Introductionمقدمة
مقدمة الفصل
ماذا ستتعلم في هذا الفصل
- What's the Internet?
- What's a protocol?
- Network edge: hosts, access net, physical media.
- Network core: packet/circuit switching, Internet structure.
- Performance: loss, delay, throughput.
- Protocol layers, service models.
أقسام الفصل
- 1What's the Internet?ما الإنترنت؟
- 2What's a protocol?ما البروتوكول؟
- 3Network edge and access networksطرف الشبكة وشبكات الوصول
- 4Hosts sending packets; physical mediaإرسال المضيف للحزم والوسائط المادية
- 5The network core: packet switchingلب الشبكة: تبديل الحزم
- 6Circuit switching vs packet switchingتبديل الدارات مقابل تبديل الحزم
- 7Internet structure: network of networksبنية الإنترنت: شبكة من الشبكات
- 8Delay and loss in packet-switched networksالتأخير والفقد
- 9Throughputالإنتاجية
- 10Protocol layers and service modelsطبقات البروتوكولات
تلميح: استخدم أسهم لوحة المفاتيح للتنقل بين الشرائح.
The Internet: "nuts and bolts" viewمكوّنات الإنترنت
What's the Internet?
Millions of connected computing devices running network apps.
Fiber, copper, radio, satellite. Transmission rate: bandwidth.
Forward packets (chunks of data): routers and switches.
- Internet: "network of networks" — interconnected ISPs.
- Protocols control sending, receiving of msgs, e.g., TCP, IP, HTTP, Skype, 802.11.
- Internet standards: RFC = Request for comments; IETF = Internet Engineering Task Force.
"Fun" internet appliances (slide 5)
- IP picture frame, web-enabled toaster + weather forecaster, Tweet-a-watt (monitor energy use), Slingbox (watch, control cable TV remotely), Internet refrigerator, Internet phones.
The Internet: a service viewالإنترنت كخدمة
What's the Internet?
Infrastructure that provides services to applications: Web, VoIP, email, games, e-commerce, social nets, …
- Provides programming interface to apps: hooks that allow sending and receiving app programs to "connect" to Internet.
- Provides service options, analogous to postal service.
What's a protocol?ما البروتوكول؟
Protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt.
Human protocolsبروتوكولات بشرية
- "what's the time?", "I have a question", introductions
- Specific msgs sent
- Specific actions taken when msgs received, or other events
Network protocolsبروتوكولات الشبكة
- Machines rather than humans
- All communication activity in Internet governed by protocols
Network edge, access networks, coreالطرف والوصول واللب
Network edge and access networks
Hosts: clients and servers (servers often in data centers).
Wired, wireless communication links.
Interconnected routers; network of networks.
Access networks connect end systems to the edge router
- Residential access nets
- Institutional access networks (school, company)
- Mobile access networks
DSL and cable accessDSL والكابل
Network edge and access networks
DSL (digital subscriber line)
- Uses existing telephone line to central office DSLAM
- Voice, data transmitted at different frequencies over dedicated line to central office
- Data → Internet; voice → telephone net
- < 2.5 Mbps upstream (typically < 1 Mbps)
- < 24 Mbps downstream (typically < 10 Mbps)
Cable network
- Frequency division multiplexing: different channels in different frequency bands
- HFC: hybrid fiber coax
- Asymmetric: up to 30 Mbps downstream, 2 Mbps upstream
- Network of cable, fiber attaches homes to ISP router
- Homes share access network to cable headend
Home, enterprise and wireless accessالمنزل والمؤسسة واللاسلكي
Network edge and access networks
Home network
- Wireless access point (54 Mbps); wired Ethernet (100 Mbps)
- Router, firewall, NAT; cable or DSL modem — often combined in single box
Enterprise access networks (Ethernet)
- Typically used in companies, universities, etc.
- 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps transmission rates
- Today, end systems typically connect into Ethernet switch
Wireless LANs
- Within building (100 ft)
- 802.11b/g (WiFi): 11, 54 Mbps transmission rate
Wide-area wireless access
- Provided by telco (cellular) operator, 10's km
- Between 1 and 10 Mbps
- 3G, 4G: LTE
Host: sends packets of dataإرسال المضيف للحزم
Hosts sending packets; physical media
Host sending function
- 1
Takes application message.
- 2
Breaks into smaller chunks, known as packets, of length L bits.
