Ethernet Switches

❒ layer 2 (frame) forwarding,

filtering using LAN
addresses
❒ Switching: A-to-B and A’-
to-B’ simultaneously, no
collisions
❒ large number of interfaces
❒ often: individual hosts,
star-connected into switch
❍ Ethernet, but no
collisions!

Ethernet Switches

❒ cut-through switching: frame forwarded from

input to output port without awaiting for assembly
of entire frame
❍ slight reduction in latency
❒ combinations of shared/dedicated, 10/100/1000
Mbps interfaces

1
Ethernet Switches (more)

Dedicated

Shared

IEEE 802.11 Wireless LAN

❒ wireless LANs: untethered (often mobile) networking

❒ IEEE 802.11 standard:
❍ MAC protocol
❍ unlicensed frequency spectrum: 900Mhz, 2.4Ghz

❒ Basic Service Set (BSS)

(a.k.a. “cell”) contains:
❍ wireless hosts
❍ access point (AP): base
station
❒ BSS’s combined to form
distribution system (DS)

2
Ad Hoc Networks

❒ Ad hoc network: IEEE 802.11 stations can

dynamically form network without AP
❒ Applications:
❍ “laptop” meeting in conference room, car
❍ interconnection of “personal” devices
❍ battlefield
❒ IETF MANET
(Mobile Ad hoc Networks)
working group

IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA: sender

- if sense channel idle for
DISF sec.
then transmit entire frame
(no collision detection)
-if sense channel busy
then binary backoff

802.11 CSMA receiver:

if received OK
return ACK after SIFS

3
IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

❒ NAV: Network Allocation
Vector
❒ 802.11 frame has
transmission time field
❒ others (hearing sata) defer
access for NAV time units

Hidden Terminal effect

❒ hidden terminals: A, C cannot hear each other

❍ obstacles, signal attenuation
❍ collisions at B
❒ goal: avoid collisions at B
❒ CSMA/CA: CSMA with Collision Avoidance

4
Collision Avoidance: RTS-CTS exchange

❒ CSMA/CA: explicit
channel reservation
❍ sender: send short
RTS: request to send
❍ receiver: reply with
short CTS: clear to
send
❒ CTS reserves channel for
sender, notifying
(possibly hidden) stations
❒ avoid hidden station
collisions

Collision Avoidance: RTS-CTS exchange

❒ RTS and CTS short:

❍ collisions less likely, of
shorter duration
❍ end result similar to
collision detection
❒ IEEE 802.11 alows:
❍ CSMA
❍ CSMA/CA: reservations
❍ polling from AP

5
Point to Point Data Link Control

❒ one sender, one receiver, one link: easier than

broadcast link:
❍ no Media Access Control
❍ no need for explicit MAC addressing
❍ e.g., dialup link, ISDN line
❒ popular point-to-point DLC protocols:
❍ PPP (point-to-point protocol)
❍ HDLC: High level data link control (Data link
used to be considered “high layer” in protocol
stack!

PPP Design Requirements [RFC 1557]

❒ packet framing: encapsulation of network-layer

datagram in data link frame
❍ carry network layer data of any network layer
protocol (not just IP) at same time
❍ ability to demultiplex upwards
❒ bit transparency: must carry any bit pattern in the
data field
❒ error detection (no correction)
❒ connection livenes: detect, signal link failure to
network layer
❒ network layer address negotiation: endpoint can
learn/configure each other’s network address

6
PPP non-requirements

❒ no error correction/recovery
❒ no flow control
❒ out of order delivery OK
❒ no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering

all relegated to higher layers!|

PPP Data Frame

❒ Flag: delimiter (framing)

❒ Address: does nothing (only one option)
❒ Control: does nothing; in the future possible
multiple control fields
❒ Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)

7
PPP Data Frame

❒ info: upper layer data being carried

❒ check: cyclic redundancy check for error
detection

Byte Stuffing
❒ “data transparency” requirement: data field must
be allowed to include flag pattern <01111110>
❍ Q: is received <01111110> data or flag?

