Hubs Interconnecting with hubs

Hubs are essentially physical-layer repeaters: ˆ Backbone hub interconnects LAN segments  bits coming from one link go out all other links ˆ Extends max distance between nodes  at the same rate ˆ But individual segment collision domains become one  no frame buffering large collision domain  no CSMA/CD at hub: adapters detect collisions ˆ Can’t interconnect 10BaseT & 100BaseT  provides net management functionality hub

twisted pair

hub hub hub hub

5: DataLink Layer 5-1 5: DataLink Layer 5-2

Switch Forwarding

switch ˆ Link layer device 1  stores and forwards Ethernet frames 2 3  examines frame header and selectively forwards frame based on MAC dest address hub  when frame is to be forwarded on segment, hub hub uses CSMA/CD to access segment ˆ transparent  hosts are unaware of presence of switches ˆ plug-and-play, self-learning  switches do not need to be configured • How do determine onto which LAN segment to forward frame? • Looks like a routing problem... 5: DataLink Layer 5-3 5: DataLink Layer 5-4

Self learning Filtering/Forwarding When switch receives a frame: ˆ A switch has a switch table ˆ entry in switch table: index switch table using MAC dest address  (MAC Address, Interface, Time Stamp) if entry found for destination  stale entries in table dropped (TTL can be 60 min) then{ ˆ swihitch learns which hosts can be reach ed th rough if dest on segment from which frame arrived which interfaces then drop the frame  when frame received, switch “learns” location of else forward the frame on interface indicated sender: incoming LAN segment }  records sender/location pair in switch table else flood forward on all but the interface on which the frame arrived

5: DataLink Layer 5-5 5: DataLink Layer 5-6

1 Switch example Switch example

Suppose C sends frame to D Suppose D replies back with frame to C.

address interface address interface switch switch 1 A 1 A 1 3 2 B 1 B 1 E 2 E 2 hub G 3 hub G 3 A hub hub A hub hub I I C 1 D F D F G G B C E H B C E H

ˆ Switch receives frame from C ˆ Switch receives frame from from D  notes in bridge table that C is on interface 1  notes in bridge table that D is on interface 2  because D is not in table, switch forwards frame into  because C is in table, switch forwards frame only to interfaces 2 and 3 interface 1 ˆ ˆ frame received by D 5: DataLink Layer 5-7 frame received by C 5: DataLink Layer 5-8

Switch: traffic isolation Switches: dedicated access

ˆ switch installation breaks subnet into LAN ˆ Switch with many A segments interfaces C’ ˆ switch filters packets: ˆ Hosts have direct B  same-LAN-segment frames not usually connection to switch forwarded onto other LAN segments ˆ No collisions; full duplex switch  segments become separate collision domains

switch Switching: A-to-A’ and B-to-B’ C collision simultaneously, no collisions domain B’ A’ hub hub hub

collision domain collision domain 5: DataLink Layer 5-9 5: DataLink Layer 5-10

More on Switches Institutional network

ˆ cut-through switching: frame forwarded mail server from input to output port without first to external network collecting entire frame router web server

sliggyht reduction in latency switch ˆ combinations of shared/dedicated, IP subnet 10/100/1000 Mbps interfaces

hub hub hub

5: DataLink Layer 5-11 5: DataLink Layer 5-12

2 Switches vs. Routers Summary comparison ˆ both store-and-forward devices  routers: network layer devices (examine network layer headers) hubs routers switches  switches are link layer devices ˆ routers maintain routing tables, implement routing traffic no yes yes alggmorithms isolation ˆ switches maintain switch tables, implement plug & play yes no yes filtering, learning algorithms optimal no yes no routing cut yes no yes through

5: DataLink Layer 5-13 5: DataLink Layer 5-14

Link Layer Point to Point Data Link Control

ˆ one sender, one receiver, one link: easier than ˆ 5.1 Introduction and ˆ 5.6 Hubs and switches broadcast link: services ˆ 5.7 PPP  no Media Access Control ˆ 5.2 Error detection ˆ 5.8 Link Virtualization:  no need for explicit MAC addressing and correction ATM ˆ 5. 3Multiple access  e.g ., dialup link, ISDN line protocols ˆ popular point-to-point DLC protocols: ˆ 5.4 Link-Layer  PPP (point-to-point protocol) Addressing  HDLC: High level data link control (Data link ˆ 5.5 Ethernet used to be considered “high layer” in protocol stack!

