high-bandwidth links (10s to 100s Gbps) connect hundreds to thousands of servers together, and to Internet
Links: physical media
(bit)
Propagates between transmitter/receiver pairs
Links: physical media (physical link)
What lies between transmitter & receiver
Links: physical media (guided media)
Signals propagate in solid media: copper, fiber, coax
Links: physical media (unguided media)
Signals propagate freely, e.g., radio
Links: physical media (Twisted pair (TP))
- two insulated copper wires
- Category 5: 100 Mbps, 1 Gbps
Ethernet - Category 6: 10Gbps Ethernet
- Category 5: 100 Mbps, 1 Gbps
Links: physical media (Coaxial cable)
- two concentric copper conductors
- bidirectional
- broadband:
- multiple frequency channels on cable
- 100’s Mbps per channel
- multiple frequency channels on cable
Links: physical media (Fiber optic cable)
- glass fiber carrying light pulses, each pulse a bit
- high-speed operation:
- high-speed point-to-point transmission (10’s-100’s Gbps)
- low error rate:
- repeaters spaced far apart
- immune to electromagnetic noise
Wireless access networks (Wireless local area network (WLANs))
- Typically, within or around building (~100 ft)
- 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate
Wireless access networks (Wide-area celullar access networks)
- provided by mobile, cellular network operator (10’s km)
- 10’s Mbps
- 4G cellular networks (5G being deployed)
Host: sends packets of data
host sending function:
- takes application message
- breaks into smaller chunks, known as packets, of length L bits
- transmits packet into access network at transmission rate R
- link transmission rate, aka link capacity, aka link bandwidth
The network core
- Mesh of interconnected routers
- packet-switching: hosts break application-layer messages into packets
- network forwards packets from one router to the next, across links on path from source to destination
Two key network-core functions
- Forwarding :
- aka “switching”
- local action:move arriving packets from router’s input link to appropriate router output link
- Routing:
- global action: determine source destination paths taken by packets
- Defining routing algorithms
Routing analogy
Forwarding
Packet-switching: store-and-forward
- packet transmission delay: takes L/R seconds to transmit (push out) L-bit packet into link at R bps
- store and forward: entire packet must arrive at router before it can be transmitted on next link
Packet-switching: queueing
Queueing occurs when work arrives faster than it can be serviced
Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time:
- packets will queue, waiting to be transmitted on output link
- packets can be dropped (lost) if memory (buffer) in router fills up
Alternative to packet switching: circuit switching
end-end resources allocated to, reserved for “call” between source and destination
- in diagram, each link has four circuits.
- call gets 2nd circuit in top link and 1st circuit in right link. - dedicated resources: no sharing
- circuit-like (guaranteed) performance
- circuit segment idle if not used by call
(no sharing) - commonly used in traditional
telephone networks
Circuit switching: FDM
Frequency Divison Multiplexing
- optical, electromagnetic frequencies divided into (narrow) frequency bands
- each call allocated its own band, can transmit at max rate of that narrow band
Circuit switching: TDM
Time Division Multiplexing (TDM)
- time divided into slots
- each call allocated periodic slot(s), can transmit at maximum rate of (wider) frequency band (only) during its time slot(s)
Packet switching versus circuit switching
example:
- 1 Gb/s link
- each user:
- 100 Mb/s when “active”
- active 10% of time
Q: how many users can use this network under circuit-switching and packet switching?
- circuit-switching: 10 users
- packet switching: with 35 users, probability > 10 active at same time is less than .0004 *
Q: how did we get value 0.0004?
A: HW problem (for those with course in probability only)
Internet structure a “network of networks”
- hosts connect to Internet via access Internet Service Providers (ISPs)
- access ISPs in turn must be interconnected
- so that any two hosts (anywhere!) can send packets to each other
- resulting network of networks is very complex
- evolution driven by economics, national policies
Internet structure: a “network of networks”
Question: given thousands of stub networks, how to connect them together?
Internet structure: a “network of networks”
Option: connect each access ISP to one global transit ISP?
Customer and provider ISPs have economic agreement.
Internet structure: a “network of networks”
But if one global ISP is viable business, there will be competitors ….
who will want to be connected
Internet structure: a “network of networks”
…and regional networks may arise to connect access nets to ISPs
Internet structure: a “network of networks”
… and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users
Internet structure: a “network of networks”
At “center”: small # of well-connected large networks
- “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage
- content provider networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs
Tier-1 ISP Network map: Sprint