❒ Sender: adds (“stuffs”) extra < 01111110> byte

after each < 01111110> data byte
❒ Receiver:
❍ two 01111110 bytes in a row: discard first byte,
continue data reception
❍ single 01111110: flag byte

8
 Byte Stuffing

flag byte
pattern
in data
to send

flag byte pattern plus

stuffed byte in
transmitted data

PPP Data Control Protocol

Before exchanging network-
layer data, data link peers
must
❒ configure PPP link (max.
frame length,
authentication)
❒ learn/configure network
layer information
❍ for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP
address

9
Asynchronous Transfer Mode: ATM

❒ 1980s/1990’s standard for high-speed (155Mbps

to 622 Mbps and higher) Broadband Integrated
Service Digital Network architecture
❒ Goal: integrated, end-end transport of carry voice,
video, data
❍ meeting timing/QoS requirements of voice, video
(versus Internet best-effort model)
❍ “next generation” telephony: technical roots in
telephone world
❍ packet-switching (fixed length packets, called
“cells”) using virtual circuits

ATM architecture

❒ adaptation layer: only at edge of ATM network

❍ data segmentation/reassembly
❍ roughly analagous to Internet transport layer
❒ ATM layer: “network” layer
❍ cell switching, routing
❒ physical layer

10
 ATM: network or link layer?
Vision: end-to-end
transport: “ATM from
desktop to desktop”
❍ ATM is a network
technology
Reality: used to connect
IP backbone routers
❍ “IP over ATM”
❍ ATM as switched
link layer,
connecting IP
routers

ATM Adaptation Layer (AAL)

❒ ATM Adaptation Layer (AAL): “adapts” upper

layers (IP or native ATM applications) to ATM
layer below
❒ AAL present only in end systems, not in switches
❒ AAL layer segment (header/trailer fields, data)
fragmented across multiple ATM cells
❍ analogy: TCP segment in many IP packets

11
ATM Adaption Layer (AAL) [more]

Different versions of AAL layers, depending on ATM

service class:
❒ AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation
❒ AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video
❒ AAL5: for data (eg, IP datagrams)

User data

AAL PDU

ATM cell

AAL5 - Simple And Efficient AL (SEAL)

❒ AAL5: low overhead AAL used to carry IP

datagrams
❍ 4 byte cyclic redundancy check
❍ PAD ensures payload multiple of 48bytes
❍ large AAL5 data unit to be fragmented into 48-byte
ATM cells

12
 ATM Layer
Service: transport cells across ATM network
❒ analagous to IP network layer
❒ very different services than IP network layer

Guarantees ?
Network Service Congestion
Architecture Model Bandwidth Loss Order Timing feedback

Internet best effort none no no no no (inferred

via loss)
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

ATM Layer: Virtual Circuits

❒ VC transport: cells carried on VC from source to dest

❍ call setup, teardown for each call before data can flow
❍ each packet carries VC identifier (not destination ID)
❍ every switch on source-dest path maintain “state” for each
passing connection
❍ link,switch resources (bandwidth, buffers) may be allocated
to VC: to get circuit-like perf.
❒ Permanent VCs (PVCs)
❍ long lasting connections
❍ typically: “permanent” route between to IP routers
❒ Switched VCs (SVC):
❍ dynamically set up on per-call basis

13
 ATM VCs

❒ Advantages of ATM VC approach:

❍ QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
❒ Drawbacks of ATM VC approach:
❍ Inefficient support of datagram traffic
❍ one PVC between each source/dest pair) does
not scale (N*2 connections needed)
❍ SVC introduces call setup latency, processing
overhead for short lived connections

ATM Layer: ATM cell

❒ 5-byte ATM cell header

❒ 48-byte payload
❍ Why?: small payload -> short cell-creation delay
for digitized voice
❍ halfway between 32 and 64 (compromise!)

Cell header

Cell format

14
ATM cell header

❒ VCI: virtual channel ID

❍ will change from link to link thru net
❒ PT: Payload type (e.g. RM cell versus data cell)
❒ CLP: Cell Loss Priority bit
❍ CLP = 1 implies low priority cell, can be
discarded if congestion
❒ HEC: Header Error Checksum
❍ cyclic redundancy check

ATM Physical Layer (more)

Two pieces (sublayers) of physical layer:

❒ Transmission Convergence Sublayer (TCS): adapts
ATM layer above to PMD sublayer below
❒ Physical Medium Dependent: depends on physical
medium being used

TCS Functions:
❍ Header checksum generation: 8 bits CRC
❍ Cell delineation
❍ With “unstructured” PMD sublayer, transmission of idle
cells when no data cells to send

15
 ATM Physical Layer

Physical Medium Dependent (PMD) sublayer

❒ SONET/SDH: transmission frame structure (like a container
carrying bits);
❍ bit synchronization;
❍ bandwidth partitions (TDM);
❍ several speeds: OC1 = 51.84 Mbps; OC3 = 155.52 Mbps;
OC12 = 622.08 Mbps
❒ TI/T3: transmission frame structure (old telephone
hierarchy): 1.5 Mbps/ 45 Mbps
❒ unstructured: just cells (busy/idle)