5: DataLink Layer 5-15 5: DataLink Layer 5-16

PPP Design Requirements [RFC 1557] PPP non-requirements

ˆ packet framing: encapsulation of network-layer ˆ datagram in data link frame no error correction/recovery ˆ no flow control  carry network layer data of any network layer protocol (not just IP) at same time ˆ out of order delivery OK  ability to demultiplex upwards ˆ no need to support multipoint links (e.g., polling) ˆ bit transparency: must carry any bit pattern in the data field ˆ error detection (no correction) Error recovery, flow control, data re-ordering ˆ connection liveness: detect, signal link failure to all relegated to higher layers! network layer ˆ network layer address negotiation: endpoint can learn/configure each other’s network address

5: DataLink Layer 5-17 5: DataLink Layer 5-18

3 Byte Stuffing Byte Stuffing ˆ “data transparency” requirement: data field must be allowed to include flag pattern <01111110> flag byte  Q: is received <01111110> data or flag? pattern in data to send

ˆ Sender: adds (“stuffs”) extra < 01111101> byte after each < 01111110> data byte ˆ Receiver:  two 01111101 bytes in a row: discard first byte, continue data reception flag byte pattern plus  single 01111110: flag byte stuffed byte in transmitted data

5: DataLink Layer 5-19 5: DataLink Layer 5-20

Link Layer Virtualization of networks

ˆ 5.1 Introduction and ˆ 5.6 Hubs and switches Virtualization of resources: a powerful abstraction in services ˆ 5.7 PPP systems engineering: ˆ 5.2 Error detection ˆ 5.8 Link Virtualization: ˆ computing examples: virtual memory, virtual and correction ATM and MPLS devices ˆ 5. 3Multiple access  Virtual machines: e.g ., java protocols  IBM VM os from 1960’s/70’s ˆ 5.4 Link-Layer ˆ layering of abstractions: don’t sweat the details of Addressing the lower layer, only deal with lower layers ˆ 5.5 Ethernet abstractly

5: DataLink Layer 5-21 5: DataLink Layer 5-22

The Internet: virtualizing networks The Internet: virtualizing networks Internetwork layer (IP): Gateway: 1974: multiple unconnected … differing in: ˆ addressing: internetwork ˆ “embed internetwork packets in appears as a single, uniform local packet format or extract nets  addressing conventions entity, despite underlying local them” network heterogeneity ˆ route (at internetwork level) to  ARPAnet  packet formats ˆ network of networks next gateway  data-over-cable networks  error recovery  packet satellite network (Aloha)  routing  packet radio networ k

gateway

ARPAnet satellite net ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, 5: DataLink Layer 5-23 5: DataLink Layer 5-24 May, 1974, pp. 637-648.

4 Cerf & Kahn’s Internetwork Architecture Asynchronous Transfer Mode: ATM

What is virtualized? ˆ 1990’s/00 standard for high-speed (155Mbps to ˆ two layers of addressing: internetwork and local 622 Mbps and higher) Broadband Integrated network Service Digital Network architecture ˆ new layer (IP) makes everything homogeneous at ˆ Goal: integrated, end-end transport of carry voice, internetwork layer video, data ˆ underlying local network technology  meeting timing/QoS requirements of voice, video (versus Internet best-effort model)  cable  “next generation” telephony: technical roots in  satellite telephone world  56K telephone modem  packet-switching (fixed length packets, called  today: ATM, MPLS “cells”) using virtual circuits … “invisible” at internetwork layer. Looks like a link layer technology to IP! 5: DataLink Layer 5-25 5: DataLink Layer 5-26

ATM architecture ATM: network or link layer? Vision: end-to-end transport: “ATM from IP desktop to desktop” network  ATM is a network ATM technology network Reality: used to connect IP backbone routers ˆ adaptation layer: only at edge of ATM network  “IP over ATM”  data segmentation/reassembly  ATM as switched  roughly analagous to Internet transport layer link layer, ˆ ATM layer: “network” layer connecting IP  cell switching, routing routers ˆ physical layer 5: DataLink Layer 5-27 5: DataLink Layer 5-28

ATM Adaptation Layer (AAL) ATM Layer Service: transport cells across ATM network ˆ analogous to IP network layer ˆ ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM ˆ very different services than IP network layer Guarantees ? layer below Network Service Congestion ˆ AAL present only in end systems, not in switches Architecture Model Bandwidth Loss Order Timing feedback ˆ AAL layer segment (header/trailer fields, data) Internet best effort none no no no no (inferred fragmented across multiple ATM cells via loss)  analogy: TCP segment in many IP packets ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion ATM ABR guaranteed no yes no yes minimum ATM UBR none no yes no no