- 3
Transmits packet into access network at transmission rate R.
transmission delay = L (bits) / R (bits/sec) — the time needed to transmit an L-bit packet into the link. L: packet length (bits); R: link transmission rate, aka link capacity, aka link bandwidth.
Physical mediaالوسائط المادية
Signals propagate in solid media: copper, fiber, coax.
Signals propagate freely, e.g., radio.
| Medium | Key points on the slides |
|---|---|
| Twisted pair (TP) |
|
| Coaxial cable |
|
| Fiber optic cable |
|
| Radio |
|
Radio link types
- Terrestrial microwave: up to 45 Mbps channels
- LAN (e.g., WiFi): 11 Mbps, 54 Mbps
- Wide-area (e.g., cellular): 3G cellular ~ few Mbps
- Satellite: Kbps to 45 Mbps channel; 270 msec end-end delay; geosynchronous versus low altitude
- Slide 23 is a photo of a microwave antenna tower (labelled "microwave"). Slides 42–43 link two videos ("How does the Internet work?"); their content is not on the slides.
Packet switching: store-and-forwardخزّن ثم مرّر
The network core: packet switching
- Network core: mesh of interconnected routers.
- Packet-switching: hosts break application-layer messages into packets and forward packets from one router to the next, across links on path from source to destination.
- Each packet transmitted at full link capacity.
Entire packet must arrive at router before it can be transmitted on next link. Takes L/R seconds to transmit an L-bit packet into a link at R bps; end-end delay = 2L/R (assuming zero propagation delay).
المسألة
One-hop numerical example (slide 26)
المعطيات
- L
- 7.5 Mbits
- R
- 1.5 Mbps
الصيغة
one-hop transmission delay = L/R
خطوات الحل
- 7.5 Mbits / 1.5 Mbps
النتيجة
one-hop transmission delay = 5 sec
Queueing, loss, routing and forwardingالطوابير والتوجيه والتمرير
The network core: packet switching
Queuing and loss
- If arrival rate (in bits) to link exceeds transmission rate of link for a period of time:
- packets will queue, wait to be transmitted on link
- packets can be dropped (lost) if memory (buffer) fills up
Routingالتوجيه
- Determines source-destination route taken by packets
- Routing algorithms
Forwardingالتمرير
- Move packets from router's input to appropriate router output
- Local forwarding table: header value → output link
Circuit switchingتبديل الدارات
End-end resources allocated to, reserved for "call" between source & dest.
- Dedicated resources: no sharing
- Circuit-like (guaranteed) performance
- Circuit segment idle if not used by call (no sharing)
- Commonly used in traditional telephone networks
- Slide 29 diagram: each link has four circuits; the call gets the 2nd circuit in the top link and the 1st circuit in the right link.
Packet switching versus circuit switchingالحزم مقابل الدارات
Circuit switching vs packet switching
المسألة
Slide 31: 1 Mb/s link; each user 100 kb/s when "active", active 10% of time.
خطوات الحل
- Circuit-switching: 1000k/100k = 10 users.
- Packet switching: with 35 users, probability > 10 active at same time is less than .0004.
النتيجة
Packet switching allows more users to use network!
| Circuit switching | Packet switching | |
|---|---|---|
| Resources |
|
|
| Performance | Circuit-like (guaranteed) performance | Excessive congestion possible: packet delay and loss |
| Setup | Resources allocated for the call | Simpler, no call setup |
| Idle capacity | Circuit segment idle if not used by call | لا تذكر الشرائح شيئًا هنا. |
| Good for | Traditional telephone networks | Bursty data |
| Slide 31 example (1 Mb/s link) | 10 users | 35 users (P(> 10 active) < .0004) |
Internet structure: network of networksبنية الإنترنت
End systems connect to Internet via access ISPs. Access ISPs in turn must be interconnected, so that any two hosts can send packets to each other.
Stepwise description (slides 35–40)
- Option
Connect each access ISP to every other access ISP? Doesn't scale: O(N²) connections.
- Option
Connect each access ISP to a global transit ISP. Customer and provider ISPs have economic agreement.
- 3
If one global ISP is viable business, there will be competitors …
- 4
… which must be interconnected: IXP (Internet exchange point), peering link.
- 5
Regional networks may arise to connect access nets to ISPs.
- 6
Content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users.
At the center (slide 41)
- Small # of well-connected large networks: "tier-1" commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage.
- Content provider network (e.g., Google): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs.
Four sources of packet delayمصادر التأخير الأربعة
Delay and loss in packet-switched networks
d_nodal = d_proc + d_queue + d_trans + d_prop — nodal delay is the sum of the four delays.