IP-Over-ATM IP over ATM

Classic IP only ❒ replace “network”
❒ 3 “networks” (e.g., (e.g., LAN segment)
LAN segments) with ATM network
❒ MAC (802.3) and IP ❒ ATM addresses, IP
addresses addresses
ATM
network

Ethernet Ethernet
LANs LANs

16
IP-Over-ATM
Issues:
❒ IP datagrams into ATM
ATM AAL5 PDUs network

❒ from IP
addresses to
ATM addresses
❍ just like IP
addresses to Ethernet
802.3 MAC LANs
addresses!

Datagram Journey in IP-over-ATM

Network
❒ at Source Host:
❍ IP layer finds mapping between IP, ATM dest address
(using ARP)
❍ passes datagram to AAL5
❍ AAL5 encapsulates data, segments to cells, passes to
ATM layer
❒ ATM network: moves cell along VC to destination
❒ at Destination Host:
❍ AAL5 reassembles cells into original datagram
❍ if CRC OK, datgram is passed to IP

17
ARP in ATM Nets

❒ ATM network needs destination ATM address

❍ just like Ethernet needs destination Ethernet
address
❒ IP/ATM address translation done by ATM ARP
(Address Resolution Protocol)
❍ ARP server in ATM network performs
broadcast of ATM ARP translation request to
all connected ATM devices
❍ hosts can register their ATM addresses with
server to avoid lookup

X.25 and Frame Relay

Like ATM:
❒ wide area network technologies
❒ virtual circuit oriented
❒ origins in telephony world
❒ can be used to carry IP datagrams
❍ can thus be viewed as Link Layers by IP
protocol

18
X.25

❒ X.25 builds VC between source and destination for

each user connection
❒ Per-hop control along path
❍ error control (with retransmissions) on each
hop using LAP-B
• variant of the HDLC protocol
❍ per-hop flow control using credits
• congestion arising at intermediate node
propagates to previous node on path
• back to source via back pressure

IP versus X.25

❒ X.25: reliable in-sequence end-end delivery

from end-to-end
❍ “intelligence in the network”
❒ IP: unreliable, out-of-sequence end-end
delivery
❍ “intelligence in the endpoints”
❒ gigabit routers: limited processing possible
❒ 2000: IP wins

19
Frame Relay

❒ Designed in late ‘80s, widely deployed in the ‘90s

❒ Frame relay service:
❍ no error control
❍ end-to-end congestion control

Frame Relay (more)

❒ Designed to interconnect corporate customer LANs

❍ typically permanent VC’s: “pipe” carrying aggregate
traffic between two routers
❍ switched VC’s: as in ATM
❒ corporate customer leases FR service from public
Frame Relay network (eg, Sprint, ATT)

20
Frame Relay (more)

flags address data CRC flags

❒ Flag bits, 01111110, delimit frame

❒ address:
❍ 10 bit VC ID field
❍ 3 congestion control bits
• FECN: forward explicit congestion
notification (frame experienced congestion
on path)
• BECN: congestion on reverse path
• DE: discard eligibility

Frame Relay -VC Rate Control

❒ Committed Information Rate (CIR)

❍ defined, “guaranteed” for each VC
❍ negotiated at VC set up time
❍ customer pays based on CIR

❒ DE bit: Discard Eligibility bit

❍ Edge FR switch measures traffic rate for each
VC; marks DE bit
❍ DE = 0: high priority, rate compliant frame;
deliver at “all costs”
❍ DE = 1: low priority, eligible for discard when
congestion

21
Frame Relay - CIR & Frame Marking

❒ Access Rate: rate R of the access link between

source router (customer) and edge FR switch
(provider); 64Kbps < R < 1,544Kbps

❒ Typically, many VCs (one per destination router)

multiplexed on the same access trunk; each VC has
own CIR
❒ Edge FR switch measures traffic rate for each
VC; it marks
❒ (ie DE <= 1) frames which exceed CIR (these may
be later dropped)

Chapter 5: Summary

❒ principles behind data link layer services:

❍ error detection, correction
❍ sharing a broadcast channel: multiple access
❍ link layer addressing, ARP
❒ various link layer technologies
❍ Ethernet
❍ hubs, bridges, switches
❍ IEEE 802.11 LANs
❍ PPP
❍ ATM
❍ X.25, Frame Relay
❒ journey down the protocol stack now OVER!
❍ Next stops: security, network management

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