5: DataLink Layer 5-29 5: DataLink Layer 5-30

5 ATM Layer: Virtual Circuits ATM VCs

ˆ VC transport: cells carried on VC from source to dest ˆ Advantages of ATM VC approach:  call setup, teardown for each call before data can flow  each packet carries VC identifier (not destination ID)  QoS performance guarantee for connection  every switch on source-dest path maintain “state” for each mapped to VC (bandwidth, delay, delay jitter) passing connection ˆ Drawbacks of ATM VC approach:  link,,wswitch resou rces ( band width , buff ers )my) may be allocated to  Inefficient support of datagram traffic VC: to get circuit-like perf.  one PVC between each source/dest pair) does ˆ Permanent VCs (PVCs) not scale (N*2 connections needed)  long lasting connections  SVC introduces call setup latency, processing  typically: “permanent” route between to IP routers overhead for short lived connections ˆ Switched VCs (SVC):  dynamically set up on per-call basis

5: DataLink Layer 5-31 5: DataLink Layer 5-32

IP-Over-ATM IP-Over-ATM IP over ATM Classic IP only ˆ replace “network” app ˆ 3 “networks” (e.g., (e.g., LAN segment) app transport LAN segments) with ATM network transport IP IP IP AAL AAL ˆ ˆ ATM addresses, IP Eth MAC (802.3) and IP Eth ATM ATM addresses addresses phy phy phy ATM phy ppyhy ATM ATM network phy

Ethernet Ethernet LANs LANs 5: DataLink Layer 5-33 5: DataLink Layer 5-34

Datagram Journey in IP-over-ATM Network IP-Over-ATM

ˆ at Source Host:  IP layer maps between IP, ATM dest address (using ARP) Issues: ATM  passes datagram to AAL5 ˆ IP datagrams into network  AAL5 encapsulates data, segments cells, passes to ATM layer ATM AAL5 PDUs ˆ ATM network: moves cell along VC to destination ˆ from IP addresses ˆ at Destination Host: to ATM addresses  AAL5 reassembles cells into original datagram  just like IP  if CRC OK, datagram is passed to IP addresses to 802.3 MAC Ethernet LANs addresses!

5: DataLink Layer 5-35 5: DataLink Layer 5-36

6 Chapter 5: Summary Chapter 6: Wireless and Mobile Networks

ˆ principles behind data link layer services: Background:  error detection, correction ˆ # wireless (mobile) phone subscribers now  sharing a broadcast channel: multiple access exceeds # wired phone subscribers!  link layer addressing ˆ computer nets: laptops, palmtops, PDAs, ˆ instantiation and implementation of various link Internet-enabled ppphone promise any time layer tthliechnologies untethered Internet access  Ethernet ˆ two important (but different) challenges  switched LANS  communication over wireless link  PPP  handling mobile user who changes point of attachment to network  virtualized networks as a link layer: ATM

5: DataLink Layer 5-37 6: Wireless and Mobile Networks 6-38

Chapter 6 outline Elements of a wireless network

6.1 Introduction Mobility wireless hosts ˆ 6.5 Principles: ˆ laptop, PDA, IP phone ˆ run applications Wireless addressing and routing to mobile users ˆ may be stationary ˆ 6.2 Wireless links, (non-mobile) or mobile characteristics ˆ 6.6 Mobile IP network  wireless does not infrastructure  CDMA ˆ 6.7 Handling mobility in always mean mobility ˆ 6.3 IEEE 802.11 cellular networks wireless LANs (“wi-fi”) ˆ 6.8 Mobility and higher- ˆ 6.4 Cellular Internet layer protocols Access  architecture 6.9 Summary  standards (e.g., GSM)

6: Wireless and Mobile Networks 6-39 6: Wireless and Mobile Networks 6-40

Elements of a wireless network Elements of a wireless network

base station wireless link ˆ typically connected to ˆ typically used to wired network connect mobile(s) to ˆ relay - responsible base station for sending packets ˆ also used as backbone between wired link network network and wireless network ˆ multiple access infrastructure host(s) in its “area” infrastructure protocol coordinates  e.g., cell towers link access 802.11 access ˆ various data rates, points transmission distance

6: Wireless and Mobile Networks 6-41 6: Wireless and Mobile Networks 6-42

7 Characteristics of selected wireless link standards 54 Mbps 802.11{a,g} 5-11 Mbps 802.11b .11 p-to-p link 1 Mbps 802.15

3G 384 Kbps UMTS/WCDMA, CDMA2000 2G 56 Kbps IS-95 CDMA, GSM

Indoor Outdoor Mid range Long range outdoor outdoor

10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km

6: Wireless and Mobile Networks 6-43

8