Check bit errors; determine output link; typically < msec.
Time waiting at output link for transmission; depends on congestion level of router.
d_trans = L/R — L: packet length (bits); R: link bandwidth (bps).
d_prop = d/s — d: length of physical link; s: propagation speed in medium (~2×10⁸ m/sec).
Caravan analogyتشبيه القافلة
Delay and loss in packet-switched networks
- Car ~ bit; caravan ~ packet.
- Toll booth takes 12 sec to service car (bit transmission time).
- Cars "propagate" at 100 km/hr; 100 km between toll booths.
المسألة
How long until the ten-car caravan is lined up before the 2nd toll booth?
خطوات الحل
- Time to "push" entire caravan through toll booth onto highway = 12×10 = 120 sec.
- Time for last car to propagate from 1st to 2nd toll booth: 100 km/(100 km/hr) = 1 hr.
النتيجة
62 minutes
المسألة
Slide 49: cars now "propagate" at 1000 km/hr and the toll booth takes one min to service a car. Will cars arrive to 2nd booth before all cars are serviced at the first booth?
النتيجة
Yes! After 7 min, 1st car arrives at second booth; three cars still at 1st booth.
Packet lossفقد الحزم
Delay and loss in packet-switched networks
- Queue (aka buffer) preceding link in buffer has finite capacity.
- Packet arriving to full queue dropped (aka lost).
- Lost packet may be retransmitted by previous node, by source end system, or not at all.
Throughput and the bottleneck linkالإنتاجية وعنق الزجاجة
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.
Link on end-end path that constrains end-end throughput.
المسألة
Internet scenario (slide 53): 10 connections (fairly) share backbone bottleneck link R bits/sec; each has a server link Rs and a client link Rc.
الصيغة
per-connection end-end throughput = min(Rc, Rs, R/10)
النتيجة
In practice: Rc or Rs is often bottleneck.
Why layering?لماذا الطبقات؟
Protocol layers and service models
Each layer implements a service via its own internal-layer actions, relying on services provided by layer below.
Why layering? Dealing with complex systems:
- Explicit structure allows identification, relationship of complex system's pieces — layered reference model for discussion.
- Modularization eases maintenance, updating of system: change of implementation of layer's service transparent to rest of system (e.g., change in gate procedure doesn't affect rest of system).
- The slide also asks: layering considered harmful?
Internet protocol stack and OSIمكدّس الإنترنت وOSI
Protocol layers and service models
| Layer | Job (slide 59) | Examples (slide 59) | Unit (slide 61) |
|---|---|---|---|
| Application | Supporting network applications | FTP, SMTP, HTTP | Message (M) |
| Transport | Process-process data transfer | TCP, UDP | Segment (Ht M) |
| Network | Routing of datagrams from source to destination | IP, routing protocols | Datagram (Hn Ht M) |
| Link | Data transfer between neighboring network elements | Ethernet, 802.11 (WiFi), PPP | Frame (Hl Hn Ht M) |
| Physical | Bits "on the wire" | لا تذكر الشرائح شيئًا هنا. | لا تذكر الشرائح شيئًا هنا. |
Presentationالعرض
- Allow applications to interpret meaning of data
- e.g., encryption, compression, machine-specific conventions
Sessionالجلسة
- Synchronization, checkpointing, recovery of data exchange
Encapsulationالتغليف
Protocol layers and service models
At the source, each layer adds its header
- Application
message M
- Transport
segment Ht M
- Network
datagram Hn Ht M
- Link
frame Hl Hn Ht M
- In the slide 61 figure, a switch processes the link and physical layers; a router processes the network, link and physical layers.
- At the destination the headers are removed layer by layer until M reaches the application.
الخلاصة
أنهيت الدرس
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ما راجعته في هذا الدرس
- What's the Internet?ما الإنترنت؟
- What's a protocol?ما البروتوكول؟
- Network edge and access networksطرف الشبكة وشبكات الوصول
- Hosts sending packets; physical mediaإرسال المضيف للحزم والوسائط المادية
- The network core: packet switchingلب الشبكة: تبديل الحزم
- Circuit switching vs packet switchingتبديل الدارات مقابل تبديل الحزم
- Internet structure: network of networksبنية الإنترنت: شبكة من الشبكات
- Delay and loss in packet-switched networksالتأخير والفقد
- Throughputالإنتاجية
- Protocol layers and service modelsطبقات البروتوكولات